CMX909BD5 - CML Microcircuits (USA) Inc.

CML Microcircuits

COMMUNICATION SEMICONDUCTOR

CMX909B

GMSK

Packet Data Modem

D/909B/2 November 2008

Features

• GMSK Modulation/Demodulation • On-chip Packet Detection

• Rx or Tx up to 38.4kbits/sec • Parallel Host Processor Interface

• Full and Short Data Packet Framing • Low Power 3.0V/5.0V Operation

• Mobitex Compatible including

R14N Short Block Frames • Flexible Operating and Powersave

Modes

1. Brief Description

The CMX909B is a half-duplex Gaussian Minimum Shift Keyed (GMSK) BT=0.3 modem data pump with

on-chip packet data handling. GMSK modulation optimises the data throughput for a given bandwidth RF

channel and the on-chip packet data handling relieves the host µC of regular processing tasks, such as

maintaining Bit and Frame Synchronisation, Block Formatting, CRC and FEC Error Processing, Data

Interleaving and Scrambling. The demodulator uses decision feedback equalisation techniques to reduce

the channel distortion effects and enhance the receiver performance without the computational overhead

of maximum likelihood (Viterbi) estimation methods.

The CMX909B is pin, function and software backwards compatible with the FX909A and MX909A

modems and also uses the same external components. It offers improved performance, higher data rates

and lower voltage operation, as well as handling the recent R14N extension to Mobitex for short block

frames. The CMX909B also offers 2-strength Xtal driver circuitry – for wider choice of xtals, optional zero-

error or one-error frame sync. detection, multiple powersave options – for intelligent power management,

and availability in both 24-pin TSSOP and SSOP low-height package options.

The CMX909B is ideally suited to wireless data applications such as Mobitex terminals, wireless

telemetry, licence-free radio data and ISM band radio schemes.

GMSK Packet Data Modem CMX909B

CONTENTS

Section Page

1. Brief Description.................................................................................................1

1.1. History......................................................................................................4

2. Block Diagram.....................................................................................................5

3. Signal List............................................................................................................6

4. External Components.........................................................................................8

5. General Description............................................................................................9

5.1. Description of Blocks ...............................................................................9

5.1.1. Data Bus Buffers.........................................................................9

5.1.2. Address and R/W Decode..........................................................9

5.1.3. Status and Data Quality Registers..............................................9

5.1.4. Command, Mode and Control Registers....................................9

5.1.5. Data Buffer................................................................................10

5.1.6. CRC Generator/Checker ..........................................................10

5.1.7. FEC Generator/Checker...........................................................10

5.1.8. Interleave/De-interleave Buffer.................................................10

5.1.9. Frame Sync Detect...................................................................10

5.1.10. Rx I/P Amp................................................................................10

5.1.11. Tx/Rx Low Pass Filter...............................................................10

5.1.12. Tx Output Buffer........................................................................10

5.1.13. Rx Level/Clock Extraction.........................................................12

5.1.14. Clock Oscillator and Dividers....................................................12

5.1.15. Scramble/De-scramble .............................................................12

5.2. Modem - µC Interaction .........................................................................13

5.3. Data Formats .........................................................................................14

5.3.1. General Purpose Formats ........................................................14

5.3.2. Mobitex Frame Structure ..........................................................14

5.4. The Programmer’s View ........................................................................16

5.4.1. Data Buffer................................................................................16

5.4.2. Command Register...................................................................16

5.4.3. Control Register........................................................................27

5.4.4. Mode Register...........................................................................30

5.4.5. Status Register .........................................................................31

5.4.6. Data Quality Register................................................................33

5.5. CRC, FEC, Interleaving and Scrambling Information:...........................34

5.5.1. CRC 34

5.5.2. FEC 35

5.5.3. Interleaving ...............................................................................35

5.5.4. Scrambling................................................................................36

6. Application Notes .............................................................................................37

6.1. Transmit Frame Example.......................................................................37

6.2. Receive Frame Example........................................................................39

6.3. Clock Extraction and Level Measurement Systems..............................41

GMSK Packet Data Modem CMX909B

6.4. AC Coupling...........................................................................................42

6.5. Radio Performance................................................................................44

7. Performance Specification...............................................................................45

7.1. Electrical Performance...........................................................................45

7.1.1. Absolute Maximum Ratings......................................................45

7.1.2. Operating Limits........................................................................45

7.1.3. Operating Characteristics .........................................................46

7.2. Packaging ..............................................................................................50

Table Page

Table 1 DOC Capacitance v. Data Rate............................................................................8

Table 2 CMX909B Registers ...........................................................................................16

Table 3 Mobitex Modem Tasks........................................................................................19

Table 4 Transmit Mode Timing........................................................................................25

Table 5 Receive Mode Timing.........................................................................................26

Table 6 Xtal/Clock Frequency v. Data Rates...................................................................28

Figure Page

Figure 1 Block Diagram .....................................................................................................5

Figure 2 Recommended External Components ................................................................8

Figure 3 Typical Modem µC Connections..........................................................................9

Figure 4 Typical Modem µC Connections........................................................................11

Figure 5 Transmitted Signal Eye Diagram (after the external RC filter)..........................12

Figure 6 Mobitex Over Air Signal Format ........................................................................15

Figure 7 The Transmit Process........................................................................................20

Figure 8 The Receive Process.........................................................................................21

Figure 9 Transmit Mode Timing Diagram........................................................................25

Figure 10 Receive Mode Timing Diagram.......................................................................26

Figure 11 Low Pass Filter Delay......................................................................................27

Figure 12 Typical Data Quality Reading v. S/N...............................................................34

Figure 13 Interleaving - Input/Output...............................................................................36

Figure 14 Transmit Process.............................................................................................38

Figure 15 Receive Process..............................................................................................40

Figure 16 Bit Clock and Level Acquisition Example ........................................................42

Figure 17 Typical Bit Error Rates.....................................................................................43

Figure 18 Decay Time - AC Coupling..............................................................................43

Figure 19 Typical System Installation..............................................................................44

Figure 20 µC Parallel Interface Timings ..........................................................................47

Figure 21 Typical Bit Error Rate (noise in bit rate bandwidth).........................................49

Figure 22 E2 Mechanical Outline: Order as part no. CMX909BE2 ................................50

Figure 23 D5 Mechanical Outline: Order as part no. CMX909BD5................................50

Figure 24 P4 Mechanical Outline: Order as part no. CMX909BP4 ................................51

GMSK Packet Data Modem CMX909B

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

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

1.1. History

Version Changes Date

1 • Original document. Aug 2000

2 • History section introduced

• Corrections to Figure 15

• Updates to parametric specification, section 1.7.1: 'TBA's removed

• Package drawings, logos, etc. updated

• Document renumbered

Nov 2008

GMSK Packet Data Modem CMX909B

2. Block Diagram

Figure 1 Block Diagram

GMSK Packet Data Modem CMX909B

3. Signal List

Package

E2/D5/P4 Signal Description

Pin No. Name Type

1 IRQN O/P A ‘wire-ORable’ output for connection to the host

μC's Interrupt Request input. This output has a

low impedance pull down to VSS when active

and is high impedance when inactive.

2 D7 BI )

3 D6 BI )

4 D5 BI )

5 D4 BI

) 8-bit bidirectional 3-state μC interface data

6 D3 BI ) lines.

7 D2 BI )

8 D1 BI )

9 D0 BI )

10 RDN I/P Read. An active low logic level input used to

control the reading of data from the modem into

the host μC.

11 WRN I/P Write. An active low logic level input used to

control the writing of data into the modem from

the host μC.

12 Vss Power The negative supply rail (ground).

13 CSN I/P Chip Select. An active low logic level input to

the modem, used to enable a data read or write

operation.

14 A0 I/P ) Two logic level modem register select

15 A1 I/P ) inputs.

16 XTALN O/P The output of the on-chip oscillator.

17 XTAL/CLOCK I/P The input to the on-chip oscillator, for external

Xtal circuit or clock.

19 DOC 2

DOC 1 O/P

O/P ) Connections to the Rx level measurement

) circuitry. A capacitor should be connected

) from each pin to VSS.

GMSK Packet Data Modem CMX909B

Package

E2/D5/P4 Signal Description

Pin No. Name Type

20 TXOP O/P The Tx signal output from the modem.

21 VBIAS O/P A bias line for the internal circuitry, held at

½ VDD. This pin must be decoupled to VSS by a

capacitor mounted close to the device pins.

22 RXIN I/P The input to the Rx input amplifier.

23 RXFB O/P The output of the Rx input amplifier and the

input to the Rx filter.

24 VDD Power The positive supply rail. Levels and voltages are

dependent upon this supply. This pin should be

decoupled to VSS by a capacitor.

Notes: I/P = Input

O/P = Output

BI = Bidirectional

To achieve good noise performance, VDD and VBIAS decoupling and protection of the receive

path from extraneous in-band signals are very important. It is recommended that the printed

circuit board is laid out with a ground plane in the CMX909B area to provide a low impedance

connection between the VSS pin and the VDD and VBIAS decoupling capacitors. It is also

important to achieve a low impedance connection between the Xtal capacitors (C3 and C4) and

the ground plane.

GMSK Packet Data Modem CMX909B

4. External Components

Figure 2 Recommended External Components

R1 See section 5.1.10 C1 0.1 µF C5 See section 5.1.12

R2 100k ohm C2 0.1 µF C6 See note below

R3 1M ohm * C3 See section 5.4.3* C7 See note below

R4 See

section 5.1.12 C4 See section 5.4.3* X1 See section 5.4.3*

* Refer also to section 5.1.14

Tolerances: R4 ±5%

R2, C5 ±10%

all other components ±20%

C6 and C7 values should satisfy the following: C (in Farads) x data rate (bits/second) = 120 x 10-6

e.g.

Data Rate (kbits/sec) C6/C7 (nF)

4.0 30

4.8 22

8.0 15

9.6 12

16.0 6.8

19.2 6.8

32.0 3.9

38.4 3.3

Table 1 DOC Capacitance v. Data Rate

GMSK Packet Data Modem CMX909B

5. General Description

This product has been designed to be compliant with the appropriate sections of the "Mobitex

Interface Specification" including Short Block Frame formatting for the extended battery saving

protocol. References to ‘data blocks’ in this document apply to both the normal (18 byte) Data

Block and the smaller (4 byte) Short Data Block.

5.1. Description of Blocks

5.1.1. Data Bus Buffers

8 bidirectional 3-state logic level buffers between the modem’s internal registers and the host

µC's data bus lines.

5.1.2. Address and R/W Decode

This block controls the transfer of data bytes between the µC and the modem's internal

registers, according to the state of the Write and Read Enable inputs (WRN and RDN), the Chip

Select input (CSN) and the Register Address inputs A0 and A1.

The Data Bus Buffers, Address and R/W Decode blocks provide a byte-wide parallel µC

interface, which can be memory-mapped, as shown in Figure 3.

Figure 3 Typical Modem µC Connections

5.1.3. Status and Data Quality Registers

8-bit registers which the µC can read to determine the status of the modem and the received

data quality.

5.1.4. Command, Mode and Control Registers

The values written by the µC to these 8-bit registers control the operation of the modem.

GMSK Packet Data Modem CMX909B

5.1.5. Data Buffer

An 18-byte buffer used to hold receive or transmit data to or from the µC.

5.1.6. CRC Generator/Checker

A circuit which generates (in transmit mode) or checks (in receive mode) the Cyclic

Redundancy Checksum bits, which are included in transmitted Mobitex data blocks so that the

receive modem can detect transmission errors.

5.1.7. FEC Generator/Checker

In transmit mode this circuit calculates and adds the Forward Error Correction (4 bits) to each

byte presented to it. In receive mode the FEC information is used to correct most transmission

errors that have occurred in Mobitex data blocks or in the Frame Head control bytes.

5.1.8. Interleave/De-interleave Buffer

This circuit interleaves data bits within a data block before transmission and de-interleaves the

received data block so that the FEC system is best able to handle short noise bursts or signal

fades.

5.1.9. Frame Sync Detect

This circuit, which is only active in receive mode, is used to look for the user specified 16-bit

Frame Synchronisation pattern which is transmitted to mark the start of every frame.

5.1.10. Rx I/P Amp

This amplifier allows the received signal input to the modem to be set to the optimum level by

suitable selection of the external components R1 and R2. The value of R1 should be calculated

to give 0.2 x VDD pk-pk at the RXFB pin for a received ‘...11110000...’ sequence.

A capacitor may be fitted if ac coupling of the received signal is desired (see section 6.4),

otherwise the dc level of the received signal should be adjusted so that the signal at the

modem's RXFB pin is centred around VBIAS (½ VDD).

5.1.11. Tx/Rx Low Pass Filter

This filter, which is used in both transmit and receive modes, is a low pass transitional Gaussian

filter having a loss of 3dB at 0.3 times the selected bit rate (BT = 0.3). See Figure 4.

In transmit mode, the bits are passed through this filter to eliminate the high frequency

components which would otherwise cause interference into adjacent radio channels.

In receive mode this filter is used with an increased BT factor (0.56) to reject HF noise, so that

the signal is suitable for extraction of the received data.

5.1.12. Tx Output Buffer

This is a unity gain amplifier used in transmit mode to buffer the output of the Tx low pass filter.

In receive mode, the input of this buffer is connected to VBIAS. When changing from Rx to Tx

mode the input to this buffer will be connected to VBIAS for 2 bit periods to prevent unwanted

signals, from the low pass filter, at the output. When the modem is set to power save mode, the

buffer is turned off and the TXOP pin connected to VBIAS via a high value resistance. When

exiting from power save mode the Tx output is only reconnected to the buffer after 2 bit periods,

to prevent unwanted signals, from the low pass filter, at the output.

Note: The RC low pass filter formed by the external components R4 and C5 between the Tx

Output Buffer and the input to the radio's frequency modulator forms an important part of the

transmit signal filtering. These components may form part of any dc level-shifting and gain

GMSK Packet Data Modem CMX909B

adjustment circuitry. The ground connection to the capacitor C5 should be positioned to give

maximum attenuation of high frequency noise into the modulator. R4 and C5 should be chosen

so that the product of the resistance of R4 (in Ohms) and capacitance of C5 (in Farads) is

0.34/bit rate (bit rate in bits per second). R4 should be not less than 47kΩ and the value used

for the external capacitor should take into account parasitic capacitance.

Suitable values being:

R4 C5

8000 bits/sec 100kΩ 430pF

4800 bits/sec 100kΩ 710pF

The signal at the TXOP pin is centred around VBIAS and is approx 0.2 x VDD pk-pk, going

positive for a logic ‘1’ and negative for a logic ‘0’, if the modem is not inverting the Tx data.

A capacitor may be fitted if ac coupling of the input to the frequency modulator is desired, see

section 6.4.

The ‘eye’ diagram of the transmitted signal (after the external R4/C5 network) is shown in

Figure 5.

Figure 4 Typical Modem µC Connections

GMSK Packet Data Modem CMX909B

Figure 5 Transmitted Signal Eye Diagram (after the external RC filter)

5.1.13. Rx Level/Clock Extraction

These circuits, which operate only in receive mode, extract a bit rate clock from the received

signal and measure the received signal amplitude and dc offset. This information is then used

to extract the received bits and also to provide an input to the received Data Quality measuring

circuit. The external capacitors C6 and C7 form part of the received signal level measuring

circuit.

5.1.14. Clock Oscillator and Dividers

This circuit derives the transmit bit rate (and the nominal receive bit rate) by frequency division

of a reference frequency which may be generated by the on-chip Xtal oscillator or applied from

an external source.

Note: If the on-chip xtal oscillator is to be used, then the external components X1, C3, C4 and

R3 are required. If an external clock source is to be used, then it should be connected to the

XTAL/CLOCK input pin, the XTALN pin should be left unconnected, and X1, C3, C4 and R3 not

fitted.

5.1.15. Scramble/De-scramble

This block may be used to scramble/de-scramble the transmitted/received data blocks. It does

this by modulating the data with a 511-bit pseudorandom sequence, as described in section

5.5.4. Scrambling smoothes the transmitted spectrum, especially when repetitive sequences

are to be transmitted.

GMSK Packet Data Modem CMX909B

5.2. Modem - µC Interaction

In general, data is transmitted over air in the form of messages, or ‘Frames’, consisting of a

‘Frame Head’ optionally followed by one or more formatted data blocks. The Frame Head

includes a Frame Synchronisation pattern designed to allow the receiving modem to identify the

start of a frame. The following data blocks are construct ed f rom t he ‘raw’ dat a using a

combination of CRC (Cyclic Redundancy Checksum) generation, Forward Error Correction

coding, Interleaving and Scrambling. Details of the message formats handled by this modem

are given in section 5.3.

To reduce the processing load on the host µC, this modem has been designed to perform as

much as possible of the computationally intensive work involved in Frame formatting and de-

formatting and (when in receive mode) in searching for and synchronising onto the Frame

Head. In normal operation the modem will only require servicing by the µC once per received or

transmitted data block.

Thus, to transmit a block, the host µC has only to load the unformatted (raw) binary data into

the modem's data buffer then instruct the modem to format and transmit that data. The modem

will then calculate and add the CRC bits as required, encode the result with Forward Error

Correction coding, interleave then scramble the bits before transmission.

In receive mode, the modem can be instructed to assemble a block’s worth of received bits, de-

scramble and de-interleave the bits, check and correct them (using the FEC coding) and check

the resulting CRC before placing the received binary data into the Data Buffer for the µC to

read.

The modem can also handle the transmission and reception of unformatted data, to allow the

transmission of special Bit and Frame Synchronisation sequences or test patterns.

GMSK Packet Data Modem CMX909B

5.3. Data Formats

5.3.1. General Purpose Formats

In a proprietary system the user may employ the data elements provided by this device to

construct a custom, over-air data structure.

For example, 16 bits of bit sync + 2 bytes of frame sync + 4 bytes of receiver and sender

address + n data blocks would be sent as:

TQB (bit and frame sync) + TQB (addresses) + (n x TDB) + TSB

And received as:

SFS + RSB + RSB + RSB + RSB + (n x RDB)

Note that it is important to have established frame synchronisation before receiving data to

enable the receiving device to decode synchronously. Also the user may add, by way of

algorithms performed on the controlling device, additional data correction with the bytes in the

data block task.

5.3.2. Mobitex Frame Structure

The Mobitex format for transmitted data is in the form of a Frame Head immediately followed by

either 1 Short Data Block or a number of Data Blocks (0 to 32).

The Frame Head consists of 7 bytes:

2 bytes of bit sync:

1100110011001100 from base,

0011001100110011 from mobile

bits are transmitted from left to right

2 bytes of frame sync:

System specific.

2 bytes of control data.

1 byte of FEC code, 4 bits for each of the control bytes:

bits 7-4 (leftmost) operate on the first control byte.

bits 3-0 (rightmost) operate on the second control byte.

Each byte in the Frame Head is transmitted bit 7 (MSB) first to bit 0 (LSB) last.

The data blocks consist of:

18 bytes of data (Data Block) or 4 bytes of data (Short Data Block).

2 bytes of CRC calculated from the data bytes.

4 bits of FEC code for each of the data and CRC bytes

The resulting data block bits are interleaved and scrambled before transmission.

GMSK Packet Data Modem CMX909B

Figure 6 shows how the over air signal is built up from Frame Sync and Bit Sync patterns,

Control bytes and Data Blocks.

The binary data transferred between the modem and the host µC is that shown enclosed by the

thick dashed rectangles near the top of the diagram.

Figure 6 Mobitex Over Air Signal Format

GMSK Packet Data Modem CMX909B

5.4. The Programmer’s View

The modem appears to the programmer as 4 write only 8-bit registers shadowed by 3 read only

registers, individual registers being selected by the A0 and A1 chip inputs:

A1 A0 Write to Modem Read from Modem

0 0 Data Buffer Data Buffer

0 1 Command Register Status Register

1 0 Control Register Data Quality Register

1 1 Mode Register not used

Table 2 CMX909B Registers

5.4.1. Data Buffer

This is an 18-byte read/write buffer which is used to transfer data (as opposed to command,

status, mode, data quality and control information) between the modem and the host µC.

It appears to the µC as a single 8-bit register; the modem ensuring that sequential µC reads or

writes to the buffer are routed to the correct locations within the buffer.

The µC should only access this buffer when the Status Register BFREE (Buffer Free) bit is ‘1’.

The buffer should only be written to while in Tx mode and read from while in Rx mode (except

when loading Frame Sync detection bytes while in Rx mode).

5.4.2. Command Register

Writing to this register tells the modem to perform a specific action or actions, depending on the

setting of the TASK, AQLEV and AQBC bits. The ENV and EOP bits are used to indicate the

presence of signals in the receive path.

When it has no action to perform (but is not ‘powersaved’), the modem will be in an ‘idle’ state.

If the modem is in transmit mode the input to the Tx filter will be connected to VBIAS. In receive

mode the modem will continue to measure the received data quality and extract bits from the

received signal, supplying them to the de-interleave buffer, but will otherwise ignore the

received data.

GMSK Packet Data Modem CMX909B

Command Register B7: AQBC - Acquire Bit Clock

This bit has no effect in transmit mode.

In receive mode, whenever a byte with the AQBC bit set to ‘1’ is written to the Command

bit timing synchronisation with the received signal as quickly as possible. This involves setting

the Phase Locked Loop of the received bit timing extraction circuits to its widest bandwidth,

then gradually reducing the bandwidth as timing synchronisation is achieved, until it reaches the

'normal' value set by the PLLBW bits of the Control Register.

Setting this bit to ‘0’ (or changing it from ‘1’ to ‘0’) has no effect, however note that the

acquisition sequence will be re-started every time that a byte written to the Command Register

has the AQBC bit set to ‘1’.

The AQBC bit will normally be set up to 12 bits before an SFS (Search for Frame Sync) or SFH

(Search for Frame Head) task, however it may also be used independently to re-establish clock

synchronisation quickly after a long fade. Alternatively, a SFS or SFH task may be written to the

Command Register with the AQBC bit ‘0’ if it is known that clock synchronisation does not need

to be re-established. More details of the bit clock acquisition sequence are given in section 6.3.

Command Register B6: AQLEV - Acquire Receive Signal Levels

This bit has no effect in transmit mode.

In receive mode, whenever a byte with the AQLEV bit set to ‘1’ is written to the Command

measure the amplitude and dc offset of the received signal as rapidly as possible. This

sequence involves setting the measurement circuits to respond quickly at first, then gradually

increasing their response time, hence improving the measurement accuracy, until the ‘normal’

value set by the LEVRES bits of the Control Register is reached.

Setting this bit to ‘0’ (or changing it from ‘1’ to ‘0’) has no effect, however note that the

acquisition sequence will be re-started every time that a byte written to the Command Register

has the AQLEV bit set to ‘1’.

The AQLEV bit will normally be set up to 12 bits before an SFS (Search for Frame Sync) or

SFH (Search for Frame Head) task is initiated, however it may also be used independently to

re-establish signal levels quickly after a long fade. Alternatively, a SFS or SFH task may be

written to the Command Register with the AQLEV bit at ‘0’ if it is known that there is no need to

re-establish the received signal levels. More details of the level measurement acquisition

sequence are given in section 6.3.

The error rate is highest immediately after a AQBC and AQLEV sequence is triggered and

rapidly reduces to its static value soon after. These erroneous bits could incorrectly trigger the

frame sync detection circuits and so it is suggested that a SFH or SFS task is set 12 bits after

setting either of the AQLEV or AQBC sequences.

GMSK Packet Data Modem CMX909B

Command Register B5: - EOP End of Packet Detector

This bit has no effect in transmit mode.

In receive mode, whenever this bit is set to ‘1’, a circuit monitors the receive wave form. If the

received signal remains close to the centre of the received data levels (as stored on the DOC

capacitors) for more than approximately 3 bit times then the logic output will be set high and bit

‘0’ of the Status Register will be set according to the table below. If the input signal level goes

toward either of the DOC capacitor values the logic output will be immediately set low. Note: If

this bit is set when a data signal is not being received and the DOC capacitors have discharged

or if there are high levels of noise, its output will be unreliable. It should be used in conjunction

with bit 4 of the Command register.

Command Register B4: - ENV Envelope Detector

This bit has no effect in transmit mode.

In receive mode, whenever this bit is set to ‘1’, a circuit monitors the DOC voltage levels. If the

DOC voltages are more than 4% of VDD apart (0.2V when VDD = 5.0V) then the logic output will

be set high and bit ‘0’ of the Status Register will be set according to the table below. Note: If

this bit is set the ENV output will also be triggered when receiving high levels of noise or other

in-band signals.

(EOP) B4

(ENV) Status Register B0

(state set to output of): IRQ triggered on:

(If enabled)

0 0 0 -

0 1 ENV detector 0 Æ 1

1 0 EOP detector 0 Æ 1

1 1

(ENV) AND (EOP) 0 Æ 1 or

1 Æ 0

If both B4 and B5 are set the Status Register will be set whenever: (envelope detector output =

high) AND (end of packet output = low). The Status Register B0 will then be high during the

most likely time that a packet is being received. In this mode the IRQ bit will be set on both

edges, thus indicating the likely time of the start and end of packets.

If either B5 or B4 are set high then B0 and B1 of the Data Quality register will directly follow the

outputs of the ENV and EOP circuits respectively. If B5 and B4 are set low the Data Quality

bit will not be set in this case. The following actions will restore the Data Quality bits B0 and B1

to indicating the data quality value: Issuing a RESET command, setting TXRXN bit = ‘1’ or by

entering Power Save mode. Note: Because the least significant bits of the Data Quality

the host processor can either ignore or mask out the 2 least significant bits when reading the

DQ value.

Note: The setting of the AQLEV and LEVRES bits is important to the correct operation of these

circuits. See section 6.3 for more details.

GMSK Packet Data Modem CMX909B

Command Register B3, B2, B1, B0: TASK - Task

Operations such as transmitting a data block are treated by the modem as ‘tasks’ and are

initiated when the µC writes a byte to the Command Register with the TASK bits set to one of

the data handling commands (marked BOLD in Table 3).

B3 B2 B1 B0 Receive Mode Transmit Mode

0 0 0 0 NULL NULL

0 0 0 1

SFH Search for Frame Head T7H Transmit 7 byte Frame Head

0 0 1 0

R3H Read 3 byte Frame Head Reserved

0 0 1 1

RDB Read Data Block TDB Transmit Data Block

0 1 0 0

SFS Search for Frame Sync TQB Transmit 4 Bytes

0 1 0 1

RSB Read Single Byte TSB Transmit Single Byte

0 1 1 0

LFSB Load Frame Sync Bytes TSO Transmit Scrambler Output

0 1 1 1 RESET Cancel any current action RESET Cancel any current action

1 0 0 1

SFHZ SFH with zero errors Reserved

1 0 1 1

RSD Read Short Data Block TSD Transmit Short Data Block

1 1 0 0

SFSZ SFS with zero errors Reserved

1 1 1 0 PSBias Turn off Bias during power save

1 1 1 1 PSBiXt T urn off Bias and Xtal during power save

Table 3 Mobitex Modem Tasks

Note: All other bit patterns are reserved.

Bold text indicates a ‘data handling command’

The µC should not write a data handling command to the Command Register or write to or read

from the Data Buffer when the BFREE (Buffer Free) bit of the Status Register is ‘0’.

Different tasks apply in receive and transmit modes.

When the modem is in transmit mode, all data handling commands other than TSO instruct the

modem to transmit data from the Data Buffer, formatting it as required. For these tasks the µC

should wait until the BFREE (Buffer Free) bit of the Status Register is ‘1’, before writing the data

to the Data Buffer, then it should write the desired task to the Command Register. If more than

1 byte needs to be written to the Data Buffer, byte number 0 of the block should be written first.

Once the byte containing the desired task has been written to the Command Register, the

modem will:

Set the BFREE (Buffer Free) bit of the Status Register to ‘0’.

Take the data from the Data Buffer as quickly as it can - transferring it to the Interleave

Buffer for eventual transmission. This operation will start immediately if the modem is

‘idle’ (i.e. not transmitting data from a previous task), otherwise it will be delayed until

there is sufficient room in the Interleave Buffer.

Once all of the data has been transferred from the Data Buffer the modem will set the

BFREE and IRQ bits of the Status Register to ‘1’, (causing the chip IRQN output to go low

if the IRQNEN bit of the Mode Register has been set to ‘1’) to tell the µC that it may write

new data and the next task to the modem.

GMSK Packet Data Modem CMX909B

In this way the µC can write a task and the associated data to the modem while the modem is

still transmitting the data from the previous task. See Figure 7.

When the modem is in receive mode, the µC should wait until the BFREE bit of the Status

Once the byte containing the desired task has been written to the Command Register, the

modem will:

Set the BFREE bit of the Status Register to ‘0’.

Wait until enough received bits are in the De-interleave Buffer.

Decode them as needed, and transfer any resulting data to the Data Buffer.

Then the modem will set the BFREE and IRQ bits of the Status Register to ‘1’, (causing

the IRQN output to go low if the IRQNEN bit of the Mode Register has been set to ‘1’) to

tell the µC that it may read from the Data Buffer and write the next task to the modem. If

more than 1 byte is contained in the Data Buffer, byte number ‘0’ of the data will be read

first.

In this way the µC can read data and write a new task to the modem while the received bits

needed for this new task are being stored in the De-interleave Buffer. See Figure 8.

The above is not true for loading the Frame Sync detection bytes (LFSB): the bytes to be

compared with the incoming data must be loaded prior to the task bits being written.

Detailed timings for the various tasks are giv en in Figure 9 and Figure 10.

Figure 7 The Transmit Process

GMSK Packet Data Modem CMX909B

Figure 8 The Receive Process

NULL - No effect

This task is provided so that a AQBC or AQLEV command can be initiated without loading a

new task.

SFH - Search for Frame Head

Causes the modem to search the received signal for a Frame Head. The Frame Head will

consist of a 16-bit Frame Sync followed by control data (see Figure 6). The search will continue

until a Frame Head has been found, or until the RESET task is loaded.

The search is carried out by first attempting to match the incoming bits against the previously

programmed (task LFSB) 16-bit Frame Sync pattern (allowing up to any one bit (of 16) in error).

When a match has been found, the modem will read the next 3 received bytes as Frame Head

bytes, these bytes will be checked, and corrected if necessary, using the FEC bits. The two

Frame Head Data bytes are then placed into the Data Buffer.

The BFREE and IRQ bits of the Status Register will then be set to a logic ‘1’ to indicate that the

µC may read the 2 Frame Head Data bytes from the Data Buffer and write the next task to the

Command Register. If the FEC indicates uncorrectable errors the modem will set the CRCFEC

bit in the Status Register to a logic ‘1’. The MOBAN bit (Mobile or Base) in the Status Register

will be set according to the polarity of the 3 bits preceding the Frame Sync pattern.

R3H - Read 3-byte Frame Head

This task, which would normally follow an SFS task, will place the next 3 bytes directly into the

Data Buffer. It also causes the modem to check the 3 bytes as Frame Head control data bytes

and will set the CRCFEC bit to a logic ‘1’ (high) only if the FEC bits indicate uncorrectable

errors. Note: This task will not correct any errors and, due to the Mobitex FEC specification, will

not detect all possible uncorrectable error patterns The BFREE and IRQ bits of the Status

from the Data Buffer and write the next task to the modem's Command Register.

The CRCFEC bit in the Status Register will be set according to the validity of the received FEC

bits.

GMSK Packet Data Modem CMX909B

RDB - Read Data Block

This task causes the modem to read the next 240 bits as a Mobitex Data Block.

It will de-scramble and de-interleave the bits, FEC correct and CRC check the resulting 18 data

bytes and place them into the Data Buffer, setting the BFREE and IRQ bits of the Status

Data Buffer and write the next task to the modem’s Command Register. The CRCFEC bit will

be set according to the outcome of the CRC check.

Note: in receive mode the CRC checksum circuits are initialised on completion of any task other

than NULL.

SFS - Search for Frame Sync

This task, which is intended for special test and channel monitoring purposes, performs the first

part only of a SFH task. It causes the modem to search the received signal for a 16-bit

sequence which matches the Frame Synchronisation pattern with up to any 1 bit in error.

When a match is found the modem will set the BFREE and IRQ bits of the Status Register to ‘1’

and update the MOBAN bit. The µC may then write the next task to the Command Register.

RSB - Read Single Byte

This task causes the modem to read the next 8 bits and translate them directly (without de-

interleaving or FEC) to an 8-bit byte which is placed into the Data Buffer (B7 will represent the

earliest bit received). The BFREE and IRQ bits of the Status Register will then be set to ‘1’ to

indicate that the µC may read the data byte from the Data Buffer and write the next task to the

Command Register.

This task is intended for special tests and channel monitoring - perhaps preceded by an SFS

task.

LFSB - Load Frame Sync Bytes

This task takes 2 bytes from the Data Buffer and updates the Frame Sync detect bytes. The

MSB of byte ‘0’ is compared to the first bit of a received Frame Sync pattern and the LSB of

byte ‘1’ is compared to the last bit of a received Frame Sync pattern. This task does not enable

Frame Sync detection.

Unlike other Rx tasks, the data buffer must be loaded before the task is issued and the task

must only be issued ‘between’ received messages, i.e. before the first task for receiving a

message and after the last data is read out of the data buffer.

Once the modem has read the Frame Sync bytes from the Data Buffer, the BFREE and IRQ

bits of the Status Register will be set to ‘1’, indicating to the µC that it may write the next task to

the modem.

SFHZ - Search for Frame Head with Zero Errors

This performs the same task as SFH task but allowing no bits to be in error over the 16-bit

Frame Sync pattern.

GMSK Packet Data Modem CMX909B

RSD - Read Short Data Block

This task causes the modem to read the next 72 bits as a Mobitex Short Data Block.

It will de-scramble and de-interleave the bits, FEC correct and CRC check the resulting 4 data

bytes and place them into the Data Buffer, setting the BFREE and IRQ bits of the Status

Data Buffer and write the next task to the modem’s Command Register. The CRCFEC bit will

be set according to the outcome of the CRC check.

Note: in receive mode the CRC checksum circuits are initialised on completion of any task other

than NULL.

SFSZ - Search for Frame Sync with Zero Errors

This performs the same task as SFS task but allowing no bits to be in error over the 16-bit

Frame Sync pattern.

T7H - Transmit 7-byte Frame Head

This task takes 6 bytes of data from the Data Buffer, calculates and appends 8 bits of FEC from

bytes ‘4’ and ‘5’ then transmits the result as a complete Mobitex Frame Head.

Bytes ‘0’ and ‘1’ form the bit sync pattern, bytes ‘2’ and ‘3’ form the frame sync pattern and

bytes ‘4’ and ‘5’ are the frame head control bytes. Bit 7 of byte ‘0’ of the Data Buffer is sent first,

bit 0 of the FEC byte last.

Once the modem has read the data bytes from the Data Buffer, the BFREE and IRQ bits of the

Status Register will be set to ‘1’, indicating to the µC that it may write the next task and its data

to the modem.

TQB - Transmit 4 Bytes

This task takes 4 bytes of data from the Data Buffer and transmits them, bit 7 first.

Once the modem has read the data bytes from the Data Buffer, the BFREE and IRQ bits of the

Status Register will be set to ‘1’, indicating to the µC that it may write the next task and its data

to the modem.

TDB - Transmit Data Block

This task takes 18 bytes of data from the Data Buffer, calculates and applies a 16-bit CRC and

forms the FEC for the 18 data bytes and the CRC. This data is then interleaved and passed

through the scrambler, if enabled, before being transmitted as a Mobitex Data Block.

Once the modem has read the data bytes from the Data Buffer, the BFREE and IRQ bits of the

Status Register will be set to ‘1’, indicating to the µC that it may write the next task and its data

to the modem.

Note: In transmit mode the CRC checksum circuit is initialised on completion of any task other

than NULL.

TSB - Transmit Single Byte

This task takes a byte from the Data Buffer and transmits the 8 bits, bit 7 first.

Once the modem has read the data byte from the Data Buffer, the BFREE and IRQ bits of the

Status Register will be set to ‘1’, indicating to the µC that it may write the next task and its data

to the modem.

GMSK Packet Data Modem CMX909B

TSO - Transmit Scrambler Output

This task, intended for channel set-up, enables the scrambler and transmits its output.

When the modem has started the task the Status Register bits will not be changed and hence

an IRQ will not be raised. The µC may write the next task and its data to the modem at any

time and the scrambler output will stop when the new task has produced its first data.

TSD - Transmit Short Data Block

This task takes 4 bytes of data from the Data Buffer, calculates and applies a 16-bit CRC and

forms the FEC for the 4 data bytes and the CRC. This data is then interleaved and passed

through the scrambler, if enabled, before being transmitted as a Mobitex Data Block.

Once the modem has read the data bytes from the Data Buffer, the BFREE and IRQ bits of the

Status Register will be set to ‘1’, indicating to the µC that it may write the next task and its data

to the modem.

Note: In transmit mode the CRC checksum circuit is initialised on completion of any task other

than NULL.

RESET - Stop any current action

This task takes effect immediately, and terminates any current action (task, AQBC or AQLEV)

the modem may be performing and sets the BFREE bit of the Status Register to ‘1’, without

setting the IRQ bit. It should be used when VDD is applied to set the modem into a known state.

Note that due to delays in the internal switched capacitor filter, it will take approximately 3 bit

times for any change to become apparent at the TXOP pin.

PSBias - PowerSave Bias Circuit

If the TASK bits are in this setting when B3 of the Mode register is set to ‘1’ the device will

power down the Bias chain in addition to powering down those circuits described in section

5.4.4. The voltage on VBIAS will decay to 0V as will the level on the TXOP, DOC1 and DOC2

pins.

PSBiXt - PowerSave Bias and Xtal Circuit

If the TASK bits are in this setting when B3 of the Mode register is set to ‘1’ the device will

power down the Bias chain and stop the Xtal oscillator in addition to powering down those

circuits described in section 5.4.4. The voltage on VBIAS will decay to 0V as will the level on the

TXOP, DOC1 and DOC2 pins. The voltage on the XTALN pin will go to a logic ‘1’ regardless of

the level at the XTAL pin.

Task Timings

The device should not write to the Command Register whenever PSBiXt and PSAVE bits are

set and for at least 2 bit times after the following:

Changing from powersave state to normal operation.

Changing the Tx/Rx bit.

Resetting or after power is applied to the device.

This is to ensure that the internal operation of the device is initialised correctly for the new task.

Note that this only applies to the Command Register, the other registers may be accessed as

normal.

GMSK Packet Data Modem CMX909B

Figure 9 Transmit Mode Timing Diagram

Task Typical time

(bit-times)

t1 Time from writing first task (modem in ‘idle’

state) to application of first transmit bit to Tx

Low Pass filter

Any

t2 Time from application of first bit of T7H 36

task to Tx Low Pass filter until BFREE TQB 24

goes to a logic ‘1’ (high) TDB 20

TSB 1

TSD 6

t3 Time to transmit all bits of task T7H 56

TQB 32

TDB 240

TSB 8

TSD 72

t4 Max time allowed from BFREE going to a T7H 18

logic ‘1’ (high) for next task (and data) to TQB 6

be written to modem TDB 218

TSB 6

TSD 64

Table 4 Transmit Mode Timing

GMSK Packet Data Modem CMX909B

Figure 10 Receive Mode Timing Diagram

Task Typical time

(bit-times)

t3 Time to receive all bits of task SFH 56

R3H 24

RDB 240

RSB 8

RSD 72

t6 Maximum time between first bit of task SFH 14

entering de-interleave circuit and task R3H 18

being written to modem RDB 218

RSB 6

RSD 64

t7 Time from last bit of task entering de-interleave

circuit to BFREE going to a logic ‘1’ (high) Any 1

Table 5 Receive Mode Timing

GMSK Packet Data Modem CMX909B

Tx/Rx Low Pass Filter Delay

The previous task timing figures are based on the signal at the input to the Tx Low Pass filter (in

transmit mode) or the input to the de-interleave buffer (in receive mode). There is an additional

delay of about 2 bit times in both transmit and receive modes due to the Tx/Rx Low Pass filter,

as illustrated in Figure 11.

Figure 11 Low Pass Filter Delay

5.4.3. Control Register

This 8-bit write only register controls the modem’s bit rate, the response times of the receive

clock extraction and signal level measurement circuits and the internal analogue filters.

GMSK Packet Data Modem CMX909B

Control Register B7, B6: CKDIV - Clock Division Ratio and

B5: HILON - Xtal Range Selection

These bits control a frequency divider driven from the clock signal present at the XTALN pin,

and hence determine the nominal bit rate. The table below shows how bit rates of 4000 to

38400 bits/sec may be obtained from common Xtal frequencies:

Xtal/Clock Frequency (MHz)

1 8.192

9.8304 4.096

(12.288/3) 4.9152 2.048

(6.144/3) 2.4576

(12.288/5)

0 4.096

(12.288/3) 4.9152 2.048

(6.144/3) 2.4576

(12.288/5) 1.024 1.2288

Division Ratio:

B7 B6 XtalFrequency

Data Rate Data Rate (bits per second)

0 0 256 128 32000 38400 16000 19200 8000 9600

0 1 512 256 16000 19200 8000 9600 4000 4800

1 0 1024 512 8000 9600 4000 4800

1 1 2048 1024 4000 4800

Table 6 Xtal/Clock Frequency v. Data Rates

Note: Device operation is not guaranteed below 4000 or above 38400 bits/sec.

The values used for C3 and C4 should be suitable for the frequency of the crystal X1. As a

guide; C3 = C4 = 33pF for X1 < 5MHz, and C3 = C4 = 18pF for X1 > 5MHz.

Control Register B4: DARA - Data Rate and Mode Register B0 - HIBW

These bits operate in both transmit and receive modes, optimising the modem's internal signal

filtering according to the relevant bit rate.

Control Register:

B4 (DARA) Mode Register:

B0 (HIBW) Data Rate

(bits/sec)

0 0 <10k

0 1 Reserved

1 0 10k – 20k

1 1 >20k

GMSK Packet Data Modem CMX909B

Control Register B3, B2: LEVRES - Level Measurement Response Time

These two bits have no effect in transmit mode.

In receive mode, they set the ‘normal’ response time of the Rx signal amplitude and dc offset

measuring circuits. This setting will be temporarily overridden by the automatic sequencing of

an AQLEV command.

B3 B2 Setting Action

0 0 Hold Keep current values of amplitude and offset

0 1 Peak Averaging Track input signal using bit peak averaging

1 0 Peak Detect Track input signal using peak detect

1 1 Lossy Peak Detect Track input signal using lossy peak detection

For Mobitex systems, and most general purpose applications using the modem, these bits

should normally be set to ‘Peak Averaging’ except when the µC detects a receive signal fade,

when ‘Hold’ should be selected.

The ‘Lossy Peak Detect’ setting is intended for systems where the µC cannot detect signal

fades or the start of a received message, as it allows the modem to respond quickly to fresh

messages and recover rapidly after a fade without µC intervention - although at the cost of

reduced Bit Error Rate versus Signal to Noise performance.

Note that, since the measured levels are stored on the external capacitors C6 and C7, they will

decay gradually towards VBIAS when the ‘Hold’ setting is chosen, the discharge time-constant

being approximately 2000 bit times. More details of the level measurement system are given in

section 6.3.

Control Register B1, B0: PLLBW

These two bits have no effect in transmit mode.

In receive mode, they set the ‘normal’ bandwidth of the Rx clock extraction Phase Locked Loop

circuit. This setting will be temporarily overridden by the automatic sequencing of an AQBC

command.

B1 B0 PLL Bandwidth Suggested use

0 0 Hold Signal fades

0 1 Narrow ± 20ppm or better Xtals

1 0 Medium Wide tolerance Xtals or long preamble acquisition

1 1 Wide Quick acquisition

The ‘hold’ setting is intended for use during signal fades, otherwise the minimum bandwidth

consistent with the transmit and receive modem bit rate tolerances should be chosen.

The wide and medium bandwidth settings are intended for systems where the µC cannot detect

signal fades or the start of a received message, as they allow the modem to respond rapidly to

fresh messages and recover rapidly after a fade without µC intervention - although at the cost

of reduced Bit Error Rate versus Signal to Noise performance.

Note: More details of the clock extraction system are given in section 6.3.

GMSK Packet Data Modem CMX909B

5.4.4. Mode Register

The contents of this 8-bit write only register control the basic operating modes of the modem:

Mode Register B7: IRQNEN - IRQN Output Enable

When this bit is set to ‘1’, the IRQN chip output pin is pulled low (to Vss) whenever the IRQ bit

of the Status Register is a ‘1’.

Mode Register B6: INVBIT - Invert Bits

This bit controls inversion of transmitted and received bit voltages. When set to ‘1’ all data is

inverted in the Tx and Rx data paths so a transmitted '1' is a voltage below VBIAS at the TXOP

pin and a received '0' is a voltage above VBIAS at the RXIN pin. Data will be inverted

immediately after this bit is set to ‘1’.

Mode Register B5: TXRXN - Tx/Rx Mode

Setting this bit to ‘1’ puts the modem into Transmit mode, clearing it to ‘0’ puts the modem into

Receive mode. When changing from Rx to Tx there must be a 2-bit pause before setting a new

task to allow the filter to stabilise. (See also PSAVE bit).

Note that changing between receive and transmit modes will cancel any current task

Mode Register B4: SCREN - Scramble Enable

The scrambler only takes effect during the transmission or reception of a Mobitex Data Block,

Short Data Block and during a TSO task. Setting this bit to ‘1’ enables scrambling, clearing it to

‘0’ disables scrambling.

The scrambler is only operative, if enabled by this control bit, during TSO, RDB, RSD, TSD or

TDB, it is held in a reset state at all other times.

This bit should not be changed while the modem is decoding or transmitting a Mobitex Data

Block.

Mode Register B3: PSAVE - Powersave

When this bit is a ‘1’, the modem will be in a ‘powersave’ mode in which the internal filters, the

Rx bit and Clock extraction circuits and the Tx o/p buffer will be disabled, and the TXOP pin will

be connected to VBIAS through a high value resistance. If the PSBias or PSBiXt bit patterns are

set in the Command Register, the VBIAS and Xtal/Clock circuits will be powersaved in

accordance with the description in section 5.4.2. Setting the PSAVE bit to ‘0’ restores power to

all of the chip circuitry. Note that the internal filters will take about 2 bit times to settle after the

PSAVE bit is taken from ‘1’ to ‘0’.

GMSK Packet Data Modem CMX909B

Mode Register B2: DQEN - Data Quality IRQ Enable

In receive mode, setting this bit to ‘1’ causes the IRQ bit of the Status Register to be set to ‘1’

whenever a new Data Quality reading is ready. (The DQRDY bit of the Status Register will also

be set to ‘1’ at the same time.)

In transmit mode this bit has no effect.

Mode Register B1: HIXTL - High Xtal Drive

This bit controls the gain of the on chip Xtal driver. For 3V operation and crystals >5MHz it

should be set to ‘1’. At 5V, or whenever using crystals <5MHz, it should be set to ‘0’.

Mode Register B0: HIBW - High Filter Bandwidth

This bit controls the internal filtering of the device. See Control Register B4 for the setting of

this bit.

5.4.5. Status Register

This register may be read by the µC to determine the current state of the modem.

Status Register B7: IRQ - Interrupt Request

This bit is set to ‘1’ by:

The Status Register BFREE bit going from ‘0’ to ‘1’, unless this is caused by a

RESET task or by a change to the Mode Register PSAVE or TXRXN bits.

or The Status Register IBEMPTY bit going from ‘0’ to ‘1’, unless this is caused by a

RESET task or by changing the Mode Register PSAVE or TXRXN bits.

or The Status Register DQRDY bit going from ‘0’ to ‘1’ (If DQEN = ‘1' ).

or The Status Register DIBOVF bit going from ‘0’ to ‘1’.

or The Status Register EOP/ENV bit going from ‘0’ to ‘1’ if ENV or EOP bits (not

both) are set in the Command Register.

or The Status Register EOP/ENV bit going from ‘0’ to ‘1’ or ‘1’ to ‘0’ if both ENV and

EOP bits are set in the Command Register.

The IRQ bit is cleared to ‘0’ immediately after a read of the Status Register.

If the IRQEN bit of the Mode Register is ‘1’, then the chip IRQN output will be pulled low (to

Vss) whenever the IRQ bit is ‘1’.

GMSK Packet Data Modem CMX909B

Status Register B6: BFREE - Data Buffer Free

This bit reflects the availability of the Data Buffer and is cleared to ‘0’ whenever a task other

than NULL, RESET or TSO is written to the Command Register.

In transmit mode, the BFREE bit will be set to ‘1’ (also setting the Status Register IRQ bit to ‘1’)

by the modem when the modem is ready for the µC to write new data to the Data Buffer and the

next task to the Command Register.

In receive mode, the BFREE bit is set to ‘1’ (also setting the Status Register IRQ bit to ‘1’) by

the modem when it has completed a task and any data associated with that task has been

placed into the Data Buffer. The µC may then read that data and write the next task to the

Command Register.

The BFREE bit is also set to ‘1’, but without setting the IRQ bit, by a RESET task or when the

Mode Register PSAVE or TXRXN bits are changed.

Status Register B5: IBEMPTY - Interleave Buffer Empty

In transmit mode, this bit will be set to ‘1’, also setting the IRQ bit, when less than two bits

remain in the Interleave Buffer. Any transmit task written to the modem after this bit goes to ‘1’

will be too late to avoid a gap in the transmit output signal.

The bit is also set to ‘1’ by a RESET task or by a change of the Mode Register TXRXN or

PSAVE bits, but in these cases the IRQ bit will not be set.

The bit is cleared to ‘0’ by writing a task other than NULL, RESET or TSO to the Command

Note: When the modem is in transmit mode and the Interleave Buffer is empty, a mid-level

voltage (VBIAS) will be applied to the Tx low pass filter.

In receive mode this bit will be ‘0’.

Status Register B4: DIBOVF - De-Interleave Buffer Overflow

In receive mode this bit will be set to ‘1’ (also setting the IRQ bit) when a task is written to the

Command Register too late to allow continuous reception.

The bit is cleared to ‘0’ by reading the Status Register or by writing a RESET task to the

Command Register or by changing the PSAVE or TXRXN bits of the Mode Register.

In transmit mode this bit will be ‘0’.

Status Register B3: CRCFEC - CRC or FEC Error

In receive mode this bit will be updated at the end of a Mobitex Data Block task, after checking

the CRC, and at the end of receiving Frame Head control bytes, after checking the FEC. A ‘0’

indicates that the CRC was received correctly or the FEC did not find uncorrectable errors, a ‘1’

indicates that errors are present.

The bit is cleared to ‘0’ by a RESET task or by changing the PSAVE or TXRXN bits of the Mode

In transmit mode this bit will be ‘0’.

GMSK Packet Data Modem CMX909B

Status Register B2: DQRDY - Data Quality Reading Ready

In receive mode, this bit is set to ‘1’ whenever a Data Quality reading has been completed. See

section 5.4.6.

The bit is cleared to '0' by a read of the Data Quality Register.

Immediately after a RESET task, or a change in the PSAVE or TXRXN bits to ‘0’, the DQRDY

bit may be set and generate an interrupt. The value in the Data Quality Register will not be

valid in this case.

Status Register B1: MOBAN - Mobile or Base Bit Sync Received

In receive mode this bit is updated at the end of the SFS and SFH tasks. This bit is set to ‘1’

whenever the 3 bits immediately preceding a detected Frame sync are ‘011’ (received left to

right), with up to any one bit in error. The bit is set to ‘0’ if the bit pattern is ‘100’, again with up

to any one bit in error. Thus, if this bit is set to ‘1’ then the received message is likely to have

originated from a Mobile and if it is set to ‘0’ from a Base Station. See section 5.3.

In transmit mode this bit is a logic ‘0’.

Status Register B0: EOP/ENV - End of Packet/Envelope Detect

This bit indicates the status of the End of Packet and Envelope detector circuits as indicated in

the description of Command Register bits B5 and B4.

In transmit mode this bit will be ‘0’.

5.4.6. Data Quality Register

This is intended to indicate the quality of the receive signal during a Mobitex Data Block or 30

single bytes. In receive mode, the modem measures the ‘quality’ of the received signal by

comparing the actual received zero crossing time against an internally generated time. This

value is averaged over 240 bits and at the end of the measurement the Data Quality Register

and the DQRDY bit in the Status Register is updated. Note: An interrupt will only occur at this

time if the DQEN bit = ‘1’.

To provide synchronisation with Data Blocks, and hence ensure the Data Quality Register is

updated in preparation to be read when the RDB task finishes, the measurement process is

reset at the end of tasks SFH, SFS, RDB and R3H.

The least significant 2 bits (B0 and B1) will be set to the output of the Envelope and End of

Packet detector circuits respectively if either of the ENV or EOP bits have been set in the

Command Register. After a RESET or if the transmit or power save modes have been set

these bits will indicate the least significant bits of the quality reading. The state of the ENV and

EOP bits have no other effect on the operation of the Data Quality Register.

In transmit mode all bits of the Data Quality Register will be ‘0’.

Figure 12 shows how the value (0-240) read from the Data Quality Register varies with received

signal to noise ratio.

GMSK Packet Data Modem CMX909B

Figure 12 Typical Data Quality Reading v. S/N

(reading taken after 240 bits, noise in bit rate bandwidth)

5.5. CRC, FEC, Interleaving and Scrambling Information:

5.5.1. CRC

This is a 16-bit CRC code used in both the Mobitex Data Block and Short Data Block. In

transmit it is calculated by the modem from the data block bytes using the following generator

polynomial:

g(x) = x16 + x 12 + x5 + 1

i.e. CRC - CCITT X.25.

This code detects all (single) error bursts of up to 16 bits in length and about 99.998% of all

other error patterns.

The CRC register is initialised to all ‘1s’ and the CRC is calculated octet by octet starting with

the least significant bit of ‘byte 0’. The CRC calculated is bit-wise inverted and appended to the

data bytes with the most significant bit transmitted earliest.

In receive mode, a 16-bit CRC code is generated from the data bytes of each Mobitex Data

Block or Short Data Block as above and the bit-wise inverted value is compared with the

received CRC bytes. If a mis-match is present, then an error has been detected.

GMSK Packet Data Modem CMX909B

5.5.2. FEC

In transmit mode, during T7H, TSD and TDB, the modem generates a 4-bit Forward Error

Correction code for each coded byte. The FEC is defined by the following H matrix:

7_______0 3___0

11101100 1000

H = 11010011 0100

10111010 0010

01110101 0001

Generation of the FEC consists of logically ANDing the byte to be transmitted with bits 7 to 0 of

each row of the H matrix. Even parity is generated for each of the 4 results and these 4 parity

bits, in the positions indicated by the last 4 columns of the H matrix, form the FEC code.

In checking the FEC, the received 12-bit word is logically ANDed with each row of the H matrix

(earliest bit received compared with the first column). Again even parity is generated for the 4

resulting words and these parity bits form a 4-bit nibble. If this nibble is all zero then no errors

have been detected. Other results ‘point’ to the bit in error or indicate that uncorrectable errors

have occurred.

This code can correct any single error that has occurred in each 12-bit (8 data + 4 FEC) section

of the message.

Example:

If the byte to be coded is ‘00101100’ then the FEC is derived as follows:

H matrix row: 1 2 3 4

A 11101100 11010011 10111010 01110101

B 00101100 00101100 00101100 00101100

A AND B 00101100 00000000 00101000 00100100

Even Parity: 1 0 0 0

where A is bits 7 - 0 of one row of the H matrix and B is the byte to be coded. The even parity

bits apply to the result of ‘A AND B’.

So the word formed will be: ‘00101100 1000’ sent left to right

When the same process is carried out on these 12 bits as above, using all 12 bits of each H

matrix row, the resulting 4 parity bits will be ‘0000’.

5.5.3. Interleaving

All the bits of transmitted Mobitex Data Blocks and Short Data Blocks are interleaved by the

modem to give protection against noise bursts and short fades. Interleaving is not performed

on any bits in the Mobitex Frame Head.

In the Mobitex Data Block case, considering the 240 bits to be numbered sequentially before

interleaving as 0 to 239 (‘0’ = bit 7 of byte 0, ‘11’ = bit 0 of FEC for byte 0, ... , ‘239’ = bit 0 of

FEC for byte 19 - see Figure 6), then they will be transmitted as shown in Figure 13. The

Mobitex Short Data Block is interleaved in a similar way; referring to Figure 13 consider bytes 4

and 5 as the CRC data and ignore bits 72 to 239 in the lower part of the diagram. i.e. the last bit

to be transmitted will be ‘71’.

The modem performs the inverse operation (de-interleaving) in receive mode on both Mobitex

Data Blocks and Short Data Blocks.

GMSK Packet Data Modem CMX909B

Figure 13 Interleaving - Input/Output

5.5.4. Scrambling

All formatted bits of both Mobitex Data Blocks and Short Data Blocks are passed through a 9-bit

scrambler. This scrambler is initialised at the beginning of the first data block in every Frame.

The 511-bit sequence is generated with a 9-bit shift register with the output of the 5th and 9th

stages XOR’ed and fed back to the input of the first stage. The scrambler is disabled during all

other tasks, apart for TSO.

GMSK Packet Data Modem CMX909B

6. Application Notes

6.1. Transmit Frame Example

If the device is required to send a Mobitex Frame the following control signals and data should

be issued to the modem, assuming the device is not starting from a powersave state, TXRXN is

set to ‘1’ and that the relevant control bits have been set as required after power was applied to

the device:

1. 6 bytes forming the Frame Head are loaded into the Data Buffer, followed by a 2-bit pause

to let the filter stabilise, followed by setting T7H task.

2. Device interrupts host µC with IRQN when the 6th byte is read from the Data Buffer.

3. Status Register is read and 18 bytes are loaded, followed by setting TDB task.

4. Device interrupts host µC with IRQN when 18th byte is read from the Data Buffer.

5. Status Register is read, host may load data and set next task as required:

GOTO ‘1’ if the last Data Block for this Frame has been transmitted

and another Frame is to be immediately transmitted

GOTO ‘3’ if another Data Block in this Frame is to be transmitted

GOTO ‘6’ if no more data is to be immediately sent

6. 1 byte representing the ‘hang byte’ is loaded into the Data Buffer, followed by setting the

TSB task.

If the ‘hang byte’ has been transmitted and no more data is to be sent then a new task need not

be written and the µC can wait for the IBEMPTY interrupt when, after a few bits to allow for the

Tx filter delay, it can shut down the Tx RF circuits.

A top level flowchart of the transmit process is shown in Figure 14.

Hang Byte

The filtering required to reduce the transmitted bandwidth causes energy from each bit of

information to be smeared across 3 bit times. To ensure that the last bit transmitted is received

correctly it is necessary to add an 8-bit ‘hang byte’ to the end of each message. Thus the tasks

required to transmit an isolated Mobitex frame are:

T7H + (n x TDB) + TSB

When receiving this data, the extra byte can be ignored as its only function is to ensure integrity

of the last bit and not to carry any information itself.

It is suggested that a ‘00110011’ or ‘11001100’ pattern is used for this ‘hang byte’.

GMSK Packet Data Modem CMX909B

Figure 14 Transmit Process

GMSK Packet Data Modem CMX909B

6.2. Receive Frame Example

If the device is required to decode a Mobitex Frame the following control signals should be

issued to the modem, assuming the device is initially not in powersave, PLLBW, LEVRES,

SCREN are set as required, TXRXN bit is set to ‘0’, the Frame Sync bytes have not been set

and the carrier has been detected, or a Frame Head is imminently expected:

1. 2 Frame Sync bytes are loaded.

2. 2 bits after the carrier has been detected, a LFSB task is loaded, along with setting the

AQLEV and AQBC bits, to initiate the level acquisition and bit clock extraction sequences.

3. Device interrupts host µC with IRQN when 2nd byte is read from Data Buffer.

4. Status Register is read, 12 bits later task is set to SFH to search for a Mobitex Frame Head.

5. Device will interrupt host µC with IRQN when valid Frame Sync is detected and header

bytes decoded.

6. Host µC reads Status Register, checks MOBAN and CRCFEC bit and reads out 2 Frame

Head control bytes.

7. Host µC sets the task to RDB to receive a Mobitex Data Block.

8. Device will interrupt host µC with IRQN when the Data Block has been received and the

CRC has been calculated.

9. Host µC reads Status Register, checks CRC validity and reads 18 Data Block bytes. The

Data Quality Register can also be read to obtain the received S/N level.

10. Host µC sets task if more information is expected:

GOTO ‘4’ if last Data Block and another Frame Head imminently expected.

GOTO ‘7’ if another Mobitex Data Block expected.

If the last Data Block has been decoded and no more information is expected then the task bits

need not be set as the device will automatically select the idle state.

A top level flowchart of the receive process is shown in Figure 15.

GMSK Packet Data Modem CMX909B

Figure 15 Receive Process

GMSK Packet Data Modem CMX909B

6.3. Clock Extraction and Level Measurement Systems

The modem needs to make accurate measurements of the received signal amplitude, dc offset

and bit timing to achieve reasonable error rates. Accurate measurements, especially in the

presence of noise, are best made by averaging over a relatively long time.

However, in most cases the modem will be used to receive isolated messages from a distant

transmitter that is only turned on for a very short time before the message starts. Also, the

received baseband signal out of the radio's frequency discriminator will have a dc offset due to

small differences between the receiver and transmitter reference oscillators and hence their

‘carrier’ frequencies.

To cater for this situation, AQBC and AQLEV (Acquire Bit Clock and Level) commands are

provided which, when triggered, causes the modem to follow an automatic sequence designed

to perform these measurements as quickly as possible.

The AQLEV sequence always starts with a measurement of the average signal voltage over a

period of 1 bit time. The sequence continues by measuring the positive going and negative

going peaks of the signal. The attack and decay times used in this ‘Lossy Peak Detect’ mode

are such that a sufficiently accurate measurement can be made within 16 bits of a ‘1100 ...’

pattern (i.e. the bit sync sequence) to allow the bit clock extraction circuits to operate.

If SFH or SFS is set within 28 bit times of AQLEV the device will switch to the Residual setting

when Frame Sync is found. If a SFH or SFS task is not set then the Residual setting will be

active 30 bits after AQLEV was set. The Residual setting is that programmed in the LEVRES

bits and is either ‘Lossy Peak Detect’, ‘Peak Detect’, ‘Peak Averaging’ or ‘Hold’. Note: For

normal operation the LEVRES bits would only be set to 'hold' for the duration of a fade.

If SFH or SFS is set within 14 bit times of AQBC the device will switch to the Medium setting

when Frame Sync is found. If a SFH or SFS task is not set then the Medium setting will be

active 16 bits after AQBC was set. The PLLBW will change to the Residual setting 30 bits later.

The complete AQBC and AQLEV sequence is illustrated below, for the situation where the µC

can detect the received carrier so that it knows when to issue the AQBC and AQLEV

commands. Note that due to the delay through the Rx low pass filter, the AQBC and AQLEV

sequences should not be started until about 2 bit times after the received carrier has been

detected at the discriminator output. See Figure 16.

In a system where the host µC is not able to detect the received carrier, the AQBC and AQLEV

sequences may be started at any time - possibly when no carrier is being received. However, in

this case the clock and level acquisition will take longer since the circuits will have to recover

from the change from a large amplitude noise signal at the output of the frequency discriminator

to the wanted signal, probably with a dc offset. In this type of system, the time between the turn-

on of the transmitter and the start of the Frame Sync pattern should be extended - preferably by

extending the Bit Sync sequence to 32 or even 48 bits.

Note that the clock extraction circuits work by detecting the timing of edges, i.e. a change from

‘0’ to ‘1’ or ‘1’ to ‘0’. They will eventually fail if ‘1’ or ‘0’ is transmitted continuously. Similarly, the

level measuring circuits require ‘00’and ‘11’ bit pairs to be received at reasonably frequent

intervals.

GMSK Packet Data Modem CMX909B

AQLEV Sequence

SFH or SFS is set up to 28 bits after AQLEV;

Frame Sync is being searched for:

1 bit of clamp.

Lossy Peak detect until Frame Sync is detected.

Residual setting.

SFH or SFS is not set; Frame Sync is not being

searched for:

1 bit of clamp.

30 bits of Lossy Peak Detect.

Residual setting.

AQBC Sequence

SFH or SFS is set up to 14 bits after AQBC;

Frame Sync is being searched for:

‘Wide’ setting until Frame Sync detected.

30 bits of ‘Medium’ setting.

Residual setting.

SFH or SFS is not set; Frame Sync is not being

searched for:

16 bits of ‘Wide’ setting.

30 bits of ‘Medium’ setting.

Residual setting.

Figure 16 Bit Clock and Level Acquisition Example

6.4. AC Coupling

For a practical circuit, ac coupling from the modem’s transmit output to the Frequency

Modulator and between the receiver’s Frequency Discriminator and the receive input of the

modem may be desired. There are, however, two problems.

Firstly, ac coupling of the signal degrades the Bit Error Rate performance of the modem. See

Figure 17.

GMSK Packet Data Modem CMX909B

Figure 17 Typical Bit Error Rates

(at 8kbits/sec, without FEC, for different degrees of ac coupling)

Secondly, any ac coupling at the receive input will transform any step in the voltage at the

discriminator output to a slowly decaying pulse which can confuse the modem’s level

measuring circuits. As illustrated below, the time for this step to decay to 37% of its original

value is ‘RC’ where:

RC = 1/( 2 x π x the 3dB cut-off frequency of the RC network )

and is 8 msec - or 64 bit times at 8kbits/sec for a 20Hz network. See Figure 18.

Figure 18 Decay Time - AC Coupling

For these reasons the maximum 3dB cut-off frequencies would seem to be around 5Hz in the

Tx path and 20Hz in the Rx path at 8kbits/sec.

GMSK Packet Data Modem CMX909B

6.5. Radio Performance

The maximum data rate that can be transmitted over a radio channel using this modem

depends on:

- RF channel spacing.

- Allowable adjacent channel interference.

- Bit rate.

- Peak carrier deviation (modulation index).

- Tx and Rx reference oscillator accuracies.

- Modulator and demodulator linearity.

- Receiver IF filter frequency and phase characteristics.

- Use of error correction techniques.

- Acceptable error rate.

As a guide, 8000 bits/sec can be achieved (subject to local regulatory requirements) over a

system with 12.5kHz channel spacing if the transmitter frequency deviation is set to ± 2kHz

peak for a repetitive ‘1100…’ pattern and the maximum difference between transmitter and

receiver ‘carrier’ frequencies is less than 1500Hz.

The modulation scheme employed by this modem is designed to achieve high data throughput

by exploiting as much as possible of the RF channel bandwidth. This does, however, place

constraints on the performance of the radio. In particular, attention must be paid to:

- Linearity, frequency and phase response of the Tx Frequency Modulator.

- The bandwidth and phase response of the receiver’s IF filters.

- Accuracy of the Tx and Rx reference oscillators, as any difference will shift the received

signal towards the skirts of the IF filter response and cause a dc offset at the discriminator

output.

Viewing the received signal eye pattern, using the output of the frequency discriminator, gives a

good indication of the overall transmitter/receiver performance.

Figure 19 Typical System Installation

GMSK Packet Data Modem CMX909B

7. Performance Specification

7.1. Electrical Performance

7.1.1. Absolute Maximum Ratings

Exceeding these maximum ratings can result in damage to the device.

Min. Max. Units

Supply (VDD - VSS) -0.3 7.0 V

Voltage on any pin to VSS -0.3 VDD + 0.3 V

Current into or out of VDD and VSS pins -30 +30 mA

Current into or out of any other pin -20 +20 mA

E2 Package Min. Max. Units

Total Allowable Power Dissipation at Tamb = 25°C - 1000 mW

... Derating - 10.0 mW/°C

Storage Temperature -55 +125 °C

Operating Temperature -40 +85 °C

D5 Package Min. Max. Units

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

P4 Package Min. Max. Units

Total Allowable Power Dissipation at Tamb = 25°C - 1660 mW

... Derating - 16.6 mW/°C

Storage Temperature -55 +125 °C

Operating Temperature -40 +85 °C

7.1.2. Operating Limits

Correct operation of the device outside these limits is not implied.

Notes Min. Max. Units

Supply (VDD - VSS) 2.7 5.5 V

Operating Temperature -40 +85 °C

Xtal Frequency 1.0 10.0 MHz

GMSK Packet Data Modem CMX909B

7.1.3. Operating Characteristics

For the following conditions unless otherwise specified:

Xtal Frequency = 4.096MHz, Bit Rate = 8k bits/sec, Noise Bandwidth = Bit Rate,

DD = 3.0V to 5.5V, Tamb = -40°C to +85°C.

Notes Min. Typ. Max. Units

DC Parameters

IDD (Running, VDD = 3.0V) 1 – 2.0 – mA

IDD (Running, VDD = 5.0V) 1 – 2.9 – mA

IDD (Powersave, VDD = 3.0V) 1 – 0.4 – mA

IDD (Powersave, VDD = 5.0V) 1 – 0.9 – mA

IDD (Powersave + No Bias) 1 – 0.75 – mA

IDD (Powersave + No Bias or Xtal) 1 – 10 – µA

AC Parameters

Tx Output

TXOP Impedance (not powersaved) 2 – 1.0 2.5 kΩ

TXOP Impedance (powersaved) 2 – 300 – kΩ

Signal Level 3 0.9 1.0 1.1 V pk-pk

Tx Data Delay 4 – 4 6 Bits

Rx Input

RXIN Impedance (at 100Hz) 10.0 – – MΩ

RXIN Amp Voltage Gain (I/P = 1mVrms at 100Hz) – 500 – V/V

Input Signal Level 5 0.7 1.0 1.3 V pk-pk

Rx Data Delay 6 – 3.5 – Bits

Xtal/Clock Input

‘High’ Pulse Width 7 40 – – ns

‘Low’ Pulse Width 7 40 – – ns

Input Impedance (at 100Hz) 10.0 – – MΩ

Gain (I/P = 1mVrms at 100Hz) 20 – – dB

µC Interface

Input Logic ‘1’ Level 8, 9 70% – – VDD

Input Logic ‘0’ Level 8, 9 – – 30% VDD

Input Leakage Current (Vin = 0 to VDD) 8, 9 -5.0 – +5.0 µA

Input Capacitance 8, 9 – 10.0 – pF

Output Logic ‘1’ Level (lOH = 120µA) 9 90% – – VDD

Output Logic ‘0’ Level (lOL = 360µA) 9,10 – – 10% VDD

‘Off’ State Leakage Current (Vout = VDD)

10 – – 10 µA

Notes: 1. Tamb = 25°C, not including any current drawn from the modem pins by external circuitry

other than R3, X1, C3 and C4.

2. Small signal impedance, at VDD = 5.0V and Tamb = 25°C.

3. For ‘…1111000011110000...’ bit sequence, at VDD = 5.0V and Tamb = 25°C

(output level is proportional to VDD).

4. Measured between issuing first task after idle and the centre of the first bit at TXOP

(See Figure 7).

5. For optimum performance, measured at RXFB pin,

for a ‘...11110000...’ bit sequence, at VDD = 5.0V and Tamb = 25°C.

6. Measured between centre of last bit of an Rx single byte or Frame Sync at RXIN and an

IRQ interrupt to the host µC.

GMSK Packet Data Modem CMX909B

7. Timing for an external input to the XTAL/CLOCK pin.

8. WRN, RDN, CSN, A0 and A1 pins.

9. D0 - D7 pins.

10. IRQN pin.

Timing Diagrams

Figure 20 µC Parallel Interface Timings

GMSK Packet Data Modem CMX909B

For the following conditions unless otherwise specified:

Xtal Frequency = 4.096MHz, VDD = 3.0V to 5.5V, Tamb = -40°C to +85°C.

Notes Min. Typ. Max. Units

μC Parallel Interface Timings (ref. Figure 20)

tACSL Address valid to CSN low time 0 - - ns

TAH Address hold time 0 - - ns

tCSH CSN hold time 0 - - ns

TCSHI CSN high time 12 6 - - clock cycles

TCSRWL CSN to WRN or RDN low time 0 - - ns

TDHR Read data hold time 0 - - ns

tDHW Write data hold time 0 - - ns

tDSW Write data setup time 90 - - ns

tRHCSL RDN high to CSN low time (write) 0 - - ns

tRACL Read access time from CSN low 11 - - 175 ns

tRARL Read access time from RDN low 11 - - 145 ns

tRL RDN low time 200 - - ns

tRX RDN high to D0-D7 3-state time - - 50 ns

tWHCSL WRN high to CSN low time (read) 0 - - ns

tWL

WRN low time 200 - - ns

Notes: 11. With 30pF max to VSS on D0 - D7 pins.

12. Xtal/Clock cycles at the XTAL/CLOCK pin.

GMSK Packet Data Modem CMX909B

Figure 21 Typical Bit Error Rate (noise in bit rate bandwidth)

N.B. A block is deemed to be in error if the CRC fails.

GMSK Packet Data Modem CMX909B

7.2. Packaging

Figure 22 E2 Mechanical Outline: Order as part no. CMX909BE2

Figure 23 D5 Mechanical Outline: Order as part no. CMX909BD5

GMSK Packet Data Modem CMX909B

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.

Figure 24 P4 Mechanical Outline: Order as part no. CMX909BP4