Datasheet

AD7879/AD7889
Rev. C | Page 31 of 40
SYNCHRONIZING THE AD7879/AD7889 TO THE
HOST CPU
The two methods for synchronizing the AD7879/AD7889 to its
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 should 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
0
1
0
1
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 YES
07667-037
Figure 38. Master Mode Operation