AD7873ACPZ datasheet(19/29 Pages) AD

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AD7873

Data Sheet

Rev. F | Page 18 of 28

PEN INTERRUPT REQUEST

The pen interrupt equivalent circuitry is outlined in Figure 33.

By connecting a pull-up resistor (10 kΩ to 100 kΩ) between +VCC

and this CMOS logic open-drain output, the PENIRQ output

remains high normally. If PENIRQ is enabled (see Table 8), when

the touch screen connected to the AD7873 is touched by a pen

or finger, the PENIRQ output goes low, initiating an interrupt to

a microprocessor. This can then instruct a control word to be

written to the AD7873 to initiate a conversion. This output can

also be enabled between conversions during power-down (see

Table 8), allowing power-up to be initiated only when the

screen is touched. The result of the first touch screen coordinate

conversion after power-up is valid, assuming any external

reference is settled to the 12-bit or 8-bit level as required.

Figure 34 assumes that the PENIRQ function was enabled in

the last write or that the part was just powered up so PENIRQ is

enabled by default. Once the screen is touched, the PENIRQ

output goes low a time tPEN later. This delay is approximately

5 µs, assuming a 10 nF touch screen capacitance, and varies

with the touch screen resistance actually used. Once the START

bit is detected, the pen interrupt function is disabled and

the PENIRQ cannot respond to screen touches. The PENIRQ

output remains low until the fourth falling edge of DCLK after

the START bit is clocked in, at which point it returns high as

soon as possible, irrespective of the touch screen capacitance.

This does not mean that the pen interrupt function is now

enabled again because the power-down bits have not yet been

loaded to the control register. Regardless of whether PENIRQ is

to be enabled again, the PENIRQ output normally always idles

high. Assuming the PENIRQ is enabled again as shown in

Figure 34, then once the conversion is complete, the PENIRQ

output again responds to a screen touch. The fact that PENIRQ

returns high almost immediately after the fourth falling edge of

DCLK means the user avoids any spurious interrupts on the

microprocessor or DSP, which can occur if the interrupt request

line on the micro/DSP were unmasked during or toward the

end of conversion and the PENIRQ pin was still low. Once the

next start bit is detected by the AD7843, the PENIRQ function

is again disabled.

If the control register write operation overlaps with the data

read, a start bit is always detected prior to the end of

conversion, meaning that even if the PENIRQ function is

enabled in the control register, it is disabled by the start bit

again before the end of the conversion is reached, so

the PENIRQ function effectively cannot be used in this mode.

However, as conversions are occurring continuously,

the PENIRQ function is not necessary and is therefore

redundant.

Figure 33. PENIRQ Functional Block Diagram

Figure 34. PENIRQ Timing Diagram

Y+

Y–

X+

TOUCH

SCREEN

EXTERNAL

PULL-UP

PENIRQ

ENABLE

+VCC

PENIRQ

100k

Ω

MODE

SER/

DFR

SCREEN

TOUCHED

HERE

NO RESPONSE TO TOUCH

tPEN

PENIRQ

DCLK

DIN

(START)

PD1 = 1, PD0 = 0, PENIRQ

ENABLED AGAIN

INTERRUPT

PROCESSOR

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