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ADA and MASK Timing on WM9712

WOLFSON MICROELECTRONICS plc

www.wolfsonmicro.com

March 2004, Rev 1.0.

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)

(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.

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

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

DEL

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

SETUP

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

DEL

PULSE

DEL

Conv

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

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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IMPORTANT NOTICE

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EH11 2QW

Tel :: +44 (0)131 272 7000

Fax :: +44 (0)131 272 7001

Email :: apps@wolfsonmicro.com