AD7879 datasheet(29/36 Pages) AD

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AD7879

Rev. 0 | Page 29 of 36

When the screen is touched, PENIRQ goes low. This generates

an interrupt request to the host. When the screen touch ends,

and if the ADC is idle, PENIRQ immediately goes high. If the

ADC is converting, PENIRQ goes high when the ADC becomes

idle. The PENIRQ operation for these two conditions is shown

Figure 36

SYNCHRONIZING THE AD7879 TO THE HOST CPU

The two recommended methods for synchronizing the AD7879

to its host CPU are slave mode (in which the mode bits can be

either 01b or 10b) and master mode (in which the mode bits

are 11b).

In master mode (ADC mode bits = 11b), PENIRQ mode 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 to convert in any mode

and read the results after the conversions are completed.

SCREEN

PENIRQ

ADC

STATUS

TOUCHED

NOT

TOUCHED

NOT

TOUCHED

NOT

TOUCHED

NOT

TOUCHED

ADC IDLE

SCREEN

PENIRQ

ADC

STATUS

TOUCHED

ADC IDLE

ADC

CONVERTING

ADC IDLE

RELEASE NOT

DETECTED

PENIRQ

DETECTS

RELEASE

PENIRQ

DETECTS

RELEASE

PENIRQ

DETECTS

TOUCH

PENIRQ

DETECTS

TOUCH

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 in Mode 11b 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

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

the programmed sequence of conversions automatically begins.

When the INT or DAVsignal asserts, the host reads the new

data available in the AD7879 results registers and returns to

sleep mode. This method can significantly reduce the load on the

host.

Figure 36. PENIRQ Operation for ADC Idle and ADC Converting

Figure 37 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/

DAV/INT pin can output only DAV or INT 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 37. Master Mode Operation

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