AD7879 datasheet(26/36 Pages) AD

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AD7879

Rev. 0 | Page 26 of 36

PM1

PM0

GPIO

DAT

GPIO

DIR

GPIO

POL

AVG1 AVG0 MED1 MED0

SW/

RST

FCD3

FCD2 FCD1 FCD0

SER/

DFR

Figure 30. Control Register 2

CONTROL REGISTER 2

Control Register 2 (Address 0x02) contains the power

management bits, the GPIO settings, the SER/DFR bit

(to choose single or differential methods of touch screen

measurement), the averaging and median filter settings, a bit

that allows resetting the part, and the first conversion delay

bits. Its power-on default value is 0x4040. See the

section for more information on the

control registers.

Detailed

First Conversion Delay (Control Register 2, Bits[3:0])

The first conversion delay (FCD) bits in Control Register 2

program a delay from 128 μs (default) up to 4.096 ms before

the first conversion to allow the ADC time to power up. This

delay also occurs before conversion of the X and Y coordinate

channels to allow extra time for screen settling, and after the

last conversion in a sequence to precharge PENIRQ.

Table 20. First Conversion Delay Selection

FCD

Function

0000

128 μs

0001

256 μs

0010

384 μs

0011

512 μs

0100

640 μs

0101

768 μs

0110

896 μs

0111

1.024 ms

1000

1.152 ms

1001

1.280 ms

1010

1.536 ms

1011

1.792 ms

1100

2.048 ms

1101

2.560 ms

1110

3.584 ms

1111

4.096 ms

Power Management (Control Register 2, Bits[15:14])

The power management (PM) bits in Control Register 2 allow

the power management features of the ADC to be programmed.

If the PM bits are 00, the ADC is permanently powered down.

This overrides any setting of the mode bits in Control Register 1.

If the PM bits are 01, both the ADC and the reference power

down when the ADC is not converting. If the PM bits are 10 or

11, the analog blocks to be powered down depend on the ADC

mode settings. Power management overrides the ADC modes.

Table 21. Power Management Selection

PM1

PM0

Function

Full shutdown; ADC, oscillator, BIAS, and

temperature sensor are all turned off. The only

way of coming out of this mode is to write to the

part over the serial interface and change the PM

bits. This setting overrides any other setting on

the part, including the ADC mode bits.

The analog blocks to be powered down depend

on the ADC mode settings. If the ADC mode is set

to master mode, the ADC, BIAS, temperature

sensor, and oscillator are powered down and

must wake up when the user touches the screen.

If the ADC mode is set to slave mode, the ADC

and the TEMP sensor are powered down while

not being used. They wake up automatically

when required. The oscillator and BIAS are

powered up because they are needed to measure

time. This also applies to the single-conversion

mode.

ADC, BIAS, and the oscillator are powered up

continuously irrespective of ADC mode.

As 01.

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