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ADA and MASK Timing on WM9712
WOLFSON MICROELECTRONICS plc
www.wolfsonmicro.com
March 2004, Rev 1.0.
Copyright 2004 Wolfson Microelectronics plc
INTRODUCTION
This applications note has been created to detail the timing of the MASK and ADA (ADC Data
Available) signals on the WM9712 device. This applications note should be used in conjunction with
the latest WM9712 datasheet.
The WM9712 has many additional functions to aid the design of portable applications. Pin 47 of the
device can be configured for either MASK input, ADA output or standard GPIO functionality. The
MASK function can be used to pause and control auxiliary ADC conversions. The ADA function can
be used to indicate when data becomes available from the auxiliary ADC. The timing associated
with both signals is detailed in the following sections of this document.
WM9712 SETUP
The WM9712 device must be configured to setup the function of pin 47. The three possible setups
are:
MASK Input Function
ADA Function
General GPIO
To set the device into one of the three above modes the following configurations should be used.
PIN 47 MODE REGISTER 56H BIT 4 SETTING
(GPIO FUNCTION SELECT)
REGISTER 4CH BIT 4 SETTING
(PIN DEFINED AS INPUT OR OUTPUT)
MASK 1 1 (Input)
ADA 1 0 (Output)
GPIO 0 1 or 0 (GPIO can be Input or Output)
Table 1 WM9712 Pin 47 Configuration
MASK FUNCTION
Sources of glitch noise, such as the signals driving an LCD display, may feed through to the
touchscreen plates and affect measurement accuracy. In order to minimise this effect, a signal may
be applied to MASK (pin 47) to delay or synchronise the sampling of any input to the ADC. The effect
of the MASK signal depends on the MSK[1-0] bits of register 78h, as described below.
MSK[1-0] EFFECT OF SIGNAL ON MASK PIN
00 MASK has no effect on conversions GPIO input disabled (default)
01 Static; ‘hi’ on MASK pin stops conversions, ‘lo’ has no effect.
10 Edge triggered; rising or falling edge on MASK pin delays conversions
by an amount set in the DEL[3-0] register. Conversions are asynchronous to
the MASK signal.
11 Synchronous mode; conversions wait until rising or falling edge on MASK
initiates cycle; screen starts to be driven when the edge arrives, the
conversion sample being taken a period set by DEL[3-0] after the edge.
Table 2 Controlling the MASK Feature
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The delay described in Table 2 refers to the length of time delay there will be from the edge of the
MASK signal to the start of the next auxiliary ADC conversion. This delay is defined as shown in
Table 3 and Table 4 below.
REGISTER
ADDRESS
BIT LABEL DEFAULT DESCRIPTION
76h 7:4 DEL 0000
(1 frame)
Delay Time
Table 3 Delay Time Control (1)
DEL DELAY
(AC-LINK FRAMES)
DELAY
(TIME)
0000 1 20.8µs
0001 2 41.7µs
0010 4 83.3µs
0011 8 167µs
0100 16 333µs
0101 32 667µs
0110 48 1ms
0111 64 1.33ms
1000 96 2ms
1001 128 2.67ms
1010 160 3.33ms
1011 192 4ms
1100 224 4.67ms
1101 256 5.33ms
1110 288 6ms
1111 No delay, switch matrix always on
Table 4 Delay Time Control (2)
The delay associated to MASK is the same as the settling delay associated with the actual auxiliary
ADC measurement.
For example, when MODE is Edge triggered, an edge on the MASK input will cause the next auxiliary
ADC measurement to be delayed by the value set in register 76h. After this delay for MASK the
auxiliary ADC conversion will be started.
Figure 1 details the timing associated with the delay set to 2 frames for each of the active MASK
modes.
With MASK = 11, a conversion will be commanded and triggered by an edge of the MASK signal.
The delay from MASK edge to conversion is set by the DEL register setting. The delay is started on
the first SYNC pulse after an edge is detected on MASK (see figure 2 for further details).
With MASK = 10, a previously commanded conversion is paused by a delay reflected in the DEL
register setting.
With MASK = 01, any commanded conversions will be delayed while the MASK signal is HIGH.
Once the MASK signal returns low normal operation will resume.
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INTERFACE TIMING
MASK
Frames
(SYNC)
AUXADC Conversion
(MASK = 11)
AUXADC Conversion
(MASK = 10)
AUXADC Conversion
(MASK = 01)
Conv
DelayConv Conv
Conv
Conv
Conv
t
DEL
t
DEL
t
DEL
t
DEL
Delay
Delay
Conv Delay Conv Delay Conv
Delay Conv Delay
Figure 1 MASK Delay Timings (tDEL = 2 frames)
PARAMETER SYMBOL MIN TYP MAX UNIT
Frame Delay set by register 76h tDEL 0 288 frames
MASK
BCLK
SYNC
t
SETUP
t
SETUP
t
HOLD
t
HOLD
Figure 2 MASK Delay Timings
PARAMETER SYMBOL MIN TYP MAX UNIT
Setup time from MASK edge to SYNC 1 tSETUP 162.8 ns
Hold time of MASK level from SYNC
rising edge.
tHOLD 81.4 ns
Note:
1. There must be at least two BCLK's between the rising edge of MASK and the rising edge of SYNC. Therefore, once MASK is
high there must be at least two BCLK rising edges prior to the SYNC pulse.
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ALTERNATIVE MASK FUNCTION
When MASK = 11, synchronous mode, conversions are commanded on the rising and falling edges
of MASK input signal. However, there is an alternative operation which allows a pulse to be applied
to the MASK pin resulting in only one conversion taking place. In this setup the edges of the pulse
are too close together to allow two separate conversions to be commanded. Figure 3 details this
operation and the timing associated.
MASK
Frames
(SYNC)
AUXADC Conversion
(MASK = 11)
Conv
t
DEL
t
PULSE
t
DEL
Conv
t
DEL
Figure 3 MASK PULSE delay timings (tDEL = 2 frames)
PARAMETER SYMBOL MIN TYP MAX UNIT
MASK Pulse Width tPULSE 100ns < tDEL ns
Frame Delay set by register 76h tDEL 0 288 frames
ADA TIMING
Whenever data becomes available from the touchpanel ADC, the internal ADA signal goes high and
remains high until the data has been read from register 7Ah (if SLEN = 0) or until it has been sent out
on an AC-Link slot (if SLEN = 1).
If ADA is configured to be output on pin 47 the timing associated with this signal will be as follows.
BCLK
SYNC
ADA
t
ADA
t
ADA
Figure 4 ADA Timings
PARAMETER SYMBOL MIN TYP MAX UNIT
Delay from SYNC to ADA tADA 200ns ns
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SUMMARY
Pin 47 of the WM9712 has three possible functions which are:
MASK input,
ADA output or
General GPIO.
The modes and timings associated with MASK and ADA are detailed in this document. Further
information on these functions and the GPIO operation is detailed in the WM9712 datasheet.
APPLICATION SUPPORT
If you require more information or require technical support please contact Wolfson Microelectronics
Applications group through the following channels:
Email: apps@wolfsonmicro.com
Telephone: +44 (0)131 272 7070
Fax: +44 (0)131 272 7001
Mail: Applications at the address on last page.
or contact your local Wolfson representative.
Additional information may be made available from time to time on our web site at
http://www.wolfsonmicro.com
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