AD7889 datasheet(32/40 Pages) AD

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AD7879/AD7889

Data Sheet

Rev. D | Page 32 of 40

SYNCHRONIZING THE AD7879/AD7889 TO THE

HOST CPU

The two methods for synchronizing the AD7879/AD7889 to the

host CPU are slave mode (in which the mode bits are set to 01

or 10) and master mode (in which the mode bits set to 11).

In master mode (ADC mode bits = 11), PENIRQ can be used

as an interrupt to the host. When PENIRQ goes low to indicate

that the screen has been touched, the host is awakened. The host

can then program the AD7879/AD7889 to convert in any mode

and read the results after the conversions are completed.

In master mode, INT or DAV can also be used as an interrupt to

the host. The host must first define a conversion sequence in

Control Register 3, initialize the AD7879/AD7889 in Mode 11,

and enable INT or DAV using Bit 15 in Control Register 1 and

Bit 13 in Control Register 3. The host can then enter sleep mode

to conserve power. The wake-up on touch feature of the AD7879/

AD7889 is active in this mode; therefore, when the screen is

touched, the programmed sequence of conversions automatically

begins. When the INT or DAV signal is asserted, the host reads

the new data available in the AD7879/AD7889 result registers and

returns to sleep mode. This method can significantly reduce the

load on the host.

Figure 38 shows how the PENIRQ circuit is enabled. The wake-up

on touch circuit and the PENIRQ circuit are enabled only in master

mode (ADC mode = 11). In slave mode, the PENIRQ/INT/DAV

pin can output only INT or DAV signals.

ADC MODE = 11?

MASTER MODE

DAV

(END OF CONVERSION SEQUENCE)

INT

(GPIO ALERT/OUT OF LIMITS)

INT/DAV/GPIO ALERT

TOUCH SCREEN TOUCHED

CONTROL REGISTER 3

BIT 13

CONTROL REGISTER 1

BIT 15

PENIRQ/INT/DAV PIN

TO THE DIGITAL CORE

ENABLE

WAKE-UP

ON TOUCH

ENABLE

PENIRQ

DETECTION

CIRCUIT

TOUCH SCREEN TOUCHED

YES

Figure 38. Master Mode Operation

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