CMX881E1 - CML Microcircuits

CMX881

Baseband Processor for

PMR and Trunked Radios

D/881/17 April 2009

Full-Feature Audio-Processing, Signalling and Data for

Half Duplex Dual-Mode Analogue PMR and Trunked Radios

Features

• Automatic signal type scanning and IRQ on

detection of valid Rx signals, level or RSSI • Voice processing facilities, including Tx and Rx

gain setting and voice/subaudio filtering

• Tone generator for caller recognition tunes • C-BUS serial host interface

• Programmable powerdown control • RF interface allowing 1 or 2 point modulation

• Programmable signal detection thresholds • Programmable soft limiter

• Low Power operation with ‘Zero Power’ mode • Programmable Selcall Codec

• Zero ‘Talkdown’ CTCSS decoder performance

prevents dropouts

• Uncommitted Aux ADC with switchable input

to monitor signals • DTMF transmitter

• Meets ETS 300 086, MPT1327, PAA1382 and

ETS 300 230 specifications • Standard (39-tone) CTCSS and 23/24 bit DCS

Codecs

• Robust half-duplex FFSK/MSK modem, 1200/2400bps with CRC and parity generator/checker, gets

data through when signal is too degraded for voice – for text messaging/paging, location transfer,

etc. applications.

1.1 Brief Description

CMX881, a full-function half-duplex audio and signalling processor IC for both PMR and Trunked radio

systems suitable for complex and simple end-designs. Under the control of the host µC, all voiceband

requirements are catered for: voiceband and sub-audio filtering, pre/de-emphasis and audio routing and

global level setting with single or two-point modulation in the transmit path.

The combination of CTCSS and DCS functions and Selcall operation of this product offer, under software

control, increased functionality, versatility and privacy.

To cater for call setup and system signalling the CMX881 provides an embedded 1200/2400 bps

MSK/FFSK free-format modem for text messaging/paging, passing GPS location data with checksum

generation and reception and sync detection in the Rx path.

PMR Baseband Processor CMX881

With ultra low power requirements and graduated powersave, this product requires a smaller, lower-

power µC than existing PMR or trunked radio solutions. It is available in compact SSOP and TSSOP

packages.

PMR Baseband Processor CMX881

CMX881 Functions and Facilities

Half Duplex Operation

Working in a half duplex mode, when the product is in Tx the Rx sections can be powered down to extend

battery life, conversely in Rx major sections of the Tx can be treated in the same manner.

Serial Control and Data Interface

C-BUS: Serial control, data and command program interface compatible with SCI, SPI and Microwire type

interfaces.

Power Requirements and Economy

With an ultra low power requirement, the CMX881 operates from a single 2.7 to 5.5 Volt supply with

graduated ‘Sleep Mode’ powersaving facilities for both Rx and Tx modes.

Signalling:

CTCSS

Zero ‘Talkdown’ performance eliminates unwanted breaks in communication. The CMX881 is pre-

programmed with 39 standard CTCSS (+ Notone and DCS ‘turn off’ tone) frequencies, any one of which

can be selected for reception or transmission. Decoding is aided by the use of adjustable decode

PMR Baseband Processor CMX881

bandwidths and threshold levels. Decoding is carried out rapidly thus avoiding the loss of the beginning

of speech or data signals.

DCS

The DCS code is in NRZ format and is transmitted at 134.4b/s in either 23 or 24 bit patterns. The code,

for transmission or reception is programmed via the host µC with the ‘turn off’ tone being supplied from

the CTCSS facility. Decoding is carried out rapidly thus avoiding the loss of the beginning of speech or

data signals.

Selcall

This product implements a fully programmable Selcall encoder and decoder employing normal and

special tones. Tone frequencies, decoder bandwidths and thresholds are programmed by the host µC. In

receive the CMX881 scans its internal tone table for a match, reporting its results to the µC.

FFSK/MSK Data

An MPT packet and free-format half duplex FFSK/MSK modem is implemented. In receive this device

can decode data at either automatic or manually selected 1200b/s or 2400b/s rates. Additionally, in

receive, a 16-bit programmed frame sync (MPT packet-type) pattern can be detected. Formatting control

and data transfers to and from the modem is under the control of the host µC.

DTMF Tx

The CMX881, under µC control, can generate and transmit all standard DTMF tone-pairs.

Signal Monitor

An auxiliary circuit intended for the monitoring of any signal or level; both internal and external. This

function can be used in conjunction with the host µC to allow such activities as: VOX operation and/or the

‘wake-up’ of powered-down circuitry.

Audio Processing:

Adjustable Gain Input Amplifiers

Selectable, component adjustable inputs are available for microphone or line voiceband or discriminator

inputs. In either mode (Tx or Rx) the selected input can be further level adjusted under the control of the

host µC prior to signal- or audio- processing.

Voiceband and Sub-Audio Filtering with Limiting

Both Rx and Tx paths present voiceband filtering; the Tx path filter can be configured to either 12.5 or 25

kHz channel spacing whilst the Rx path also includes a sub-audio passband filter.

Voiceband Pre-emphasis and De-emphasis

Voiceband pre-emphasis is selectable to either 12.5 or 25 kHz channel configurations in the Tx path; de-

emphasis at -6dB/ octave is selectable in the Rx path.

Software Adjustable Gains, Volume, Mixing and Routing

Providing total flexibility of operation, this product, under µC control has the ability to select and route

functions and audio and signal paths, set bandwidths and threshold levels, mix audio and sub bands and

vary both input and out gain/attenuation levels. Output levels from all analogue ports can be ‘ramped’ up

and down at independently programmed rates.

Attenuation-Adjustable Single/Two-Point Modulation Outputs

To facilitate a wide range of transmitter types, the CMX881 has the ability to provide, independently

programmable, modulation outputs; for single or two-point modulation schemes.

PMR Baseband Processor CMX881

CONTENTS

Section Page

1.0 Features.............................................................................................................................1

1.1 Brief

Description.................................................................................................................1

1.2 Block Diagram....................................................................................................................7

1.3 Signal List...........................................................................................................................8

1.4 External Components.......................................................................................................10

1.4.1 PCB Layout Guidelines and Power Supply Decoupling.....................................11

1.4.2 Modulator Outputs...............................................................................................12

1.5 General Description .........................................................................................................13

1.5.1 Sleep Mode and Auto Start Up...........................................................................14

1.5.2 Auxiliary ADC......................................................................................................14

1.5.3 Receive Mode.....................................................................................................15

1.5.4 Transmit Mode....................................................................................................22

1.5.5 FFSK/MSK

Data packeting.................................................................................26

1.5.6 C-BUS Operation................................................................................................28

1.6 C-BUS

1.7 Application Notes.............................................................................................................52

1.7.1 CRC and Parity information................................................................................52

1.8 Performance Specification...............................................................................................53

1.8.1 Electrical Performance........................................................................................53

1.8.2 Packaging ...........................................................................................................63

Table Page

Table 1 Concurrent Rx Signalling Modes Supported by the CMX881..........................................15

Table 2 CTCSS Tones..................................................................................................................18

Table 3 DCS Modulation Modes...................................................................................................18

Table 4 DCS 23 Bit Codes............................................................................................................19

Table 5 Selcall Tones....................................................................................................................20

Table 6 Concurrent Tx Modes Supported by the CMX881...........................................................22

Table 7 Data Frequencies for each Baud Rate ............................................................................25

Table 8 DTMF Tone Pairs and Corresponding Tx Programming Codes .....................................26

Table 9 Maximum Data Transfer Latency.....................................................................................27

Figure Page

Figure 1 Block Diagram...................................................................................................................7

Figure 2 Recommended External Components............................................................................10

Figure 3 Power Supply Connections and De-coupling.................................................................11

Figure 4 Modulator output components to achieve -100dB/decade roll-off..................................12

Figure 5 Rx Audio Filter Frequency Response.............................................................................16

Figure 6 De-emphasis Curve for TIA/EIA-603 Compliance..........................................................16

Figure 7 Low Pass Sub-Audio Band Filter for CTCSS and DCS..................................................17

Figure 8 25kHz Channel Audio Filter Response Template ..........................................................23

Figure 9 12.5kHz Channel Audio Filter Response Template .......................................................23

Figure 10 Audio Frequency Pre-emphasis Template...................................................................24

Figure 11 Modulating Waveforms for 1200 and 2400 Baud FFSK/MSK Signals.........................25

Figure 12 C-BUS Transactions.....................................................................................................28

PMR Baseband Processor CMX881

Figure 13 Possible PMR Configuration.........................................................................................52

Figure 14 C-BUS Timing...............................................................................................................61

Figure 15 Typical FFSK/MSK Bit Error Rate Graph .....................................................................62

Figure 16 Mechanical Outline of 28-pin SSOP (D6): Order as part no. CMX881D6..................63

Figure 17 Mechanical Outline of 28-pin TSSOP (E1): Order as part no. CMX881E1................63

It is always recommended that you check for the latest product datasheet version from the Datasheets

page of the CML website: [www.cmlmicro.com].

PMR Baseband Processor CMX881

1.2 Block Diagram

Figure 1 Block Diagram

PMR Baseband Processor CMX881

1.3 Signal List

Package

D6, E1 Signal Description

Pin No. Name Type

23 VDD(D) Power The digital positive supply rail. This pin should be decoupled

to VSS(D) by a capacitor mounted close to the device pins.

5 VSS(D) Power The negative supply rail (digital ground).

18 VDD(A) Power The analogue positive supply rail. Levels and thresholds

within the device are proportional to this voltage. This pin

should be decoupled to VSS(A) by a capacitor mounted

close to the device pins.

9, 21 VSS(A) Power The negative supply rail. Both pins must be connected to

analogue ground.

1, 2 NC No connection should be made to these pins.

3 IRQN O/P A 'wire-Orable' output for connection to the Interrupt Request

input of the host. This output is pulled down to VSS(D) when

active and is high impedance when inactive. An external

pull-up resistor is required.

4 REPLY_DATA T/S The C-BUS serial data output to the host. This output is held

at high impedance when not sending data to the host.

6 SERIAL_CLOCK I/P The C-BUS serial clock input from the host.

7 CMD_DATA I/P The C-BUS serial data input from the host.

8 CSN I/P The C-BUS data loading control function. Data transfer

sequences are initiated, and completed by the CSN signal.

PMR Baseband Processor CMX881

1.3 Signal List (continued)

Package

D6, E1 Signal Description

Pin No. Name Type

10 VBIAS O/P Internally generated bias voltage of approximately VDD(A)/2,

except when bias is power-saved when VBIAS will discharge

to VSS(A). This pin should be decoupled to VSS(A) by a

capacitor mounted close to the device pins.

11 DISC I/P Input terminal of discriminator input amplifier.

12 DISC_FB O/P Output / feedback terminal of discriminator input amplifier.

13 INPUT_2 I/P Input terminal of amplifier 2, for either a second microphone

or discriminator input.

14 INPUT_2_FB O/P Output / feedback terminal of input amplifier 2.

15 MIC I/P Input terminal of microphone input amplifier.

16 MIC_FB O/P Output / feedback terminal of microphone input amplifier.

17 SIG_MONITOR I/P Signal Monitor input to the internal level detecting circuit.

19 MOD_1 O/P Modulator 1 output.

20 MOD_2 O/P Modulator 2 output.

22 AUDIO O/P Output of the audio section.

24 CLOCK/XTAL I/P The input to the on-chip oscillator for an external crystal or a

clock circuit.

25 CLOCK_OUT O/P Buffered (un-inverted) clock output available for use by other

devices in the system.

26 I/P Test input, connect to VSS(D).

27, 28 NC No connection should be made to these pins.

Notes: I/P = Input

O/P = Output

T/S = 3-state Output

NC = No Connection

PMR Baseband Processor CMX881

1.4 External Components

VDD(D)

R1 NC 128

NC 227

IRQN 326

VSS(D) X1

REPLY_DATA 425

CLOCK_OUT

C- BUS VSS(D) 524

CLOCK/XTAL VSS(D)

Interface SERIAL_CLOCK 623

VDD(D) C2

CMD_DATA 722

AUDIO R2 Loudspeaker amp

Discriminator CSN 821

VSS(A)

VSS(A) 920

MOD_2 R3 Modulator reference

C8 R5 C5 VBIAS 10 19 MOD_1 R4 Modulator control

DISC 11 18 VDD(A) C1 C3 C4

DISC_FB 12 17 SIG_MONITOR

R6 INPUT_2 13 16 MIC_FB VSS(A)

INPUT_2_FB 14 15 MIC

C7 C9* D1* R10* C10*

C6 R7 R8 R9*

Microphone VSS(A)

CMX881

Figure 2 Recommended External Components

R1 100kΩ R9 See note 6 C6 See note 4

R2 100kΩ R10 See note 6 C7 200pF

R3 100kΩ C8 See note 4

R4 100kΩ C1 100pF C9/10 See note 6

R5 See note 2 C2 1nF

R6 100kΩ C3 100pF X1 18.432MHz See note 1

R7 See note 3 C4 100pF

R8 100kΩ C5 100pF D1 See note 6

Resistors ±5%, capacitors and inductors ±20% unless otherwise stated.

Notes:

1. X1 can be a crystal or an external clock generator; this will depend on the application. The clock drift

requirement is defined in section 1.8.1. The tracks between the crystal and pin 24 and pin 5 should

be as short as possible to achieve maximum stability and best start up performance.

2. R5 should be selected to provide the desired dc gain (assuming C8 is not present) of the

discriminator input, as follows:

⏐GAINDisc⏐ = 100kΩ / R5

The gain should be such that the resultant output at the DISC_FB pin is within the discriminator input

signal range specified in section 1.8.1.

3. R7 should be selected to provide the desired dc gain (assuming C6 is not present) of the microphone

input, as follows: ⏐GAINMic⏐ = 100kΩ / R7

The gain should be such that the resultant output at the MIC_FB pin is within the microphone input

signal range specified in section 1.8.1. For optimum performance with low signal microphones, an

additional external gain stage may be required.

4. C6 and C8 should be selected to maintain the lower frequency roll-off of the microphone and

discriminator inputs as follows: C6 ≥ 30nF × ⏐GAINMic⏐

C8 ≥ 1μF × ⏐GAINDisc⏐

PMR Baseband Processor CMX881

5. INPUT_2 and INPUT_2_FB connections allow the user to have a second discriminator or

microphone input. Component connections and values are as for the networks around pins 11 and

12 or pins 15 and 16 respectively. If this input is not required pin 13 must be connected to pin 14.

6. The circuit formed by D1, C9, C10, R9 and R10 is a peak detector, this is only required when the

signal monitor is connected to an ac signal (e.g. microphone or received signal). For a dc type signal

(e.g. RSSI) these components are not required. The values of C9 and R10 set the attack time, C9

and R9 set the decay time. D1 can be any suitable small signal diode. R10 should be a high enough

value so as not to distort the signal source.

1.4.1 PCB Layout Guidelines and Power Supply Decoupling

Digital G round

128 Digital

227C11C12

ground plane

326 +

425 Cloc k O ut put

VSS(D) 524 L1

623

V DD(D) Digital + ve Suppl y

provis ion for 7 22

wire link 821

V S S (A) A nalogue Ground

VSS(A) 920

C13 C14

VBIAS 10 19 +L2

C15 11 18 V DD(A) Anal ogue + ve S upply

12 17

13 16 Analogue

14 15 ground plane

CMX881

Figure 3 Power Supply Connections and De-coupling

C11 10nF C14 10μF L1 100nH See note 7

C12 10μF C15 100nF L2 100nH See note 7

C13 10nF

Resistors ±5%, capacitors and inductors ±20% unless otherwise stated.

Notes:

7. The inductors L1 and L2 can be omitted but this may degrade system performance.

It is important to protect the analogue pins from extraneous inband noise and to minimise the impedance

between the CMX881 and the supply and bias de-coupling capacitors. The de-coupling capacitors C11,

C12, C13 and C14 should be as close as possible to the CMX881, particularly C11 and C13. It is

therefore recommended that the printed circuit board is laid out with separate ground planes for the

VSS(A) and VSS(D) in the area of the CMX881, with provision to make a link between them close to the

CMX881.

VBIAS is used as an internal reference for detecting and generating the various analogue signals. It must

be carefully decoupled, to ensure its integrity, so apart from the decoupling capacitor shown, no other

loads should be connected. If VBIAS needs to be used to set the discriminator mid-point reference, it must

be buffered with a high input impedance buffer.

The single ended microphone input(s) and audio output must be ac coupled as shown, so their return

paths can be connected to VSS(A) without introducing dc offsets. Further buffering of the audio output is

advised.

The crystal X1 can be replaced with an external clock source if required/desired. The internal clock

generating circuit can be placed in power-save mode if the clock is provided externally.

PMR Baseband Processor CMX881

1.4.2 Modulator Outputs

The combination of CMX881 and the modulator output components, R3/C3 and R4/C4, achieve roll-off

rates better than –60dB/decade. If required this can be increased to better than –100dB/decade by

replacing R3/C3 and R4/C4 with the active filter circuit shown in Figure 4.

C17/C19

MO D_1 & 2 +

R11/R13 R12/R14 C16/C18 Modul ator 1 & 2

VSS(A)

Figure 4 Modulator output components to achieve -100dB/decade roll-off

R11 120kΩ C16 220pF

R12 120kΩ C17 440pF (2 x C16)

R13 120kΩ C18 220pF

R14 120kΩ C19 440pF (2 x C18)

Resistors ±5%, capacitors and inductors ±20% unless otherwise stated.

Notes:

8. The external op-amp must be chosen to ensure that the required signal output level can be driven

within acceptable distortion limits.

PMR Baseband Processor CMX881

1.5 General Description

The CMX881 is intended for use in half duplex analogue two way land mobile radio (LMR) equipment and

is particularly suited to multi standard PMR systems. The CMX881 provides radio signal encoder and

decoder functions for: Voice, Selcall, Tx DTMF, CTCSS, DCS and FFSK/MSK data permitting simple to

sophisticated levels of tone control and data transfer. Power control facilities allow the device to be

placed in varying levels of sleep allowing the user to fine tune the power depending on system

requirements. The CMX881 includes a crystal clock generator, with buffered output, to provide a common

system clock if required. A block diagram of the CMX881 is shown in Figure 1.

Tx functions

Audio

o Single/dual microphone inputs with input amplifier and programmable gain adjustment

o Filtering selectable for 12.5kHz and 25kHz channels

o Selectable pre-emphasis

o 2-point modulation outputs with programmable level adjustment

Signalling

o Pre-programmed 39 tone CTCSS encoder

o Programmable 23/24bit DCS encoder

o Programmable audio tone generator (for custom audio tones)

o Programmable Selcall encoder

o DTMF encoder

o 1200/2400bps MSK modulator

Rx functions

Audio

o Single/dual demodulator inputs with input amplifier and programmable gain adjustment

o Voice-band and sub-audio rejection filtering

o Selectable de-emphasis

o Software volume control

Signalling

o 1 from 39 CTCSS decoder + Tone Clone mode

o 23/24bit DCS decoder

o Programmable Selcall decoder

o 1200/2400bps MSK demodulator and 16-bit frame sync detector

o Signal Monitor (RSSI / Microphone / Rx channel level monitor)

Host Interface

A serial data interface (C-BUS) is used for command, status and data transfers between the CMX881 and

the host μC; this interface is compatible with microwire, SPI etc. Interrupt signals notify the host μC when

a change in status has occurred and the μC should read the status register across the C-BUS and

respond accordingly. Interrupts only occur if the appropriate mask bit has been set. See section 1.6.15.

Auxiliary (Signal Monitor) analogue signal

The CMX881 includes a Signal level monitor. This is an 8-bit successive approximation ADC and a two

level signal sensor. The two level sensor facility can be used in conjunction with the power saving mode

to wake up powered down blocks, and issue an interrupt on the IRQN line when the Signal exceeds the

preset threshold level. The auxiliary ADC voltage reference is taken directly from the VDD(A) supply, so

the Signal level being monitored should be derived from this supply voltage.

PMR Baseband Processor CMX881

1.5.1 Sleep Mode and Auto Start Up

A power-on reset signal remains asserted for approximately 256 x xtal clock cycles after power is applied

and the clock or xtal oscillator is established. It performs the same function as the C-BUS General Reset

command ($01), further details of which are given in section 1.6.2. A temporary loss of power may cause

the power-on reset signal to be re-asserted. If this happens, both the C-BUS registers and the

Programming register block should be reprogrammed, once power has been restored and a C-BUS

General Reset command has been issued. This is to prevent any possibility of data corruption within the

device.

Power-on reset or C-BUS general reset places the CMX881 into sleep mode, which results in all internal

blocks, except the xtal clock circuit, being placed in power-saved mode. The xtal clock circuit can be

power-saved but this must be done by an explicit C-BUS command. Power saving is achieved by turning

off bias current sources or disabling local clocks, as appropriate.

During system standby periods, parts of the device can be put into sleep mode by the host to conserve

power. The Auxiliary ADC can be programmed so that when the level exceeds a threshold, an interrupt is

issued over the C-BUS and the selected mode (Tx or Rx) “woken up” within 400µs. If this time is too long

to ensure no part of the signal is lost, the DISC or MIC input and ADC path can be kept powered up whilst

in standby mode. The receive modes and transmit modes can also be activated by commands from the

C-BUS. On wake up, activation of the various signal path stages are phased appropriately to avoid

causing unwanted transients. More details are provided in section 1.6.4 on Signal Routing.

The CMX881 can be programmed to wake up its receive path automatically (automatic start-up) when the

DISC input level exceeds the ‘high’ level threshold. While the CMX881 is in automatic receive start-up

mode the DISC input must also be selected for the signal path. When not in automatic start-up mode it is

recommended that the required input is selected during Auxiliary ADC operation to avoid subsequent

switching of the input signal source.

1.5.2 Auxiliary ADC

This section of the CMX881 operates in both Tx and Rx modes and can be used to monitor one of 4

signal sources: Sig_Monitor pin, MIC1, Input_2 or DISC inputs. Activity on the selected input will

optionally issue an interrupt if host intervention is required. During idle periods the majority of the

CMX881 can be placed into low power mode. If monitoring ac signals connected to the Sig_Monitor pin

they must be rectified and filtered using passive external circuitry.

The Auxiliary ADC facility comprises an 8-bit ADC, a comparator, an 8-bit result data word and two 8-bit

threshold registers, one defining the ‘Signal high’ level and the other the ‘Signal low’ level. The two

threshold registers are combined into one 16-bit C-BUS register word. The ADC measures the Signal

level at intervals that are set by C-BUS command.

It is advised that the interval be set to <125µs while waiting for a new incoming signal so that the CMX881

and host µC can be powered up and put into the correct mode in time to avoid missing any part of the

signal. The default interval period following a reset is 20.8µs. Power dissipation of the Auxiliary ADC can

be reduced by increasing the conversion interval time.

The result of the most recent Auxiliary ADC measurement can be read over the C-BUS whenever the

Signal Processing and Aux ADC circuits are powered up.

The Auxiliary ADC compares each conversion result with the values in the ‘Signal high’ or the ‘Signal low’

threshold registers. The CMX881 can, for example, issue an interrupt to the host µC to wake up the

receive path when the Auxiliary ADC input exceeds the ‘high’ level threshold. The CMX881 can also

issue an interrupt to the host µC to indicate a weak or absent signal when it falls below the ‘low’ level

threshold. This provides a user programmable hysteresis facility. The host must ensure that the value in

the ‘low’ register is always less than that of the ‘high’ register. The options for issuing interrupts and for

automatic start-up are selected by C-BUS command.

The Auxiliary ADC options are controlled by the $B2, $B3 and $C0 C-BUS registers. The auxiliary ADC

data can be read from the $B4 C-BUS register.

The Auxiliary ADC requires the Auxiliary ADC, BIAS and Xtal clock to all be enabled in the Power Down

Control register.

PMR Baseband Processor CMX881

1.5.3 Receive Mode

The CMX881 can receive voice and various signal formats: CTCSS tone, DCS code, Selcall and FFSK/

MSK data at 1200 and 2400bps. Reception of each of these signal types can be independently

enabled/disabled by C-BUS command. If enabled, an interrupt will be issued to notify the host μC of the

presence and type of the incoming signal.

In receive mode the CMX881 performs signal type identification in 2 frequency bands, sub-audio (60 -

260Hz) and voice band (300 - 3kHz), to determine what type of signal is being received. When an

enabled signal is detected this will be indicated to the host over the C-BUS and the CMX881 will continue

to process the received signal in its band. Identification / process mode will continue in the other band.

The CMX881 can process voice and simultaneously identify and process at least 2 other signal types.

See Table 1 for valid combinations.

The receive gain and audio output amplifier gain can be adjusted by the host μC, via C-BUS command, to

provide receive signal level adjustment and output volume control or muting.

Table 1 Concurrent Rx Signalling Modes Supported by the CMX881

Sub-Audio

All combinations of: Voice band signalling

Any one of A - C:

With Rx Voice

Processing1 or

Audio Tone

generation

DCS

Inverted DCS

CTCSS

None

Selcall

1200bps FFSK/MSK

Sub-Audio

All combinations of: Voice band signalling

Any one of A - G:

No Voice

Processing or

Audio Tone

generation

DCS

Inverted DCS

CTCSS

None

Selcall

1200bps FFSK/MSK

2400bps FFSK/MSK

1200 & 2400 bps FFSK/MSK

Selcall & 1200bps FFSK/MSK

Selcall & 2400bps FFSK/MSK

Sub-Audio

All combinations of: Voice band signalling

Any one of A - H:

No Voice

Processing or

Audio Tone

generation

No Subaudio

processing

None

Selcall

1200bps FFSK/MSK

2400bps FFSK/MSK

1200 & 2400 bps FFSK/MSK

Selcall & 1200bps FFSK/MSK

Selcall & 2400bps FFSK/MSK

Selcall & 1200bps & 2400bps FFSK/MSK

1 Including optional de-emphasis

By disabling all the decoding modes, the device can be configured to receive voice only signals with no

decoding of the voice band, CTCSS or DCS signalling. This will result in reception of all signals as if they

are voice. In this case it is up to the user/host μC to respond appropriately to incoming signals.

The CMX881 operates in half duplex, so whilst in receive mode the transmit path (microphone input and

modulator output amplifiers) can be disabled and powered down if required. The AUDIO output signal

level is equalised (to VBIAS) before switching between the audio port and the modulator ports, to minimise

unwanted audible transients. The Off/Power-save level of the modulator outputs is the same as the VBIAS

pin, so the audio output level must also be at this level before switching.

PMR Baseband Processor CMX881

1.5.3.1 Receiving Voice Band Signals

When a voice based signal is being received, it is up to the μC, in response to signal status information

provided by the CMX881, to control muting/enabling of the voice band signal to the AUDIO output.

The discriminator path through the device has a programmable gain stage. Whilst in receive mode this

should normally be set to 0dB (the default) gain.

Receive Filtering

The incoming signal is filtered, as shown in Figure 5, to remove sub-audio components and to minimise

high frequency noise. When appropriate the voice signal can then be routed to the AUDIO output.

Frequency (Hz)

-60

-50

-40

-30

-20

-10

10Hz 100Hz 1000Hz 10000Hz 100000Hz

Tem

late

Filter response

250Hz

300Hz 3kHz

Figure 5 Rx Audio Filter Frequency Response

De-emphasis

Optional de-emphasis at -6dB per octave from 300Hz to 3000Hz (shown in Figure 6) can be selected to

facilitate compliance with TIA/EIA-603.

-20

-16

-12

-8

-4

100 1000 10000

Frequency (Hz)

0dB/octave

+1dB

0dB

-3dB

-18dB/octave

6dB/octave

Figure 6 De-emphasis Curve for TIA/EIA-603 Compliance

PMR Baseband Processor CMX881

1.5.3.2 Receiving and Decoding CTCSS Tones

The CMX881 is able to accurately detect valid CTCSS tones quickly to avoid losing the beginning of voice

or possibly data transmissions, and is able to continuously monitor the detected tone with minimal

probability of falsely dropping out. The received signal is filtered in accordance with the template shown

in Figure 7, to prevent signals outside the sub-audio range from interfering with the sub-audio tone

detection.

Figure 7 Low Pass Sub-Audio Band Filter for CTCSS and DCS

Once a valid CTCSS tone has been detected, the voice band signal can be passed to the audio output.

The voice band signal is extracted from the received signal by band pass filtering as shown in Figure 5.

To help decode received CTCSS tones adjustable decoder bandwidths and threshold levels permit

decode certainty and signal to noise performance to be traded when congestion or range limits the

system performance. This entails setting the tone decoder bandwidth and threshold level in P2.1 of the

Programming register ($C8) and programming the Audio & CTCSS Control register with the desired tone.

Tone Cloning™:

Tone CloningTM facilitates the detection of CTCSS tones 1 to 39 in receive mode. This allows the device

to non-predictively detect any tone in this range. The received tone number will be reported in the Tones

Status register. This tone code can then be programmed into the ‘Audio and Device Address Control’

It is recommended that the CTCSS bandwidth selected in Programming Register word P2.1 is set to be

sufficiently low to ensure no overlapping of adjacent tones.

™ Tone Cloning is a trademark of CML Microsystems Plc.

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0 200 400 600 800 1000

Fre quency (Hz)

Gai n (dB)

PMR Baseband Processor CMX881

CTCSS Tones

Table 2 lists the CTCSS tones available. The tone numbers are decimal equivalents of the numbers

written to the Audio & CTCSS Control register ($C2) and reported in the Tone Status register ($CC).

Table 2 CTCSS Tones

Tone

Number Freq.

(Hz) Tone

Number Freq.

(Hz) Tone

Number Freq.

(Hz)

001 No Tone 20 131.8 40-54 Reserved

01 67.0 21 136.5 552 Invalid

02 71.9 22 141.3 tone

03 74.4 23 146.2 >=56 Reserved

04 77.0 24 151.4

05 79.7 25 156.7

06 82.5 26 162.2

07 85.4 27 167.9

08 88.5 28 173.8

09 91.5 29 179.9

10 94.8 30 186.2

11 97.4 31 192.8

12 100.0 32 203.5

13 103.5 33 210.7

14 107.2 34 218.1

15 110.9 35 225.7

16 114.8 36 233.6

17 118.8 37 241.8

18 123.0 38 250.3

19 127.3 39 69.3

Notes:

1 Tone number 00 in the Tone Status register ($CC) indicates that none of the above subaudio

tones is being detected - see also section 1.6.19. If tone number 00 is programmed into the

Audio & CTCSS Control register ($C2) no tone will be scanned for. If CTCSS transmit is

selected this tone setting will cause the CTCSS generator to output no signal.

2 Tone number 55 is reported in the Tone Status register ($CC), when CTCSS receive is enabled

and a subaudio tone is detected that does not correspond to the selected tone. This could be a

tone in the subaudio band which is not in the table or a tone in the table which is not the selected

tone.

1.5.3.3 Receiving and Decoding DCS Codes

DCS Code is in NRZ format and transmitted at 134.4 ±0.4bps. The CMX881 is able to decode any 23 or

24 bit pattern in either of the two DCS modulation modes defined by TIA/EIA-603 and described in

Table 3. The CMX881 can detect a valid DCS Code quickly enough to avoid losing the beginning of voice

transmissions. Table 3 DCS Modulation Modes

Modulation Type: Data Bit: FM Frequency Change:

A 0 Minus frequency shift

1 Plus frequency shift

B 0 Plus frequency shift

1 Minus frequency shift

The CMX881 detects the DCS code that matches the programmed code defined in the ‘DCS Code’ words

(P2.2-3) in the Programming register ($C8).

PMR Baseband Processor CMX881

To detect the pre-programmed DCS code the signal is low pass filtered to suppress all but the sub-audio

band using the filter shown in Figure 7. Further equalisation filtering, signal slicing and level detection are

done to extract the code being received. The extracted code is then matched with the programmed 23 or

24-bit DCS code to be recognised, in the order least significant first through to most significant DCS code

bit last. Table 4 shows a selection of valid 23-bit DCS codes, this does not preclude other codes being

programmed. Recognition of a valid DCS Code will be flagged if the decode is successful (3 or less

errors). A failure to decode is indicated by a '0' flag. This flag is updated after the decoding of every 4th

bit of the incoming signal.

Once a valid DCS Code has been detected, the voice band signal can be passed to the AUDIO output

under the control of the host µC. The voice signal is extracted from the received input signal by band

pass filtering; see Figure 5. More details for programming DCS Codes are provided in section 1.6.20.3.

The end of DCS transmissions is indicated by a 134.4 ±0.5Hz tone for 150-200ms. To detect the DCS

turn off tone while receiving DCS, the DCS turn off tone option must be selected in the Audio and CTCSS

Control ($C2) register. When a DCS turn off tone is detected it will cause a DCS interrupt; the receiver

audio output can then be muted by the host.

Table 4 DCS 23 Bit Codes

DCS

Code

DCS

bits

22-12

DCS

bits

11-0 DCS

Code

DCS

bits

22-12

DCS

bits

11-0 DCS

Code

DCS

bits

22-12

DCS

bits

11-0

023 763 813 174 18B 87C 445 7B8 925

025 6B7 815 205 6E9 885 464 27E 934

026 65D 816 223 68E 893 465 60B 935

031 51F 819 226 7B0 896 466 6E1 936

032 5F5 81A 243 45B 8A3 503 3C6 943

043 5B6 823 244 1FA 8A4 506 2F8 946

047 0FD 827 245 58F 8A5 516 41B 94E

051 7CA 829 251 627 8A9 532 0E3 95A

054 6F4 82C 261 177 8B1 546 19E 966

065 5D1 835 263 5E8 8B3 565 0C7 975

071 679 839 265 43C 8B5 606 5D9 986

072 693 83A 271 794 8B9 612 671 98A

073 2E6 83B 306 0CF 8C6 624 0F5 994

074 747 83C 311 38D 8C9 627 01F 997

114 35E 84C 315 6C6 8CD 631 728 999

115 72B 84D 331 23E 8D9 632 7C2 99A

116 7C1 84E 343 297 8E3 654 4C3 9AC

125 07B 855 346 3A9 8E6 662 247 9B2

131 3D3 859 351 0EB 8E9 664 393 9B4

132 339 85A 364 685 8F4 703 22B 9C3

134 2ED 85C 365 2F0 8F5 712 0BD 9CA

143 37A 863 371 158 8F9 723 398 9D3

152 1EC 86A 411 776 909 731 1E4 9D9

155 44D 86D 412 79C 90A 732 10E 9DA

156 4A7 86E 413 3E9 90B 734 0DA 9DC

162 6BC 872 423 4B9 913 743 14D 9E3

165 31D 875 431 6C5 919 754 20F 9EC

172 05F 87A 432 62F 91A

1.5.3.4 Receiving and Decoding Selcall Tones

Selcall tones can be used to flag the start and end of a call. They may also occur during a call in which

case the tone may be audible at the receiver. If enabled, an interrupt will be issued when a signal

matching a valid Selcall tone is detected and when a present Selcall tone turns off or changes (i.e. at the

PMR Baseband Processor CMX881

start and at the end of each Selcall tone). The audio path can then be turned on and off at the appropriate

times under control of the host µC.

The CMX881 implements a fully programmable Selcall encoder / decoder. The frequency of each tone is

defined in the Program registers P1.2-18. See section 1.6.20 for programming details.

In receive the CMX881 scans through the tone table sequentially, the code reported will be the first one

that matches the incoming frequency.

Adjustable decoder bandwidths, threshold levels are programmable via the Programming register and

permits certainty of detection and signal to noise performance to be traded when congestion or range

limits the system performance. The Selcall signal is derived from the received input signal after the band

pass filtering shown in Figure 5.

Table 5 lists the Selcall codes available, these are 5 bit numbers set or reported in: Tx Tone register

($C3) and Tone Status register ($CC).

Table 5 Selcall Tones

Special / Information Tones

(5th bit = 0) Normal Tones

(5th bit = 1)

4 bit Code Frequency set in 4 bit Code Frequency set in

Dec Hex Program register: Dec Hex Program register:

0 0 No Tone 0 0 P1.31

1 1 1 1 P1.41

2 2 2 2 P1.51

3 3 3 3 P1.61

4 4 4 4 P1.71

5 5 5 5 P1.81

6 6 6 6 P1.91

7 7 7 7 P1.101

8 8 8 8 P1.111

9 9 9 9 P1.121

10 A 10 A P1.131

11 B 11 B P1.141

12 C 12 C P1.151

13 D

1-13

Reserved

13 D P1.161

14 E P1.21,2 14 E P1.171

15 F Unrecognised tone 15 F Unrecognised tone

Notes:

1 Special tone 14, and Normal tones 0 - 15 provide user programmable tone options for both

transmit and receive modes as set in the indicated Program register, for programming

information see section 1.6.20.2. To ensure correct operation tones should not be programmed

with overlapping detect bandwidths.

2 Special tone 14 is the repeat tone, this code must be used in transmit when the new code to be

sent is the same as the previous one. e.g. to send ‘333’ the sequence ‘3R3’ should be sent,

where ‘R’ is the repeat tone. When receiving Selcall tones the CMX881 will indicate the repeat

tone when it is received, it is up to the host to interpret this and decode tones accordingly.

PMR Baseband Processor CMX881

1.5.3.5 Receiving FFSK Signals

The CMX881 can decode incoming FFSK/MSK signals at either 1200 or 2400 baud data rates. It can

achieve this by deriving the baud rate from the received signal. Alternatively a control word may set the

baud rate, in which case the device only responds to signals operating at that rate. The form of

FFSK/MSK signals for these baud rates, excluding noise, is shown in Figure 11.

The received signal is filtered and data is extracted. A PLL is used to extract the clock from the recovered

serial data stream. The recovered data is stored in a 1 byte buffer and an interrupt issued to indicate

received data is ready. Data is transferred over the C-BUS, controlled by host instructions. If this data is

not read before the next data is decoded it will be overwritten. The MSK bit clock is not output externally.

It is up to the user to ensure that the data is transferred at an adequate rate following data ready being

flagged, see Table 9.

The extracted data is compared with up to three 16-bit programmed frame sync patterns (SYND, SYNC

and it's inverse SYNT). SYNC and SYND are both preset to $C4D7 following a RESET command. An

interrupt will be flagged when the programmed frame sync pattern is detected. The host may stop the

frame sync search by disabling the MSK demodulator.

FFSK may be transmitted in conjunction with a CTCSS or DCS sub-audio component. The device will

handle the sub-audio signals as already described. If a sub-audio signal turns off during reception of

FFSK, it is up to the host μC to turn off the FFSK decoding as the device will continue receiving and

processing the incoming signal until commanded otherwise by the host µC.

The host must keep track of the message length or otherwise determine the end of reception (e.g. by

using sub-audio information or the Auxiliary ADC to check for signal level) and disable the FFSK

demodulator at the appropriate time.

PMR Baseband Processor CMX881

1.5.4 Transmit Mode

The device operates in half duplex, so when the device is in transmit mode the receive path (discriminator

and audio output amplifiers) should be disabled, and can be powered down, by the host μC.

Two modulator outputs with independently programmable gains are provided to facilitate single or two-

point modulation, separate sub-audio and voice band outputs. If one of the modulator outputs is not used

it can be disabled to conserve power.

To avoid erroneous transmission of out of band frequencies when changing from Rx to Tx the MOD_1

and MOD_2 outputs are ramped to the quiescent modulator output level, VBIAS before switching.

Similarly, when starting a transmission, the transmitted signal strength is ramped up from the quiescent

VBIAS level and when ending a transmission the transmitted signal strength is ramped down to the

quiescent VBIAS level. The ramp rates are set in the Programming register P4.6. When the modulator

outputs are disabled, their outputs will be set to VBIAS. When the modulator output drivers are powered

down, their outputs will be floating (high impedance), so the RF modulator will need to be turned off.

Table 6 Concurrent Tx Modes Supported by the CMX881

Sub-Audio Voice band

CTCSS

CTCSS + Voice

CTCSS + Selcall

CTCSS + FFSK/MSK

CTCSS + DTMF

DCS

DCS + Voice

DCS + Selcall

DCS + DTMF

DCS + FFSK/MSK

Voice

Selcall

DTMF

FFSK/MSK

For all transmissions the host must only enable signals after the appropriate data and settings for those

signals are loaded into the C-BUS registers. As soon as any signalling is enabled the CMX881 will use

the settings to control the way information is transmitted.

A programmable gain stage in the microphone input path facilitates a host controlled VOGAD capability.

PMR Baseband Processor CMX881

1.5.4.1 Processing Voice Signals for Transmission over Analogue Channels

The microphone input(s), with programmable gain, can be selected as the voice input source. Pre-

emphasis is selectable with either version of the 2 analogue Tx audio filters (for 12.5kHz and 25kHz

channel spacing). These are designed for use in ETS-300-086 and/or TIA/EIA-603 compliant

applications. Both filters attenuate sub-audio frequencies below 250Hz by more than 33dB wrt the signal

level at 1kHz. These filters together with a built in limiter help ensure compliance with ETS-300-086

(25kHz and 12.5kHz channel spacing) when levels and gain settings are set up correctly in the target

system.

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10 100 1000 10000 100000

Frequency (Hz)

Gain (dB)

r eference 0dB at 1kH z

+0.5dB/-2dB wrt ref.

-33dB

3kHz

250Hz

300Hz

-14dB/octave

Figure 8 25kHz Channel Audio Filter Response Template

The filter characteristics of the 12.5kHz channel filter fits the filter template shown in Figure 9 (solid

outline). This filter also facilitates implementation of systems compliant with TIA/EIA-603 ‘A’ and ‘B’

bands. To achieve attenuation above 3kHz of better than −100dB/decade for TIA/EIA-603 ‘C’ bands

(dashed outline), additional external circuitry is required, such as suggested in section 1.4.2.

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10 100 1000 10000 100000

Frequency (Hz)

Gain (dB)

reference 0dB at 1kHz

+0.5dB/-2dB wrt ref.

-33dB

3kHz

250Hz

300Hz

- 60dB/decade wrt r ef.

2.55kHz

-100dB/d ec ade

(-50dB)

(-82.5dB)

20kH

Figure 9 12.5kHz Channel Audio Filter Response Template

PMR Baseband Processor CMX881

The CMX881 provides selectable pre-emphasis filtering of +6dB per octave from 300Hz to 3000Hz,

matching the template shown in Figure 10.

Figure 10 Audio Frequency Pre-emphasis Template

Modulator Output Routing

The sub-audio component can be combined with the voice band signal and this composite signal routed

to both MOD_1 and MOD_2 outputs, or the sub-audio and voice band signal can be output separately

(sub-audio to MOD_2 and voice band to MOD_1), in accordance with the settings of the Signal Routing

1.5.4.2 CTCSS Tone

The sub-audio CTCSS tone generated is defined in the Audio & CTCSS Control register ($C2). Table 2

lists the CTCSS tones and the corresponding value for programming the TX TONE bits.

1.5.4.3 DCS

Code

A 23 or 24-bit sub-audio DCS Code can be generated, as defined by the ‘DCS Code’ words (P2.2-3) of

the Programming register ($C8); the same DCS Code pattern is used for detection and transmission. The

DCS Code is NRZ encoded at 134.4±0.4 bits/s, low pass filtered and added to the voice band signal, prior

to passing the signal to the modulator output stages. Valid 23-bit DCS codes and the corresponding

settings for the DCS Code Register are shown in Table 4, this does not preclude other codes being

programmed. The least significant bit of the DCS code is transmitted first and the most significant bit is

transmitted last. The CMX881 is able to encode and transmit either of the two DCS modulation modes

defined by TIA/EIA-603 and described in Table 3.

To signal the end of the DCS transmission, the host should set the special sub-audio bits in the Audio &

CTCSS Control register ($C2) to enable the DCS turn off tone for 150ms to 200ms. After this time period

has elapsed the host should then disable DCS in the Mode register ($C1). Do not enable CTCSS in the

Mode Control ($C1) register when transmitting the DCS turn off tone. To summarize, detection of DCS

turn off tone requires the CTCSS decoder to be enabled, whereas generation of the DCS turn off tone

requires the CTCSS encoder to be disabled.

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-8

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100 1000 10000

Frequency (Hz)

+1dB

0dB

+12dB/octave

+6dB/octave

3dB

PMR Baseband Processor CMX881

1.5.4.4 Transmitting Selcall Tones

The Selcall tone to be generated is defined in the Tx In-Band Tones register ($C3). The tone level is set

in the Programming register (P1.0). The Selcall tone must be transmitted without other signals in the

voice band, so when either In-band signalling bit is selected, the voice path is automatically disabled. The

voice path bit should not be set to '0' at this time, as this produces anomalous results. However, the voice

path can be disabled by setting bits 4 and 5 to '00' in the Signal Routing register ($B1). Table 5 shows

valid Selcall tones, together with the values for programming the Selcall bits of the Tx In-Band Tones

Custom Selcall tone frequencies are set in the program register ($C8) P1.2-17. See section 1.6.20.2 for

programming details.

1.5.4.5 Transmitting FFSK/MSK Signals

The FFSK/MSK encoding operates in accordance with the bit settings in the Mode Control register ($C1).

When enabled the MSK modulator begins transmitting data using the settings and values in the Tx Data

enabled.

The CMX881 generates it’s own internal data clock and converts the binary data into the appropriately

phased frequencies, as shown in Figure 11 and Table 7. The binary data is taken from register $CA,

most significant bit first. The following data words must be provided over the C-BUS within certain time

limits to ensure the selected baud rate is maintained. The time limits will be dependent on the data coding

being used, see Table 9.

SIGN

LI/P

LOGIC '1' LOGIC '0'

2400 B

LOGIC '1' LOGIC '0'

R x

S I G N

L I / P

1 2 0 0 B

U D

L O G I C ' 1 ' L O G I C ' 0 ' L O G I C ' 1 ' L O G I C ' 0 '

Figure 11 Modulating Waveforms for 1200 and 2400 Baud FFSK/MSK Signals

The table below shows the combinations of frequencies and number of cycles to represent each bit of

data, for both baud rates.

Table 7 Data Frequencies for each Baud Rate

Baud Rate Data Frequency Number of Cycles

1200 baud 1 1200Hz one

0 1800Hz one and a half

2400 baud 1 1200Hz half

0 2400Hz one

Note: FFSK may be transmitted in conjunction with a CTCSS or DCS sub-audio component.

PMR Baseband Processor CMX881

1.5.4.6 Transmitting DTMF Tones

The DTMF signals to be generated are defined in the TX TONE register ($C3). Single tones and twist

(lower frequency tone reduced by 2dB) can be enabled by setting the appropriate bit in the $C3 register to

'1'. The DTMF level is set in programming register P1.0. The DTMF tones must be transmitted on their

own within the voice band, the host μC must disable other voice band signals prior to initiating

transmission of the DTMF tones, and (if required) restore the voice band signals after the DTMF

transmission is complete. Table 8 shows the DTMF tone pairs, together with the values for programming

the ‘Tone Pair’ field of the TX TONE register.

Table 8 DTMF Tone Pairs and Corresponding Tx Programming Codes

Tone Pair

Code (Hex) Key Pad

Position Low Tone

(Hz) High Tone

(Hz)

1 1 697 1209

2 2 697 1336

3 3 697 1477

4 4 770 1209

5 5 770 1336

6 6 770 1477

7 7 852 1209

8 8 852 1336

9 9 852 1477

A 0 941 1336

B * 941 1209

C # 941 1477

D A 697 1633

E B 770 1633

F C 852 1633

0 D 941 1633

Note: Only the underlined tone is generated when the 'Single Tone' bit is enabled.

1.5.5 FFSK/MSK Data packeting

The CMX881 has a built in 15 bit CRC and 1 bit parity generator / checker to ease host processing during

transmission and reception of data packets. The CRC / parity function can be used with any length

message in both Tx and Rx modes. In Tx the host may reset, add to or send the 2 byte checksum at any

byte boundary in the data sequence. In Rx the host may reset the checking circuit at any byte boundary

and the CMX881 will indicate for each subsequent byte if the preceding bytes satisfied the CRC and

parity requirements.

Tx frame example:

Write Mode reg 2

MSK transfer flag

Read Status reg 2

Write Tx Data reg 2 BS BS FS FS 0 1 2 3 0 1 2 …

Enable CRC bit 1

Tx CRC bit 1

Tx over air data BS BS FS FS 0 1 2 3 CS CS 0 1 2 …

BS = Bit sync FS = Frame sync 0, 1, … Data bytes CS = Checksum

Notes: 1 The Tx CRC and Enable CRC bits are controlled by writing to the Tx Data register

2 Actions requiring a C-BUS transfer

PMR Baseband Processor CMX881

Rx frame example:

Rx over air data BS BS FS FS 0 1 2 3 CS CS 0 1 2 …

Write Mode reg 2

Frame Sync flag

MSK transfer flag

Read Status reg 2

Checksum bit 3

Read Rx Data reg 2 0 1 2 3 CS CS 0 1 2

Enable CRC bit 2,1

BS = Bit sync FS = Frame sync 0, 1, … Data bytes CS = Checksum

Notes: 1 The Enable CRC bit is controlled by writing to the Tx Data register

2 Actions requiring a C-BUS transfer

3 The Checksum bit is read from the Rx Data register

1.5.5.1 Tx Hang bit

When transmitting FFSK/MSK data, the user should ensure that the data is terminated with a hang bit.

This is recommended regardless of whether the on-chip data formatting is used. To do this, the host must

set the 'Last Data' bit in the Tx Data register ($CA) when the message is required to end. This will

append a hang bit onto the end of the current byte and generate (if enabled) an interrupt when the last Tx

data has left the modulator.

Write Mode reg 2

MSK transfer flag

Tx MSK end flag

Read Status reg 2

Write Tx Data reg 2 BS BS FS FS 0 1 2 3

Enable CRC bit 1

Tx CRC bit 1

Last Data bit 1

Tx over air data BS BS FS FS 0 1 2 3 CS CS H

BS = Bit sync FS = Frame sync 0, 1, … Data bytes CS = Checksum H = Hang Bit

Notes: 1 The Tx CRC, Enable CRC and Last Data bits are changed by writing to the Tx Data register

2 Actions requiring a C-BUS transfer

1.5.5.2 Data Buffer Timing

Data must be transferred at the rate appropriate to the signal type and data format. The CMX881 buffers

signal data in the lower 8-bits of a 16-bit register. The CMX881 will issue interrupts to indicate when data

is available or required. The host must respond to these interrupts within the maximum allowable latency

for the signal type. Table 9 shows the maximum latencies for transferring signal data to maintain

appropriate data throughput. Table 9 Maximum Data Transfer Latency

Data type Max time to read from

or write to data buffer Data buffer

size

1200b/s MSK 6.6ms 8 bits

2400b/s MSK 3.3ms 8 bits

PMR Baseband Processor CMX881

1.5.6 C-BUS Operation

This block provides for the transfer of data and control or status information between the CMX881’s

internal registers and the µC over the C-BUS serial interface. Each transaction consists of a single

µC to be written into one of the CMX881’s Write Only Registers, or one or more data byte(s) read out from

one of the CMX881’s Read Only Registers, as illustrated in Figure 12.

Data sent from the µC on the Command Data line is clocked into the CMX881 on the rising edge of the

Serial_Clock input. Reply Data sent from the CMX881 to the µC is valid when the Serial_Clock is high.

The CSN line must be held low during a data transfer and kept high between transfers. The C-BUS

interface is compatible with most common µC serial interfaces and may also be easily implemented with

general purpose µC I/O pins controlled by a simple software routine.

The number of data bytes following an A/C byte is dependent on the value of the A/C byte. The most

significant bit of the address or data are sent first. For detailed timings see section 1.8.1.

C-BUS Write:

See Note 1 See Note 2

CSN

Serial_Clock

CMD_DATA 7 6 5 4 3 2 1 0 7 6 … 0 7 … 0

MSB LSB MSB LSB MSB LSB

Address / Command byte Upper 8 bits Lower 8 bits

REPLY_DATA

High Z state

C-BUS Read:

See Note 2

CSN

Serial_Clock

CMD_DATA 7 6 5 4 3 2 1 0

MSB LSB

Address byte Upper 8 bits Lower 8 bits

REPLY_DATA

7 6 … 0 7 … 0

High Z state MSB LSB MSB LSB

Data value unimportant Repeated cycles Either logic level valid

Figure 12 C-BUS Transactions

Notes:

1. For Command byte transfers only the first 8 bits are transferred ($01 = Reset).

2. For single byte data transfers only the first 8 bits of the data are transferred.

3. The CMD_DATA and REPLY_DATA lines are never active at the same time. The Address byte

determines the data direction for each C-BUS transfer.

4. The Serial_Clock input can be high or low at the start and end of each C-BUS transaction.

5. The gaps shown between each byte on the CMD_DATA and REPLY_DATA lines in the above

diagram are optional, the host may insert gaps or concatenate the data as required.

PMR Baseband Processor CMX881

1.6 C-BUS Register Description

1.6.1 C-BUS Register Summary

C-BUS Write Only Registers

ADDR.

(hex) REGISTER Word Size

(bits)

$01 C-BUS RESET 0

$B0 ANALOGUE GAIN 16

$B1 SIGNAL ROUTING 16

$B2 AUXILIARY ADC THRESHOLDS 16

$B3 AUXILIARY ADC CONTROL 8

$C0 POWER DOWN CONTROL 16

$C1 MODE CONTROL 16

$C2 AUDIO & CTCSS CONTROL 16

$C3 TX IN-BAND TONES 16

$C7 RESERVED REGISTER ADDRESS 16

$C8 PROGRAMMING REGISTER 16

$CA TX DATA 16

$CB RESERVED REGISTER ADDRESS 16

$CD AUDIO TONE 16

$CE INTERRUPT MASK 16

$CF RESERVED REGISTER ADDRESS 16

The C-BUS addresses $C7, $CB and $CF are allocated for production testing and must not be accessed

in normal operation.

C-BUS Read Only Registers

ADDR

(hex) REGISTER Word Size

(bits)

$B4 AUX ADC MONITOR DATA 8

$C5 RX DATA 16

$C6 STATUS 16

$C9 RESERVED REGISTER ADDRESS 16

$CC TONE STATUS 16

Interrupt Operation

The CMX881 will issue an interrupt on the IRQN line when the IRQ bit (bit 15) of the Status register and

the IRQ Mask bit (bit 15) are both set to ‘1’. The IRQ bit is set when the state of the interrupt flag bits in

the Status register change from a '0' to a '1' and the corresponding mask bit(s) in the Interrupt Mask

All interrupt flag bits in the Status register except the Programming Flag (bit 0) are cleared and the

interrupt request is cleared following the command/address phase of a C-BUS read of the Status register.

The Programming Flag bit is set to '1' only when it is permissible to write a new word to the Programming

PMR Baseband Processor CMX881

1.6.2 $01 C-BUS RESET: address only.

The reset command has no data attached to it. It sets the device registers into the states listed below.

Addr. REG. 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

$B0 ANALOGUE GAIN 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

$B1 SIGNAL ROUTING 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

$B2 AUXILIARY ADC THRESHOLDS 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0

$B3 AUXILIARY ADC TIMING 0 0 0 0 0 0 0 0

$B4 AUX ADC MONITOR DATA X X X X X X X X

$C0 POWER DOWN CONTROL 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

$C1 MODE CONTROL 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

$C2 AUDIO & CTCSS CONTROL 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

$C3 TX IN-BAND TONES 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

$C5 RX DATA XXXXXXXXXX X X X X XX

$C6 STATUS 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

$C7 Reserved Register Address 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

$C8 PROGRAMMING REGISTER 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

$CA TX DATA 00000000000 0 0 0 00

$CC TONE STATUS 00000000000 0 0 0 00

$CD AUDIO TONE 00000000000 0 0 0 00

$CE INTERRUPT MASK 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

$CF Reserved Register Address 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

Whilst the Programming Register word is cleared to zero by a general C-Bus reset, the Programming

blocks is controlled by the Power Down Control regist er (see sect ion 1.6.7).

PMR Baseband Processor CMX881

1.6.3 $B0 ANALOGUE GAIN: 16-bit write-only

Bit: 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

Inv_1 MOD_1

Attenuation Inv_2 MOD_2

Attenuation 0 Input

Gain Audio Output

Attenuation

Bits 15 and 11 set the phase of the MOD_1 and MOD_2 outputs. When set to '0' the 'true' signal (0°

phase shift) will be produced, when set to '1' the signal will be inverted (180° phase shift). This can be

useful when interfacing with rf circuitry or when generating an inverted turn off tone for CTCSS. Any

change will take place immediately after these bits are changed.

The output paths provide user programmable attenuation stages to independently adjust the output levels

of the modulators. Finer level control of the MOD_1 and MOD_2 outputs can be achieved with the FINE

OUTPUT GAIN 1 and FINE OUTPUT GAIN 2 registers (P4.2-3).

Bit 14 Bit 13 Bit 12 MOD_1 Output

Attenuation Bit 10 Bit 9 Bit 8 MOD_2 Output

Attenuation

0 0 0 >40dB 0 0 0 >40dB

0 0 1 12dB 0 0 1 12dB

0 1 0 10dB 0 1 0 10dB

0 1 1 8dB 0 1 1 8dB

1 0 0 6dB 1 0 0 6dB

1 0 1 4dB 1 0 1 4dB

1 1 0 2dB 1 1 0 2dB

1 1 1 0dB 1 1 1 0dB

Bit 7 is reserved - set to 0.

Bits 6 to 4 control the input path programmable gain stage - useful when amplifying low power voice

signals from the microphone inputs. Finer gain control can be achieved with the ‘FINE INPUT GAIN’

control register (P4.0). In receive mode it is recommended to set the gain to 0dB.

Bit 6 Bit 5 Bit 4 Input Gain Bit 3Bit 2Bit 1Bit 0 Audio Output

Attenuation

0 0 0 0dB 0 0 0 0 >60dB

0 0 1 3.2dB 0 0 0 1 44.8dB

0 1 0 6.4dB 0 0 1 0 41.6dB

0 1 1 9.6dB 0 0 1 1 38.4dB

1 0 0 12.8dB 0 1 0 0 35.2dB

1 0 1 16.0dB 0 1 0 1 32.0dB

1 1 0 19.2dB 0 1 1 0 28.8dB

1 1 1 22.4dB 0 1 1 1 25.6dB

1 0 0 0 22.4dB

1 0 0 1 19.2dB

1 0 1 0 16.0dB

1 0 1 1 12.8dB

1 1 0 0 9.6dB

1 1 0 1 6.4dB

1 1 1 0 3.2dB

1 1 1 1 0dB

Bits 3 to 0 control the output path programmable attenuation stage to adjust the volume of the audio

output signal. Finer volume control can be achieved with the ‘FINE OUTPUT GAIN 1’ control register

(P4.2).

PMR Baseband Processor CMX881

1.6.4 $B1 SIGNAL ROUTING: 16-bit write-only

Bit: 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

0 0 Tx MOD_1 and

MOD_2 Routing 0 0 0 0 0 0

Analogue

i/p select AUDIO

o/p select Ramp

Up Ramp

Down

Bits 15 and 14 reserved - set to 0.

Bits 13 and 12 select the routing of the transmit signals allowing 1 or 2 point modulation and interfaces.

Bit 13 Bit 12 Tx MOD_1 and MOD_2 routing

0 0 Tx, MOD_1 and MOD_2 outputs set to bias.

0 1 Tx, In band signals to MOD_1, Subaudio signals to MOD_2

1 0 Tx, In band and Subaudio to MOD_1, Subaudio signals to

MOD_2

1 1 Tx, In band and Subaudio to both MOD_1 and MOD_2

To route In-band and Subaudio to MOD_1 and Vbias to MOD_2, select b13 = 1 (b12 = 0 or = 1) and set

MOD_2 attenuation to >40dB in the Analogue Gain register.

‘In-Band’ in this context refers to any of the signals; Voice, Selcall tone, DTMF etc.

Bits 11 to 6 are reserved - set to 0.

Bit 5 Bit 4 Analogue Input select

0 0 No input selected (Input = VBIAS)

0 1 Input amplifier 2 (Input_2 i/p)

1 0 Microphone (MIC i/p)

1 1 Discriminator (DISC i/p)

Bit 3 Bit 2 AUDIO Output select

0 0 No output selected (Output = VBIAS)

0 1 Received Voice signal

1 0 MOD_1 signal (for Tx monitoring)

1 1 Reserved, do not use

When bits 1 or 0 are set to '1' output signals are ramped up (bit 1) or ramped down (bit 0) to reduce

transients in the transmitted signal. Time to ramp up / down is set in the 'Ramp Rate Control' section of

the Programming register (P4.6).

1.6.5 $B2 AUXILIARY ADC THRESHOLDS: 16-bit write-only

Bit: 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

High Threshold [Range: 0 to 255] Low Threshold [Range: 0 to 255]

If the selected signal level exceeds the High Threshold, the ‘Signal High’ bit of the Status register will be

set to 1. If the Signal level falls below the Low Threshold, the ‘Signal Low’ bit of the Status register will be

set to 1. If the corresponding interrupt bit is enabled, a C-BUS interrupt will be generated. These status

bits are cleared when the Status register is read. The behaviour of the CMX881 is not defined if the high

threshold is less than the low threshold.

Threshold resolution: VDD(A)/256 per LSB

Threshold accuracy: ±2 LSB

Differential linearity: ±1 LSB [monotonic]

The ‘Auxiliary ADC Thresholds’ register must not be updated whilst the Auxiliary ADC is enabled.

PMR Baseband Processor CMX881

1.6.6 $B3 AUXILIARY ADC CONTROL: 8-bit write-only

Bit: 7 6 5 4 3 2 1 0

Aux ADC i/p select Conversion Interval

The ‘Conversion Interval’ (bits 5 to 0) defines the time between measurements whilst the Auxiliary ADC is

enabled. This allows the user to trade-off device power consumption with response time.

Auxiliary ADC power = 0.5mW/VDD(A)/conversion (approximate)

Conversion Interval = 20.8μs per LSB. (approximate)

The user should set an interval to ensure that no part of a received signal is missed, so that the signal

type can be correctly identified. If using the Rx Auto start-up feature the recommended maximum

Conversion Interval is 125µs. The ‘Auxiliary ADC’ register must not be updated whilst the Aux ADC is

enabled.

The Aux ADC i/p select (bits 7 to 6) control the input to the Auxiliary ADC. Control is independent of the

Analogue i/p select bits and hence the Aux ADC can monitor any one of the 4 inputs independently.

Bit 7 Bit 6 Auxiliary ADC input from:

0 0 Signal monitor (Sig_Monitor i/p)

0 1 Input amplifier 2 (Input_2 i/p)

1 0 Microphone (MIC i/p)

1 1 Discriminator (DISC i/p)

1.6.7 $C0 POWER DOWN CONTROL: 16-bit write-only

Bit: 15 14 13 12 11 10 9 8

Input_2

amp MIC

amp Disc

amp Input

Gain Output Fine

Gain 1 Output Fine

Gain 2 O/P Coarse

Gain 1 O/P Coarse

Gain 2

Bit: 7 6 5 4 3 2 1 0

Audio

Output BIAS Signal

Processing Prog Reg

Save Disable

Xtal_N Disable

Clock_Out_N Enable

Aux ADC Rx Auto

start-up

Bits 15 to 5 provide the power control of the specified blocks. If a bit is '1', the corresponding block is on,

else it is powered down. A C-BUS or Power up reset clears all bits in this register to '0'.

If bit 5 is '0' the internal signal processing blocks are reset and placed into a power-save mode.

If bit 4 is clear to ‘0’, the program registers will be reset to the default state described below whenever the

Signal Processing block comes out of power save (b5 0 → 1). To preserve the current settings of the

Programming register values bit 4 should be set to a '1'. Setting bit 4 to ‘1’ prevents the Programming

Rx Auto start-up. This facility should only be used if all the Programming register values have been

initialised or programmed by the host μC prior to the signal processing block being put into power save.

Bits 3 and 2 control the xtal clock circuit. The xtal circuit is powered down by setting bit 3 to '1'. Note: The

Clock/Xtal pin may be driven by an external clock source regardless of the setting of these bits. The

Clock_Out pin is disabled (held low) by setting bit 2 to ‘1’. After a Power-up or C-BUS reset bits 2 and 3

are cleared to ‘0’, so that both the xtal circuit and clock output are enabled.

Bit 1 controls the Auxiliary ADC. If set to '1' the Auxiliary ADC will generate interrupts in accordance with

the settings of the interrupt mask bits. If bit 1 is '0' the Auxiliary ADC is disabled and powered down.

Bit 0 controls Rx Auto start up. If bit 0 is set to '1' and the Aux ADC input rises above the ‘High Threshold’

the device will automatically enter receive mode and initiate Rx signal type identification for those signals

enabled in the Mode register. The correct Aux ADC input, Rx signal routing and power down bits must be

set for automatic receive start up to operate, the mode control bits (b1, b0) should be set to ‘01’ (Receive)

in this case. If bit 0 is cleared to '0' the CMX881 will not automatically start-up and it is up to the host to

PMR Baseband Processor CMX881

respond to Aux ADC interrupts in this case. Bit 0 must be set to '0' whilst writing through register $C8 -

Programming Register.

Initialisation of the Programming Register Blocks

Removal of the signal processing block from reset (b5 0→ 1), with b4 kept low (= 0), will cause all of the

programming register words (P0 – P4) to be reset to zero, except the following:

P0.0 Frame SYNC LSB - - - - 0 0 0 0 1 1 0 1 0 1 1 1

P0.1 Frame SYNC MSB - - - - 0 0 0 0 1 1 0 0 0 1 0 0

P0.2 Frame SYND LSB - - - - 0 0 0 0 0 0 1 1 0 0 1 1

P0.3 Frame SYND MSB - - - - 0 0 0 0 1 0 1 1 0 1 0 0

P4.7 Transmit Limiter Control - - 1 1 1 1 1 1 1 1 1 1 1 1 1 1

This initiates the device with the MPT frame SYNC pattern of $C4D7 and the PAA frame SYND pattern of

$B433. The transmit limiter value is initialised to the maximum limit.

1.6.8 $C1 MODE CONTROL: 16-bit write-only

Bit: 15 14 13 12 11 10 9 8

Enable

Voice In band signalling:

Selcall, Tx DTMF Generate

Audio Tone Enable

CTCSS Enable DCS Enable DCS

Inverse 0

Bit: 7 6 5 4 3 2 1 0

SYNC SYND SYNT

Enable

2400b/s Enable

1200b/s 0 Mode Select

Bits 1 and 0 control the overall mode of the CMX881 according to the table below:

Bit 1 Bit 0 Device Mode

0 0 Idle

0 1 Receive Mode

1 0 Transmit Mode

1 1 Reserved - do not use

During transmit, only one signal type may be enabled for each of the sub-audio and voice bands, see

Table 6, with the exception that "enable voice" must either be selected at all times or during transmission

of audio tones or during Selcall tone transmissions or during DTMF transmissions. During receive the

CMX881 will search for all signals enabled in this register and report those that are successfully decoded.

See also Table 1 in section 1.5.3.

In transmit mode the CMX881 begins transmission of a selected signal immediately after it has been

enabled. The host µC must ensure all associated data and control bits have been set to their required

values before enabling the signal in this register.

Bits 4 and 3 control the modem functions of the CMX881 in accordance with the following table:

Bit 4 Bit 3 Tx - Transmitted signal Rx - Monitored signal(s)

0 0 None None

0 1 MSK 1k2b/s MSK 1k2b/s

1 0 MSK 2k4b/s MSK 2k4b/s

1 1 Reserved MSK 1k2 & 2k4b/s

In transmit mode data transmission will start or finish (regardless of whether all data has been

transmitted) immediately after the modem control bits are changed. To transmit a second data message

the modem control bits must be set to '0', data bytes for the following message loaded, and the required

bits set to '1'.

When MSK receive is enabled bits 5 to 7 allow the detection of the MSK SYND, SYNC and SYNT frame

sync patterns respectively. Each frame sync pattern may be individually controlled so any combination of

PMR Baseband Processor CMX881

the 3 patterns - SYND, SYNC (and it's inverse - SYNT) can be searched for. Once the frame sync pattern

has been detected, all further bits are interpreted as data. To receive a second data message, the host

must extract the length of the first message from the message, then switch back into sync detection mode

by changing either bits 0 and 1 or bits 3 and 4 to '00' for at least 350µs, before returning those bits to their

original condition. Re-acquisition of bit sync followed by frame sync will then precede the reception of the

second data message. When transmitting MSK, bits 5 to 7 should be set to '0' and the bit sync and frame

sync patterns set in the first four 8 bit transfers from the host - see section 1.5.5.

Bits 2 and 8 are reserved - set to '0'.

Bits 11 to 9 determine the sub-audio transmission / reception signalling:

Bit 11 Bit 10 Bit 9 Tx - Transmitted signal: Rx - Monitored signal(s):

0 0 0 No Sub-Audio Transmitted No Sub-audio Monitoring

0 0 1 Inverted DCS* Inverted DCS*

0 1 0 DCS DCS

0 1 1 Do not use DCS + inv DCS*

1 0 0 CTCSS CTCSS

1 0 1 Do not use CTCSS + inv DCS*

1 1 0 Do not use CTCSS + DCS

1 1 1 Do not use CTCSS + DCS + inv DCS*

* See Table 3 DCS Modulation Modes.

Bit 12 enables Audio tone generation (see section 1.6.14). This operates in transmit and receive modes.

In transmit mode this bit will only enable the Audio Generator when no other voice band signals are being

transmitted i.e. bits 14, 13, 4 and 3 set to '0'.

Bits 14 and 13 select the type of In band tone (Selcall or DTMF) to transmit or receive. When transmitting

In band signals the voice path must be enabled by setting ‘Enable Voice’ bit 15 to '1'. To de-emphasise

received in-band tones, both b15 of this register and b0 of the Programming register P1.0 must be set to

'1'. Note that the pre-emphasis of In-band signals other than voice is not allowed when the 'Enable Voice'

bit 15 is set to '1'.

Bit 14 Bit 13 Tx - Transmitted signal Rx - Monitored signal

0 0 No voice band tone transmitted No voice band tones monitored

0 1 Selcall Selcall

1 0 DTMF Reserved

1 1 Reserved Reserved

When set to '1', bit 15 enables the voice path. In transmit mode the selected audio input is routed to the

modulator outputs. In transmit mode bit 15, if set to '1', will be temporarily disabled (cleared to '0')

whenever any of the bits 3, 4, 12, 13 and 14 are set to '1'. In receive mode the voice processing path is

enabled to the audio output. In receive mode bit 15, if set to '1', will be temporarily disabled (cleared to '0')

whenever bit 12 is set to '1'. It is up to the host µC to control bit 15 when voice band signals are received.

Bits 15, 14 and 13 should not be enabled in the same command instruction.

When in receive mode, if Voice is enabled and Selcall tone detection is also selected, the voice

processing quality (SINAD) is reduced. A 350µs delay must be inserted between enabling Voice and

Selcall tone detection.

The Mode Control register ($C1) may be written to at any time (subject to C-BUS timing restrictions). If

the enable bit of the currently decoded signal is cleared, the decoder is turned off. If it is subsequently re-

enabled, the decoder will enter the appropriate signal acquisition phase.

The CMX881 will only detect signals when their amplitude is above the threshold set for each band (sub-

audio and voice), as set in the program registers. Therefore even if valid tones or signals are present the

CMX881 will ignore them unless they exceed the detect threshold. Time and level hysteresis is applied to

reduce chattering in marginal conditions.

PMR Baseband Processor CMX881

Detection strategies used by the CMX881 whilst in receive mode:

When in receive mode the CMX881 treats the received signal in two bands; Sub-audio (60-260Hz) and

voice band (300-3kHz). For the sub-audio the CMX881 can monitor and decode CTCSS and DCS

signals in parallel. Because certain FFSK bit patterns can mimic some Selcall tones the Selcall receiver

is temporarily disabled when an FFSK frame sync is detected. The host must monitor the received data

and restore Selcall (by setting bits 14 and 13 as required) when it has detected the end of data.

Configuring the CMX881 for Automatic Receive Start-Up

Prior to setting the CMX881 into Automatic Receive Start-Up mode, the Mode Control register should be

clear (Tx and Rx modes disabled). Set up the Programming Register blocks as required. Write to the

Powerdown Control register with the appropriate functions enabled or disabled, including Signal

Processing bit clear (b5 = 0), Prog Reg Save bit set (b4 = 1) and the Rx Auto Start-Up bit set (b0 = 1).

While the Signal Processing block is in its powersave condition, the Mode Control register should be set

up for the appropriate signalling schemes to be processed and Rx mode selected (b1 = 0, b0 = 1). When

the auto start-up is triggered, the signal processing bit in the Powerdown Control register will

automatically be set (=1) and the selected receiver processing will start.

Configuring and Changing Transmit or Receive Modes

Bits 15-9 and 7-3 select the device function and bits 1,0 select the mode, transmit (TX), receive (RX) or

IDLE. When IDLE mode is selected, all the function bits should also be cleared to ‘0’ by the host. If a

mode change places the device into IDLE mode, at least 350 µs should elapse before changing the

device again into an active mode (RX or TX). It is possible to transition between RX and TX modes

without going through the IDLE mode.

All the Programming register words must be configured as required, in accordance with the procedures

defined in section 1.6.20, whilst the Mode Control register ($C1) is set in IDLE mode.

In RX mode, concurrent signalling schemes are allowed, as described in section 1.5.3. Receive functions

can be switched on or off at any time while in receive mode, by setting or clearing their respective enable

bits in the Mode Control register.

In TX mode, only up to one of the in-band signalling schemes, plus "enable voice", and only up to one of

the sub-audio signalling schemes should be selected, as described in section 1.5.4, table 6.

Special consideration is needed when changing from one in-band TX function to another whilst remaining

in TX mode. To effect the change, set the current function to transmit a null signal, adjust settings for the

next function as required, wait for at least 350µs, then select the new function and turn off the current

function by writing to the Mode Control register with the current function enable bit cleared and the new

function enable bit set.

• To select a null TX voice signal function, set Signal Routing register ($B1) bits 5,4 = 0,0.

• To select a null TX Selcall tone signal function, clear Tx In-Band Tones register ($C3) = 0.

• To select a null TX DTMF tone pair signal function, clear Tx In-Band Tones register ($C3) = 0

and select TX Selcall function [or select the null TX signal of the new function].

• To select a null TX MSK signal function, set TX DATA register ($CA) ‘last data’ bit 8 = 1, as

described in sections 1.5.5.1 and 1.6.13.

• To start a new MSK data packet it is necessary to toggle between the MSK TX mode and

IDLE mode or, if CTCSS or DCS require to be maintained, toggle between the MSK TX and

null voice or null Selcall tone TX.

CTCSS sub-audio transmission is turned off by putting the device into IDLE mode (Mode Control register

= 0) [or by setting the Audio and CTCSS Control register bits 7-0 = 0, to select CTCSS No Tone]. DCS

transmission is normally ended by applying the special DCS turn-off tone. To turn off the special sub-

audio DCS turn off tone, put the device into IDLE mode [or set the Audio & CTCSS Control register bits 7-

0 = 0 and select CTCSS].

PMR Baseband Processor CMX881

Some configurations of the Audio and CTCSS Control register ($C2) and the TX Data register ($CA) are

decoded by the device at the start of the function’s operation, so they must be set up before selecting the

function in the Mode Control register.

Configuration features Register & Bits Action to instigate configuration change

Voice filter and emphasis $C2, bits 12-10 Decode triggered by entering Voice mode from

IDLE mode or from another function mode.

Selcall or DTMF using

Pre-/De-Emphasis $C2, bit 10 Decode triggered by entering the in-band

signalling mode from IDLE mode or from

another function mode.

CTCSS tone $C2, bits 5-0 Decode triggered by selecting CTCSS RX

mode. Decode of transmitted CTCSS tones is

continuous in TX mode.

MSK tx initial bit sync data $CA Decode triggered by selecting MSK TX mode.

In the case of MSK transmit functions, the data register must be updated with new data as described in

sections 1.5.5 and 1.6.13. Subsequent changes to the other configurations defined above will only take

effect when the relevant operation is re-started from IDLE mode or by switching to another mode.

Changes to other registers and configuration bits are permitted at any time, as appropriate. Normally the

device will be configured prior to selecting the transmit or receive mode in the Mode Control register.

PMR Baseband Processor CMX881

1.6.9 $C2 AUDIO & CTCSS CONTROL: 16-bit write-only

Bit: 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

0 0 0 Voice filter mode Special

Sub-Audio 0 0 CTCSS tone

Bits 5 to 0 select the CTCSS tone to be used in both Tx and Rx modes, the range of valid addresses is 0-

39 (in decimal).

In Tx the CTCSS tone number is used to select the CTCSS tone. If the tone number is outside the valid

range no signalling will occur. Decoding of the CTCSS tone number is continuous, so changing the

CTCSS tone number will cause the transmitted CTCSS tone to change accordingly.

In Rx the CTCSS tone number will be searched for when CTCSS is enabled in the Mode register and if

detected, this number will be indicated in the Tone Status register $CC. The CTCSS tone number must

be asserted before selecting CTCSS receive mode.

Bits 9 and 8 select special sub-audio tones in accordance with the following table.

Bit 9 Bit 8 Freq (Hz) Special Sub-Audio tone

0 0 - None

0 1 134.4 DCS turn off tone

1 0 - Reserved - do not set

1 1 Clone CTCSS Tone clone mode (Rx only)

• Selecting the ‘DCS turn off tone’ during DCS transmit will cause the DCS turn off tone to be

transmitted; this will override the DCS data being transmitted. Select ‘DCS turn off tone’ in this

Mode Control register to receive the ‘DCS turn off tone’.

• If the Tone CloneTM mode is selected this allows the device in Rx to non-predictively detect any

CTCSS frequency in the range of valid tones, the received tone number will be reported in the Tone

Status register $CC. The narrowest bandwidth should be selected in Programming Register word

P2.1.

The voice filter control bits 12 and 11 determine the Voice Band Filter mode applied to the voice signal

before it is transmitted or after it has been received. Bit 10 controls the de-emphasis (Rx) or pre-

emphasis (Tx) mode of the voice band filtering.

Bit 12 Bit 11 Bit 10 Voice filter mode

X X 0 Disable de/pre-emphasis

X X 1 Enable de/pre-emphasis

0 0 X No filtering applied

0 1 X In transmit mode:

HPF (to remove SA) + 12.5kHz channel filtering

In receive mode:

HPF (to remove SA) + Low pass filter

1 0 X In transmit mode:

HPF (to remove SA) + 25.0kHz channel filtering

In receive mode:

HPF (to remove SA) + Low pass filter

1 1 X Reserved – do not use

The settings of bits 10 to 12 must be asserted before voice Tx or Rx mode is selected. To change the

operating modes controlled by these bits, the Tx or Rx operation must be turned off, then after the

appropriate period (see section 1.6.8), apply the changes to this register and turn on the new mode in the

Mode Control register.

Bits 15 to 13 and 7 to 6 are reserved – set to ‘0’.

PMR Baseband Processor CMX881

1.6.10 $C3 Tx In-Band Tones: 16-bit write-only

Bit: 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

Tx Selcall tone 0 0 0 0 0 Twist Single Tx DTMF tone

Bits 15 to 11 define the tone transmitted when Tx Selcall is enabled. The frequency is as defined in Table

5 Selcall Tones.

Bits 10 to 6 are reserved – set to ‘0’.

Bit 5 controls DTMF twist, when set to '0' the two tones are sent at the same level, when set to '1' the

amplitude of the lower frequency tone is 2dB below the amplitude of the higher tone.

Bit 4 controls whether a single tone is generated w hen transmitting DTMF, when set to '0' dual tones are

sent, when set to '1' the single tone identified in Table 8 is sent on it's own.

Bits 3 to 0 define the signals produced when Tx DTMF is enabled. The frequencies are as defined in

Table 8 DTMF Tone Pairs and Corresponding Tx Programming Codes.

1.6.11 $C7 Reserved - Do not write to this register

1.6.12 $C8 PROGRAMMING REGISTER: 16-bit write-only

Bit: 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

First

Word Block

Number Block /

Data Programming Data

See section 1.6.20 for a description of this register.

1.6.13 $CA TX DATA: 16-bit write-only

Bit: 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

0 0 0 0 0

CRC Tx

CRC Last

Data Tx Data Byte

Bits 15 to 11 are reserved, set to '0'.

Bits 10 to 8 control the MPT1327 compatible CRC / parity circuit: See section 1.5.5 for timing diagrams.

En CRC (bit 10):

Tx: This bit should be changed when updating this register with new data. If this bit is set to '0' the CRC /

parity circuit will be reset, bits 7 to 0 will be passed to both the modulator and CRC / parity circuit

after it has been reset. If set to '1' the CRC / parity circuit will not be reset and bits 7 to 0 will be

passed to both the modulator and CRC / parity circuit.

Rx: In receive this bit should be changed before the interrupt for the next over-air byte occurs. If this bit is

set to '0' the next received byte will be passed to the CRC / parity circuit after it has been reset. If

this bit is set to '1' the next received byte will be passed to the CRC / parity circuit which will not be

reset.

Tx CRC (bit 9): If this bit is set to '1' the Tx Data Byte (bits 7-0) is transmitted and also passed to the CRC

and parity generator. The following 2 bytes transmitted are the 15 bits of CRC and the 1 bit of parity. The

request to load more data into the CMX881 will be raised after the 2nd byte is passed to the modulator.

Last Data (bit 8): If this bit is set to '1' then the CMX881 will ignore bits 7 to 0, finish transmitting the

current byte, append a hang bit and then turn off the FFSK modulator. At the end of transmitting the hang

bit the CMX881 will set bit 7 of the Status register to '1' and an interrupt (if enabled) will be raised, the host

may then wait a short time before shutting down the rf sections of the transmit path.

Tx Data Byte (bits 7 to 0) holds the next byte of MSK data to be transmitted. Outgoing data is continuous,

whatever data is in bits 7 to 0 will be re-transmitted if the host does not provide required data in time.

Transmission of current data will be completed before transmission of newly loaded data begins.

PMR Baseband Processor CMX881

1.6.14 $CD AUDIO TONE: 16-bit write only

Bit: 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

0 0 0 0 Audio Tone

When the required bits of the Mode Control register ($C1) are set an audio tone will be generated with the

frequency determined by bits (11-0) of this register in accordance with the formula below:

frequency = Audio Tone (i.e. 1Hz per LSB)

If bits 11-0 are programmed with '0' no tone (i.e. Vbias) will be generated when the Audio Tone is

enabled. The Audio Tone frequency must only be set to generate frequencies from 300Hz to 3000Hz.

The host must suppress other data and set the correct audio routing before generating an audio tone and

re-enable data and audio routing on completion of the audio tone. The timing of intervals between these

actions is also controlled by the host μC.

This register may be written to whilst the audio tone is being generated, any change in frequency will take

place after the C-BUS write to this register. This allows sequences (e.g. ring or alert tunes) to be

generated for the local speaker (Tx or Rx via the AUDIO pin) or transmitted signal (via the MOD1/2 pins).

PMR Baseband Processor CMX881

1.6.15 $CE INTERRUPT MASK: 16-bit write-only

Bit: 15 14 13 12 11 10 9 8

IRQ

MASK 0 Rx Selcall

detect MASK 0 Rx CTCSS

detect MASK Rx DCS

detect MASK Aux ADC

High MASK Aux ADC Low

MASK

Bit: 7 6 5 4 3 2 1 0

Tx MSK end

MASK Data transfer

MASK 0 Rx 2400b/s

detect MASK Rx 1200b/s

detect MASK 0 0

Prog Flag

MASK

Bit Value Function

15 1 Enable selected interrupts

0 Disable all interrupts (IRQN pin not activated)

14 Reserved – Set to 0

13 1 Enable interrupt when a change to a Selcall tone is detected as indicated

by a '0' to '1' change of bit 13 of the Status register

0 Disabled

12 0 Reserved - Set to 0

11 1 Enable interrupt when a change to a programmed CTCSS tone is

detected as indicated by a '0' to '1' change of bit 11 of the Status register

0 Disabled

10 1 Enable interrupt on a change in the detect status of the DCS decoder as

indicated by a '0' to '1' change of bit 10 of the Status register

0 Disabled

9, 8 1 Enable interrupt when the corresponding Aux ADC status bit changes

0 Disabled

7 1 Enable interrupt when MSK data transmission has ended

0 Disabled

6 1 Enable interrupt when an MSK data transfer is required

0 Disabled

5 0 Reserved - Set to 0

4 1 Enable interrupt when a valid 2400b/s frame sync is detected

0 Disabled

3 1 Enable interrupt when a valid 1200b/s frame sync is detected

0 Disabled

2,1 0 Reserved - Set to 0

0 1 Enable interrupt when Prog Flag bit of the Status register changes from '0'

to '1' (see Programming register $C8)

0 Disabled

PMR Baseband Processor CMX881

The following 4 registers are read only

1.6.16 $B4 AUX ADC MONITOR DATA: 8-bit read-only

Bit: 7 6 5 4 3 2 1 0

Signal Monitor Data

This data holds the result of the last measurement performed by the auxiliary ADC.

The signal processor must be on to read Aux ADC data, so Power Down Control register b5 must be set

to ‘1’. This is independent of whether Tx or Rx modes are selected.

1.6.17 $C6 STATUS: 16-bit read-only

Bit: 15 14 13 12 11 10 9 8

IRQ 0 Selcall

state

change 0 CTCSS

state

change

DCS

state

change

Aux ADC

Monitor High Aux ADC

Monitor Low

Bit: 7 6 5 4 3 2 1 0

Tx MSK

end

MSK data

transfer

required 0 Rx 2400b/s Rx 1200b/s Rx data information Programming

Flag

This word holds the current status of the CMX881: the value read out is only valid when bit 5 of the Power

Down Control register ($C0) is set to '1'. Changes in the Status register will cause the IRQ bit (bit 15) to

be set to '1' if the corresponding interrupt mask bit is enabled. An interrupt request is issued on the IRQN

pin when the IRQ bit is '1' and the IRQ MASK bit (bit 15 of register $CE) is set to '1'.

Bits 1 to 15 of the Status register are cleared to '0' after the Status register is read. Bit 0 is only cleared by

writing to the Programming Register.

Bits 14, 12 and 5 are reserved.

Bits 13, 11 and 10 indicate that a Selcall, CTCSS or DCS event caused the interrupt, the host should then

read the Tones Status register ($CC) for further information. In transmit these bits will be set to '0'.

Detection of the DCS turn off tone and removal of DCS turn off tone are both flagged as DCS events in

the Status register, not as CTCSS events.

Aux ADC High (bit 9) and Aux ADC Low (bit 8) reflect the recent history of the Aux ADC level, with respect

to the high and low thresholds. The most recent Aux ADC reading can be read from $B4.

Aux ADC

Monitor High Aux ADC

Monitor Low Aux ADC history since last reading:

0 0 Neither threshold crossed

0 1 Signal gone below low threshold

1 0 Signal gone above high threshold

1 1 Signal gone below low threshold and above high

threshold

In Tx mode bit 7 will be set when the last bit of MSK data has been transmitted. Note; this bit will only be

set if bit 8 of the Tx Data register ($CA) is set at the appropriate time. In Rx mode this bit will be set to '0'.

Bit 6 indicates that new transmit data is required (in Tx mode) or received data is ready to be read (in Rx

mode). For continuous transmission or reception of information, a data transfer should be completed

within the time appropriate for that data (see Table 9 Maximum Data Transfer Latency).

PMR Baseband Processor CMX881

Bits 4 and 3 indicate the received data rate after a valid frame sync pattern has been detected. Bits 2 and

1 indicate the received frame sync pattern detected.

Bit 4 Bit 3 Data type Bit 2 Bit 1 Received sync pattern:

0 0 none Reserved

0 0 Reserved

0 1 SYNC

1 0 SYNT

1200b/s

2400b/s 1 1 SYND

1 1

Reserved Reserved

Programming Flag, bit 0: The Programming Register ($C8) should only be written to when bit 0 is set to

'1' (with both Mode select bits set low – See register $C8). Writing to the Programming Register ($C8)

clears bit 0 to '0'. Bit 0 is restored to '1' when the programming action is complete, normally within 250 μs,

when it is then safe to write to the Programming Register.

1.6.18 $C5 RX DATA: 16-bit read-only

Bit: 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

0 0 0 0 0 0

CRC 0 Rx Data Byte

Bits 15 to 10 and 8 are reserved.

Rx CRC (bit 9) indicates the validity of the received data bytes since the En CRC bit has been set, a ‘1’

indicates a valid CRC and parity bit, a ‘0’ indicates that the received CRC and parity bits do not match the

locally calculated values - see section 1.5.5.

Rx Data Byte (bits 7 to 0) holds the most recent byte of decoded MSK data. Received data is continuous,

if the data is not read before the next data is received the current data will be over-written.

1.6.19 $CC TONES STATUS: 16-bit read-only

Bit: 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

Detected Selcall tone frequency Sub-Audio Status 0 0 Detected CTCSS code

This word holds the current status of the CMX881 sub-audio and Selcall sections. This word should be

read by the host after an interrupt caused by a DCS, CTCSS or Selcall event.

The value in bits 5 to 0, Detected CTCSS code, identifies the detected sub-audio tone by its position in

Table 2 CTCSS Tones. If bits 5 to 0 = '000000' there is no CTCSS tone currently being detected. If bits

5 to 0 = '110111' (= 55 in decimal) this indicates that an Invalid Tone has been detected. An Invalid Tone

is any tone in the subaudio band which is not the selected subaudio tone. A change in the state of bits 5-

0 to Invalid Tone from the no tone condition will not cause Status register ($C6), b11 to be set to '1'. Any

other change in the state of bits 5-0 will cause the Status register ($C6), b11 to be set to '1'.

A detected Selcall frequency is indicated by the value in bits 15 to 11, ‘Detected Selcall tone frequency’,

identifies the frequency by its position in Table 5 Selcall Tones. If bits 15 to 11 = '00000' there is no

Selcall tone currently being detected. A change in the state of bits 15 to 11 will cause bit 13 of the Status

and No Tone.

Bits 10 to 8 indicate the DCS and special sub-audio tone status. The Status register ($C6) will indicate

the type of signal detected. If DCS or special CTCSS tones are detected they will be indicated in bits 10

to 8 according to the table below and bits 5 to 0 will be set to '000000'. If a normal CTCSS tone is

detected bits 10 to 8 will be set to '000' and bits 5 to 0 will indicate the decoded tone. A change in the

state of bits 10 to 8 will cause the DCS state change bit of the Status register to be set to '1'. During DCS

receive, the device can flag an interrupt when the DCS code fails to be recognised. This may be due to

PMR Baseband Processor CMX881

code dropout. The turn off tone may be flagged shortly after, if the transmission is ending. Alternatively

the DCS link may be restored and DCS detection will be flagged again.

Bit 10 Bit 9 Bit 8 Sub-Audio status

0 0 0 No DCS or special CTCSS detected

0 0 1 Reserved

0 1 0 DCS sequence detected Only enabled with DCS

0 1 1 inverted DCS sequence detected Only enabled with DCS

1 0 0 Reserved

1 0 1 134.4Hz DCS turn off tone detected Only enabled with DCS

1 1 0 Reserved

1 1 1 Reserved

When the relevant detection mode is not enabled, the associated bits will be set to '0'. In Tx mode this

Bits 7 and 6 are reserved.

During DCS receive, the device can flag an interrupt when the DCS code fails to be recognised. This may

be due to code dropout. The turn off tone may be flagged shortly after, if the transmission is ending.

Alternatively the DCS link may be restored and DCS detection will be flagged again.

PMR Baseband Processor CMX881

1.6.20 $C8 PROGRAMMING REGISTER: 16-bit write-only

Bit: 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

First

Word Block

Num. Block Num.

or Data Programming Data

This register is used for programming various gains, levels, offset compensations, tones and codes. The

programmed values are initialised in accordance with the settings described in section 1.6.7 (Power Down

Control), when the signal processing block is taken out of reset and the Prog Reg Save bit is clear (= 0).

The Signal Processing function and the XTAL clock circuit must both be enabled in order to write to the

Programming Register, so Power Down Control register bit 5 must be set to '1' and bit 3 must be set to '0'.

All other interrupt sources should be disabled while loading the programming register blocks.

The Programming Register should only be written to when the Programming Flag bit (bit 0) of the Status

'0'). The Programming Flag is cleared when the Programming Register is written to. When the

corresponding programming action has been completed (normally within 250μs) the CMX881 will set the

flag back to '1' to indicate that it is now safe to write the next programming value. The Programming

sequence of 16-bit words to the Programming Register, in the order shown in the following tables. Writing

data to the Programming Register must be performed in the order shown for each of the blocks, however

the order in which the blocks are written is not critical. If later words in a block do not require updating the

user may stop programming that block when the last change has been performed. e.g. If only 'Fine output

gain 1' needs to be changed the host will need to write to P4.0, P4.1 and P4.2 only.

The user must not exceed the defined word counts for each block. The word P4.8 is allocated for

production testing and must not be accessed in normal operation.

The high order bits of each word define which block the word belongs to, and if it is the first word of that

block: Bit 15 Bit 14 Bit 13 Bit 12 Bit 11 – Bit 0

1 X X X 1st data for each block

0 X X X 2nd and following data

X 1 0 0 Write to block 0 (12 bit words)

X 1 0 1 Write to block 1 (12 bit words)

X 1 1 0 Write to block 2 (12 bit words)

X 1 1 1 Reserved - do not use

X 0 Write to block 4 (14 bit words)

Block 0 – Modem Configuration:

Bit: 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

P0.0 1 1 0 0 0 MSK SYNC / SYNT LSB

P0.1 0 1 0 0 0 MSK SYNC / SYNT MSB

P0.2 0 1 0 0 0 MSK SYND LSB

P0.3 0 1 0 0 0 MSK SYND MSB

PMR Baseband Processor CMX881

Block 1 –Selcall Setup:

Bit: 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

P1.0 1 1 0 1 Audio band Tx level Emph

P1.1 0 1 0 1 0 Audio band detect threshold Selcall detect bandwidth

P1.2 0 1 0 1 0 Programmable Selcall Repeat Tone

P1.3 0 1 0 1 0 Programmable Selcall Tone 0

P1.4 0 1 0 1 0 Programmable Selcall Tone 1

P1.5 0 1 0 1 0 Programmable Selcall Tone 2

P1.6 0 1 0 1 0 Programmable Selcall Tone 3

P1.7 0 1 0 1 0 Programmable Selcall Tone 4

P1.8 0 1 0 1 0 Programmable Selcall Tone 5

P1.9 0 1 0 1 0 Programmable Selcall Tone 6

P1.10 0 1 0 1 0 Programmable Selcall Tone 7

P1.11 0 1 0 1 0 Programmable Selcall Tone 8

P1.12 0 1 0 1 0 Programmable Selcall Tone 9

P1.13 0 1 0 1 0 Programmable Selcall Tone 10

P1.14 0 1 0 1 0 Programmable Selcall Tone 11

P1.15 0 1 0 1 0 Programmable Selcall Tone 12

P1.16 0 1 0 1 0 Programmable Selcall Tone 13

P1.17 0 1 0 1 0 Programmable Selcall Tone 14

P1.18 0 1 0 1 0 Reserved, set to zero

Block 2 – CTCSS and DCS Setup:

Bit: 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

P2.0 1 1 1 0 CTCSS and DCS Tx level

P2.1 0 1 1 0

DCS

24 0 CTCSS and DCS detect threshold CTCSS detect

bandwidth

P2.2 0 1 1 0 DCS Code bits 11 – 0

P2.3 0 1 1 0 DCS Code bits 23/22 – 12

P2.4 0 1 1 0 Sub-audio drop out time 0

Block 3 – Reserved. Do not use.

PMR Baseband Processor CMX881

Block 4 – Gain and Offset Setup:

Bit: 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

P4.0 1 0 Fine Input Gain

P4.1 0 0 Reserved - set to '0'

P4.2 0 0 Fine Output Gain 1

P4.3 0 0 Fine Output Gain 2

P4.4 0 0 Output 1 Offset Control

P4.5 0 0 Output 2 Offset Control

P4.6 0 0 Ramp Rate Control

P4.7 0 0 Limiter Setting (all 1's = Vbias +/- 0.5 Vdd)

P4.8 0 0 Special Programming Register (Production Test Only)

1.6.20.1 PROGRAMMING REGISTER Block 0 – Modem Configuration:

$C8 (P0.0-3) MSK Frame SYNC / SYNT and SYND

Bit: 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

P0.0 1 1 0 0 0 MSK SYNC / SYNT LSB

P0.1 0 1 0 0 0 MSK SYNC / SYNT MSB

P0.2 0 1 0 0 0 MSK SYND LSB

P0.3 0 1 0 0 0 MSK SYND MSB

Bits 7 to 0 set the three 16-bit Frame Sync patterns used in Rx MSK data. Bit 7 of the MSB is compared

to the earliest received data. Note: SYNT is the bitwise inverse of SYNC. After a power on reset SYNC is

set to $C4D7 (MPT) and SYND is set to $B433 (PAA), if Powerdown Control bit 4 is clear when signal

processing is enabled (See section 1.6.7).

PMR Baseband Processor CMX881

1.6.20.2 PROGRAMMING REGISTER Block 1 – Selcall Setup:

$C8 (P1.0) Voice band tones Tx Level

Bit: 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

P1.0 1 1 0 1 Voice band tones Tx level Emph

Bits 11 (MSB) to 1 (LSB) set the transmitted Selcall, DTMF, Audio Tone and MSK signal level (pk-pk) with

a resolution of VDD(A)/2048 per LSB (1.465mV per LSB at VDD(A)=3V). Valid range for this value is 0 to

1536.

Bit 0 controls Rx Selcall de-emphasis. When set to '0' the signal going to the Selcall tone detector is not

de-emphasised. When voice processing is enabled in the Mode register, de/pre-emphasis is enabled in

the Audio & Device Address register and this bit (b0) is set to '1', signals going to the Selcall tone detector

are de-emphasised in accordance with Figure 6.

$C8 (P1.1) Selcall Detect Bandwidth and Audio Band Detect Threshold

Bit: 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

P1.1 0 1 0 1 0 Audio band detect threshold Selcall detect bandwidth

The ‘detect threshold’ bits (bits 9 to 4) set the minimum Selcall and/or MSK signal level that will be

detected. The levels are set according to the formula:

Minimum Level = Detect Threshold × 3.63mV rms at VDD(A) = 3V

The Selcall detected bandwidth is set in accordance with the following table:

BANDWIDTH

Bit 3 Bit 2 Bit 1 Bit 0 Will Decode Will Not Decode

1 0 0 0 ±1.1% ±2.4%

Recommended for EEA ⇒ 1 0 0 1 ±1.3% ±2.7%

1 0 1 0 ±1.6% ±2.9%

1 0 1 1 ±1.8% ±3.2%

$C8 (P1.2-17) Programmable Selcall Tones

Bit: 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

P1.2-17 0 1 0 1 0 Programmable Selcall Tone

N (see below) R (see below)

These words set the programmable Selcall tones used in transmit and receive. The frequency is set in

bits 11-0 for each word according to the formula:

N = Integer part of (0.042666 x frequency)

R = (0.042666 x frequency - N) x 6000 / frequency (round to nearest integer)

Example: For 1010Hz, N = 43, R = 1. The programmed tones should only be set to frequencies between

400Hz and 3000Hz. It is possible to programme frequencies outside these limits but the programmed

transmitter signal levels and accuracy and receiver thresholds and decode bandwidths may not be

applicable – see section 1.8.1, AC parameters of the Selcall Tone Detector and of the Selcall Tone

Encoder.

PMR Baseband Processor CMX881

1.6.20.3 PROGRAMMING REGISTER Block 2 – CTCSS and DCS Setup:

$C8 (P2.0) CTCSS and DCS TX LEVEL

Bit: 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

P2.0 1 1 1 0 CTCSS and DCS Level

Bits 11 (MSB) to 0 (LSB) set the transmitted CTCSS or DCS sub-audio signal level (pk-pk) with a

resolution of VDD(A)/16384 per LSB (0.183mV per LSB at VDD(A)=3V, giving a range 0 to 749.8mV pk-

pk).

$C8 (P2.1) CTCSS TONE BW AND LEVEL

Bit: 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

P2.1 0 1 1 0

DCS

24 0 CTCSS and DCS detect threshold CTCSS detect

bandwidth

Bit 11, DCS 24, sets the length of DCS code transmitted or searched for. When this bit is set to ‘1’ 24 bit

codes are transmitted and decoded. When this bit is set to ‘0’ 23 bit codes are used.

The ‘detect threshold’ bits (bits 9 to 4) set the minimum CTCSS or DCS signal level that will be detected.

The levels are set according to the formula:

Minimum Level = Detect Threshold × 2mV rms at VDD(A) = 3V

The CTCSS detected tone bandwidth is set in accordance with the following table:

BANDWIDTH

Bit 3 Bit 2 Bit 1 Bit 0 Will Decode Will Not Decode

Recommended for Tone CloningTM ⇒ 0 1 1 0 ±0.5% ±1.8%

0 1 1 1 ±0.8% ±2.1%

Recommended for CTCSS ⇒ 1 0 0 0 ±1.1% ±2.4%

1 0 0 1 ±1.3% ±2.7%

1 0 1 0 ±1.6% ±2.9%

1 0 1 1 ±1.8% ±3.2%

$C8 (P2.2-3) DCS CODE (LOWER) and DCS CODE (UPPER)

Bit: 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

P2.2 0 1 1 0 DCS Data (bits 11-0)

P2.3 0 1 1 0 DCS Data (bits 23/22-12)

These words set the DCS code to be transmitted or searched for. The least significant bit (bit 0) of the

DCS code is transmitted or compared first and the most significant bit is transmitted or compared last.

Note that DCS Data bit 23 is only used when bit 11 (DCS 24) of P2.1 is set to ‘1’.

$C8 (P2.4) SUBAUDIO DROP OUT TIME

Bit: 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

P2.4 0 1 1 0 Subaudio Drop Out Time 0

The Subaudio Drop Out Time defines the time that the sub-audio signal detection can drop out before

loss of sub-audio is asserted. The period is set according to the formula:

Time = Subaudio Drop Out Time × 8.0ms [range 0 to 120ms]

The setting of this register defines the maximum drop out time that the device can tolerate. The setting of

this register also determines the de-response time, which is typically 90ms longer than the programmed

drop out time.

PMR Baseband Processor CMX881

1.6.20.4 PROGRAMMING REGISTER Block 3 – Reserved

1.6.20.5 PROGRAMMING REGISTER Block 4 – Gain and Offset Setup

$C8 (P4.0) FINE INPUT GAIN

Bit: 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

P4.0 1 0 Fine Input Gain (unsigned integer)

Gain = 20 × log([32768-IG]/32768)dB IG is the unsigned integer value in the ‘Fine Input Gain’ field

Fine input gain adjustment should be kept within the range 0 to -3.5dB.

$C8 (P4.1) Reserved

Bit: 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

P4.1 0 0 Reserved - set to '0'

This register is reserved and should be set to '0'.

$C8 (P4.2-3) FINE OUTPUT GAIN 1 and FINE OUTPUT GAIN 2

Bit: 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

P4.2 0 0 Fine Output Gain 1 (unsigned integer)

P4.3 0 0 Fine Output Gain 2 (unsigned integer)

Gain = 20 × log([32768-OG]/32768)dB OG is the unsigned integer value in the ‘Fine Output Gain’ field

Fine output gain adjustment should be kept within the range 0dB to -3.5dB.

$C8 (P4.4-5) OUTPUT 1 OFFSET and OUTPUT 2 OFFSET

Bit: 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

P4.4 0 0 2’s complement offset for MOD_1, resolution = VDD(A)/65536 per LSB

P4.5 0 0 2’s complement offset for MOD_2, resolution = VDD(A)/65536 per LSB

The programmed value is subtracted from the output signal. Can be used to compensate for inherent

offsets in the output path via MOD_1 (Output 1 Offset) and MOD_2 (Output 2 Offset). It is recommended

that the offset correction is kept within the range +/-50mV.

$C8 (P4.6) RAMP RATE CONTROL

Bit: 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

P4.6 0 0 Ramp Rate Up Control (RRU) Ramp Rate Down control (RRD)

The ramp-up rate and ramp-down rates can be independently programmed. The ramp rates apply to all

the analogue output ports. They only affect those ports being turned on (ramp-up) or turned off (ramp

down). The ramp rates should be programmed before ramping any outputs.

Time to ramp-up to full gain = (1 + RRU) × 1.333ms

Time to ramp down to zero gain = (1 + RRD) × 1.333ms

Ramp up starts from when transmit mode starts (Mode Control Register bit 1 set = ’1’). Ramp down starts

from when transmit mode is turned off (Mode Control Register bit 1 cleared = ’0’).

PMR Baseband Processor CMX881

$C8 (P4.7) TRANSMIT LIMITER CONTROL

Bit: 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

P4.7 0 0 Limiter Setting, resolution = VDD(A)/16384 per LSB

This unsigned number sets the clipping point (maximum deviation from the centre value) for the MOD_1

and MOD_2 pins. The maximum setting ($1FFF) is +/- VDD(A)/2 i.e. output limited from 0 to VDD(A).

The limiter is set to maximum following a C-BUS Reset or a Power Up Reset. The limiter is applied to the

composite inband and subaudio signal, not just the voice signal. The levels of internally generated signals

must be limited by setting appropriate transmit levels.

$C8 (P4.8) Special Programming Register – do not access.

PMR Baseband Processor CMX881

1.7 Application Notes

Receive

FFSK

Voice+SubAudio

CMX881

Transmit

FFSK

Voice+SubAudio

Dem odulator

2-point Tx

Modulator

Host

MicroController

Da ta Po r t

Radio Section

BUS

MIC 1

MIC 2

Audio output

Audio input 1

Audio input 2

Discriminator

input

M odulator

output 1

M odulator

output 2

Dis

Figure 13 Possible PMR Configuration

1.7.1 CRC and Parity information

15 bit CRC is used with the inbuilt data packeting with the following generator polynomial:

x15 + x14+ x13+ x11+ x 4+ x2 + x0

A 15 bit remainder is calculated for previous bytes sent. When the CMX881 is instructed to send the CRC

these 15 bits are added onto the end of the message with the least significant bit inverted.

The 16th bit of the checksum is an even parity bit calculated from the message data and 15 bit CRC result

(including the inverted last bit of the CRC).

In receive the 15 bit CRC is calculated and even parity is generated at each byte boundary. If the

calculated receive CRC is zero and the parity bits match the CRC bit is set to indicate a correctly decoded

message.

PMR Baseband Processor CMX881

1.8 Performance Specification

1.8.1 Electrical Performance

The performance data are target figures, that may change subject to the outcome of device evaluation.

Absolute Maximum Ratings

Exceeding these maximum ratings can result in damage to the device.

Min. Max. Unit

Supply: VDD(D)- VSS(D) −0.3 7.0 V

VDD(A)- VSS(A) −0.3 7.0 V

Voltage on any pin to VSS(D) −0.3 VDD(D) + 0.3 V

Voltage on any pin to VSS(A) −0.3 VDD(A) + 0.3 V

Current into or out of VDD(A), VSS(A), VDD(D) and VSS(D) −30 +30 mA

Current into or out of any other pin −20 +20 mA

Voltage differential between power supplies:

VDD(D) and VDD(A) 0 0.3 V

VSS(D) and VSS(A) 0 50 mV

D6 Package (SSOP) Min. Max. Unit

Total Allowable Power Dissipation at Tamb = 25°C 1490 mW

... Derating 14.9 mW/°C

Storage Temperature −55 +125 °C

Operating Temperature −40 +85 °C

E1 Package (TSSOP) Min. Max. Unit

Total Allowable Power Dissipation at Tamb = 25°C 1110 mW

... Derating 11.1 mW/°C

Storage Temperature −55 +125 °C

Operating Temperature −40 +85 °C

Operating Limits

Correct operation of the device outside these limits is not implied.

Notes Min. Max. Unit

Supply (VDD - VSS) 2.7 5.5 V

Operating Temperature −40 +85 °C

Clock/Xtal Frequency 11 18.3 18.6 MHz

Notes: 11 Nominal clock frequency is 18.432MHz.

PMR Baseband Processor CMX881

Operating Characteristics

For the following conditions unless otherwise specified:

External components as recommended in Figure 2.

Maximum load on digital outputs = 30pF.

Xtal Frequency = 18.432MHz ±0.01% (100ppm).

VDD = 2.7V to 5.5V; Tamb = −40°C to +85°C.

Signal levels are defined for Vdd = 3V.

Signal levels track with supply voltage, so scale accordingly

Reference Signal Level = 308mV rms at 1kHz with VDD = 3V.

Signal to Noise Ratio (SNR) in bit rate bandwidth.

Input stage gain = 0dB.

Output stage attenuation = 0dB.

DC Parameters Notes Min. Typ. Max. Unit

Supply Current

IDD(D) (VDD = 3.0V) 21 4.5 8.0 mA

IDD(A) (VDD = 3.0V) 21 1.0 2.0 mA

IDD(D) (All Power-saved) (VDD = 3.0V) 21 2.0 10 µA

IDD(A) (All Power-saved) (VDD = 3.0V) 21 2.0 10 µA

C-BUS Interface

Input Logic ‘1’ 70% VDD

Input Logic ‘0’ 30% VDD

Input Leakage Current (Logic ‘1’ or ‘0’) 21 −1.0 1.0 µA

Input Capacitance - 7.5 pF

Output Logic ‘1’ (IOH = 120µA) 90% VDD

Output Logic ‘0’ (IOL = 360µA) 10% VDD

“Off” State Leakage Current 21 10 µA

IRQN (Vout = VDD(D)) 21

−1.0 1.0 µA

REPLY_DATA (output HiZ) 21 −1.0 1.0 µA

CLOCK_OUT

Output Logic ‘1’ (IOH = 120µA) 90% VDD

(IOH = 1mA) 80% VDD

Output Logic ‘0’ (IOL = 360µA) 10% VDD

(IOL = -1.5mA) 15% VDD

CLOCK/XTAL 22

Input Logic ‘1’ 70% VDD

Input Logic ‘0’ 30% VDD

Input current (Vin = VDD) 40 µA

Input current (Vin = VSS)

−40 µA

VBIAS 23

Output voltage offset wrt VDD/2 (IOL < 1μA) -2% +2% VDD

Output impedance 22

kΩ

Notes: 21 Tamb = 25°C, not including any current drawn from the device pins by external circuitry.

22 Characteristics when driving the CLOCK/XTAL pin with an external clock source.

23 Applies when utilising VBIAS to provide a reference voltage to other parts of the system.

When using VBIAS as a reference, VBIAS must be buffered. VBIAS must always be

decoupled with a capacitor as shown in Figure 3.

PMR Baseband Processor CMX881

AC Parameters Notes Min. Typ. Max. Unit

CLOCK/XTAL Input

'High' pulse width 31 21 ns

'Low' pulse width 31 21 ns

Input impedance (at 18.432MHz)

Powered-up Resistance 150 kΩ

Capacitance 20 pF

Powered-down Resistance 300 kΩ

Capacitance 20 pF

Clock frequency 18.432 MHz

Clock stability/accuracy ±100 ppm

Clock start up (from power-save) 400 ms

CLOCK_OUT Output

CLOCK/XTAL input to CLOCK_OUT timing:

(in high to out high) 32 15 ns

(in low to out low) 32 15 ns

'High' pulse width 33 22 27.13 33 ns

'Low' pulse width 33 22 27.13 33 ns

VBIAS

Start up time (from power-save) 30 ms

Microphone, Input_2 and Disc Inputs

(MIC, INPUT_2, DISC)

Input impedance 34 1 MΩ

Input signal range 35 10 90 %VDD

Feedback load resistance (pins 12, 14 & 16) 80 kΩ

Amplifier open loop voltage gain ⎫

(I/P = 1mV rms at 100Hz) ⎭ 60 dB

Unity gain bandwidth 1.0 MHz

Programmable Input Gain Stage 36

Gain (at 0dB) −0.5 0 0.5 dB

Cumulative Gain Error ⎫

(wrt attenuation at 0dB) ⎭ −1.0 1.0 dB

Notes: 31 Timing for an external input to the CLOCK/XTAL pin.

32 CLOCK/XTAL input driven by external source.

33 18.432MHz XTAL fitted.

34 With no external components connected

35 After multiplying by gain of input circuit, with external components connected.

36 Gain applied to signal at output of buffer amplifier, pin 12, 14 or 16

PMR Baseband Processor CMX881

AC Parameters (cont.) Notes Min. Typ. Max. Unit

Modulator Outputs 1 and 2 and Audio Output

(MOD_1, MOD_2, AUDIO)

Power-up to output stable 37 50 100 µs

Modulator Attenuators

Attenuation (at 0dB) 39 −1.0 0 1.0 dB

Cumulative Attenuation Error ⎫

(wrt attenuation at 0dB) ⎭ −0.6 0.6 dB

Output Impedance ⎫ Enabled 38 600 Ω

⎭ Disabled 38 500 kΩ

Output current range (VDD = 3.0V) -125 125

μA

Output voltage range 40 0.5 VDD–0.5 V

Load resistance 20 kΩ

Audio Attenuator

Attenuation (at 0dB) 39 −1.0 0 1.0 dB

Cumulative Attenuation Error ⎫

(wrt attenuation at 0dB) ⎭ −1.0 1.0 dB

Output Impedance ⎫ Enabled 38 600 Ω

⎭ Disabled 38 500 kΩ

Output current range (VDD = 3.0V) -125 125

μA

Output voltage range 40 0.5 VDD–0.5 V

Load resistance 20 kΩ

Notes: 37 Power-up refers to issuing a C-BUS command to turn on an output. These limits

apply only if VBIAS is on and stable. At power supply switch-on, the default state

is for all blocks, except the XTAL and C-BUS interface, to be in placed in power-

save mode.

38 Small signal impedance, at VDD = 3.0V and Tamb = 25°C.

39 Wrt the signal at the feedback pin of the selected input port.

With output driving a 20kΩ load to VDD/2.

PMR Baseband Processor CMX881

AC Parameters (cont.) Notes Min. Typ. Max. Unit

Auxiliary ADC (Signal Monitor)

8 Bit ADC Mode

Resolution 8 Bits

Input Range 10% 90% VDD(A)

Conversion time 41 20.8 μs

Input impedance

Resistance 10

MΩ

Capacitance 5 pF

Zero error ⎫

(input offset to give ADC output = 0) ⎭ −20 +20 mV

Integral Non-linearity ⎫ 42 2 LSB

⎭ 43 4 LSB

Differential Non-linearity ⎫ 42 1 LSB

⎭ 43 3 LSB

Source output impedance 44 24 kΩ

Level Threshold Detect Mode

Threshold Resolution 8 Bits

Upper threshold range (VTH) 45 VTL VDD(A) V

Lower threshold range (VTL) 45 VSS(A) VTH V

Signal Monitor change to IRQ 46 120 μs

Signal Monitor change to Receiver-Turn-On 47 60 μs

Notes: 41 With clock frequency of 18.432MHz.

42 Vdd(A) >= 3.0V.

43 Vdd(A) < 3.0V.

44 Denotes output impedance of the driver of the Signal Monitor input, to ensure < 1

bit additional error under nominal conditions.

45 Upper threshold > Lower threshold

46 Time from Signal Monitor input rising above Upper Threshold or falling below

Lower Threshold, to IRQN being asserted.

47 Time from Signal Monitor input rising above Upper Threshold to receiver path

powering up, settling and starting automatic signal type identification.

PMR Baseband Processor CMX881

AC Parameters (cont.) Notes Min. Typ. Max. Unit

Receiver Signal Ty pe Identification

Probability of correctly identifying signal type

(SNR = 12dB) >>99.9 %

CTCSS Detector

Sensitivity (Pure Tone) 51 -26 dB

Response Time (Composite Signal) 52 140 250 ms

De-response Time (Composite Signal) 52, 55 210 ms

Dropout immunity 55 160 ms

Frequency Range 60 260 Hz

SELCALL Detector

Sensitivity (Pure Tone) 53 -26 dB

Response Time (Good Signal) 35 ms

De-response Time (Good Signal) 52 ms

Dropout immunity 20 ms

Frequency Range (Selcall) 56 400 3000 Hz

DCS Decoder

Sensitivity 51 58 mVp-p

Bit-Rate Sync Time 2 edges

FFSK/MSK Decoder

Signal Input Dynamic Range 54 100 800 mVrms

Bit Error Rate (SNR = 20dB) 54 <1 10-8

Receiver Synchronisation (SNR = 12dB)

Probability of bit 16 being correct >99.9 %

Notes: 51 Sub-Audio Detection Level threshold set to 16mV.

52 Composite signal = 308mV rms at 1kHz + 75mV rms Noise + 31mV rms Sub-

Audio signal. Noise bandwidth = 5kHz Band Limited Gaussian.

53 Selcall Tone Detection Level threshold set to 16mV.

54 VDD (A) = 3.0V, for a “101010101 ... 01” pattern measured at the input amplifier

feedback pin (12). Signal level scales with VDD (A). See

Figure 15 for variation of BER with SNR.

With sub-audio dropout time (P2.4) set to ≥120ms. The typical dropout immunity

is approximately 40ms more than the programmed dropout immunity. The typical

de-response time is approximately 90ms longer than the programmed dropout

immunity. See section 1.6.20.3, P2.4

The device can decode in-band tones below the 400Hz lower limit but not

necessarily within the selected bandwidth or with the selected threshold

resolution. This can result in some CTCSS tones above approximately 160Hz

being flagged in the Tone Status register as unrecognised in-band tones.

Some tones in the range 250Hz to 400Hz cannot be defined by the ‘in-band’

custom tones programming facility, but can be detected by selecting a valid close

tone and using a suitably wide bandwidth setting.

PMR Baseband Processor CMX881

AC Parameters (cont.) Notes Min. Typ. Max. Unit

CTCSS Encoder

Frequency Range 60.0 260 Hz

Tone Frequency Accuracy ±0.3 %

Tone Amplitude Tolerance 61 -1.0 +1.0 dB

Total Harmonic Distortion 62 2.0 4.0 %

Selcall Encoder

Frequency Range 65 400 3000 Hz

Tone Frequency Accuracy ±0.3 %

Tone Amplitude Tolerance 63 -1.0 +1.0 dB

Total Harmonic Distortion 62 2.0 4.0 %

DTMF Encoder

Output signal level High tone 0 dB

Low tone (twist on) 64 -2 dB

(twist off) 64 0 dB

Output distortion 2 5 %

DCS Encoder

Bit Rate 134.4 bits/s

Amplitude Tolerance 61 -1.0 +1.0 dB

FFSK/MSK Encoder

Output signal level 775 mVrms

Output level variation -1.0 +1.0 dB

Output distortion 5 %

3rd harmonic distortion 3 %

Logic 1 freq 1200baud and

2400baud 1198 1200 1202 Hz

Logic 0 freq 1200baud 1798 1800 1802 Hz

2400baud 2398 2400 2402 Hz

Isochronous distortion (0 to 1 and 1 to 0) 40 μs

Notes: 61 VDD(A) = 3.0V and TX Sub-Audio Level set to 88mV p-p (31mV rms).

62 Measured at MOD_1 or MOD_2 output.

63 VDD(A) = 3.0V and Tx Audio Level set to 871mV p-p (308mV rms).

64 With respect to high tone level

65 ‘In-band tones between 400Hz and approximately 250Hz can be programmed,

but the range is not contiguous and the transmitted tones may not be within the

tone accuracy limits.

PMR Baseband Processor CMX881

AC Parameters (cont.) Notes Min. Typ. Max. Unit

Analogue Channel Audio Filtering

Pass-band (nominal bandwidth):

Received voice 71 300 3000 Hz

12.5kHz channel transmitted voice 72 300 2550 Hz

25kHz channel transmitted voice 73 300 3000 Hz

Pass-band Gain (at 1.0kHz) 0 dB

Pass-band Ripple (wrt gain at 1.0kHz) -2 +0.5 dB

Stop-band Attenuation 33.0 dB

Residual Hum and Noise 76 -50 dB

Pre-emphasis 74 6 dB/oct

De-emphasis 75

−6 dB/oct

Notes: 71 The receiver voice filter complies with the characteristic shown in Figure 5. The

high pass filtering removes sub-audio components from the audio signal.

72 The 12.5kHz channel filter complies with the characteristic shown in Figure 9.

73 The 25kHz channel filter complies with the characteristic shown in Figure 8.

74 The pre-emphasis filter complies with the characteristic shown in Figure 10.

75 The de-emphasis filter complies with the characteristic shown in Figure 6.

76 Measured in a 30kHz bandwidth.

PMR Baseband Processor CMX881

C-BUS Timing

Figure 14 C-BUS Timing

C-BUS Timing Notes Min. Typ. Max. Unit

tCSE CSN Enable to SClk high time 100 ns

tCSH Last SClk high to CSN high time 100 ns

tLOZ SClk low to ReplyData Output Enable

Time 0.0 ns

tHIZ CSN high to ReplyData high impedance 1.0 µs

tCSOFF CSN high time between transactions 1.0 µs

tNXT Inter-byte time 200 ns

tCK SClk cycle time 200 ns

tCH SClk high time 100 ns

tCL SClk low time 100 ns

tCDS Command Data setup time 75 ns

tCDH Command Data hold time 25 ns

tRDS Reply Data setup time 50 ns

tRDH Reply Data hold time 0 ns

Notes: 1. Depending on the command, 1 or 2 bytes of COMMAND DATA are transmitted to the

peripheral MSB (Bit 7) first, LSB (Bit 0) last. REPLY DATA is read from the peripheral MSB

(Bit 7) first, LSB (Bit 0) last.

2. Data is clocked into the peripheral on the rising SERIAL_CLOCK edge.

3. Commands are acted upon at the end of each command (rising edge of CSN).

4. To allow for differing µC serial interface formats C-BUS compatible ICs are able to work

with SERIAL_CLOCK pulses starting and ending at either polarity.

5. Maximum 30pF load on IRQN pin and each C-BUS interface line.

These timings are for the latest version of C-BUS and allow faster transfers than the original C-BUS timing

specification. The CMX881 can be used in conjunction with devices that comply with the slower timings,

subject to system throughput constraints.

PMR Baseband Processor CMX881

1.0E-05

1.0E-04

1.0E-03

1.0E-02

1.0E-01

4 5 6 7 8 9 10 11 12 13 14

SNR (dB) - Noise in bit rate bandw i d th

1200 b/s

2400 b/s

Figure 15 Typical FFSK/MSK Bit Error Rate Graph

PMR Baseband Processor CMX881

1.8.2 Packaging

Figure 16 Mechanical Outline of 28-pin SSOP (D6): Order as part no. CMX881D6

Figure 17 Mechanical Outline of 28-pin TSSOP (E1): Order as part no. CMX881E1

PMR Baseband Processor CMX881

Handling precautions: This product includes input protection, however, precautions should be taken to prevent device damage

from electro-static discharge. CML does not assume any responsibility for the use of any circuitry described. No IPR or circuit

patent licences are implied. CML reserves the right at any time without notice to change the said circuitry and this product

specification. CML has a policy of testing every product shipped using calibrated test equipment to ensure compliance with this

product specification. Specific testing of all circuit parameters is not necessarily performed.