AD7879 datasheet(24/36 Pages) AD

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AD7879

Rev. 0 | Page 24 of 36

CONTROL REGISTERS

DIS

PENIRQ

CHNL

ADD2

CHNL

ADD1

CHNL

ADD0

ADC

MODE1

ADC

MODE0

ACQ1 ACQ0 TMR7 TMR6 TMR5 TMR4 TMR3 TMR2 TMR1 TMR0

Figure 29. Control Register 1

CONTROL REGISTER 1

Control Register 1 (Address 0x01) contains the ADC channel

address and the ADC mode bits. It sets the acquisition time and

the timer. It also contains a bit to disable the pen interrupt.

Control Register 1 should always be the last register programmed

prior to starting conversions. Its power-on default value is

0x0000. To change any parameter after conversion has begun,

the part should first be put into Mode 00. Make the changes;

then reprogram Control Register 1, ensuring that it is always the

last register programmed before conversions begin.

Timer (Control Register 1, Bits[7:0])

The TMR bits in Control Register 1 enable the ADC to repeat-

edly perform a conversion or to perform a conversion sequence

only once or at intervals of 35 μs from 550 μs up to 9.440 ms. In

slave mode, the timer starts as soon as the conversion sequence

is finished. In master mode, the timer starts at the end of a

conversion sequence only if the screen remains touched. If the

touch is released at any stage, then the timer stops. The next

time the screen is touched, a conversion sequence immediately

begins.

Table 16. Control Register 1 Timer Selection

TMR

Function

00000000

Convert one time only (default)

00000001

Every 550 μs

00000010

Every 585 μs

00000011

Every 620 μs

…

11111101

Every 9.370 ms

11111110

Every 9.405 ms

11111111

Every 9.440 ms

Acquisition Time (Control Register 1, Bits[9:8])

The ACQ bits in Control Register 1 allow the selection of

acquisition times for the ADC of 2 μs (default), 4 μs, 8 μs,

or 16 μs. The user can program the ADC with an acquisition

time suitable for the type of signal being sampled. For example,

signals with large RC time constants can require longer acquisi-

tion times.

Table 17. Acquisition Time Selection

ACQ1

ACQ0

Function

4 clock periods (2 μs)

8 clock periods (4 μs)

16 clock periods (8 μs)

32 clock periods (16 μs)

ADC Mode (Control Register 1, Bits[11:10])

The mode bits select the operating mode of the ADC. The

AD7879 has three operating modes. These are selected by

writing to the mode bits in Control Register 1. If the mode

bits are 00, no conversion is performed.

Table 18. Control Register 1 Mode Selection

ADC

MODE1

ADC

MODE0

Function

Do not convert (default)

Single-channel conversion; the AD7879 is

in slave mode

Sequence 0; the AD7879 is in slave mode

Sequence 1; the AD7879 is in master mode

If the mode bits are 01, a single conversion is performed on

the channel selected by writing to the channel bits of Control

TMR bits in Control Register 1 are set to 00000000, the mode

bits revert to 00 and the ADC returns to no convert mode until

a new conversion is initiated by the host. Setting the TMR bits

to a value other than 00000000 causes the conversion to be

repeated.

The AD7879 can also be programmed to automatically convert

a sequence of selected channels. The two modes for this type of

conversion are slave mode and master mode.

For slave mode operation, the channels to be digitized are selected

by setting the corresponding bits in Control Register 3. Conver-

sion is initiated by writing 10b to the mode bits of Control

stores the results in the corresponding results registers. At the

end of the conversion, if the TMR bits in Control Register 1 are

set to 00000000, the mode bits revert to 00 and the ADC returns

to no convert mode until a new conversion is initiated by the

host. Setting the TMR bits to a code other than 00000000 causes

the conversion sequence to be repeated.

For master mode operation, the channels to be digitized are

written to the Control Register 3. Master mode is then selected

by writing 11 to the mode bits in Control Register 1. In this

mode, the wake-up on touch feature is active; therefore, conver-

sion does not immediately begin. The AD7879 waits until the

screen is touched before beginning the sequence of conversions.

The ADC then digitizes the selected channels; and the results

are written to the result registers. The AD7879 waits for the

screen to be touched again, or for a timer event if the screen

remains touched, before beginning another sequence of

conversions.

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