AD7843 datasheet(20/21 Pages) AD

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AD7843

Rev. B | Page 19 of 20

PEN INTERRUPT REQUEST

The pen interrupt equivalent output circuitry is outlined in

Figure 28. 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 7), when the touch screen connected to the AD7843

is touched via a pen or finger, the PENIRQ output goes low,

initiating an interrupt to a microprocessor that can then instruct

a control word to be written to the AD7843 to initiate a conver-

sion. This output can also be enabled between conversions

during power-down (see Table 7 ), 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- or 8-bit

level as required.

TOUCH

SCREEN

PENIRQ

ENABLE

EXTERNAL

PULL-UP

PENIRQ

Y+

100k

Ω

+VCC

Y–

X+

Figure 28. PENIRQ Functional Block Diagram

Figure 29 assumes that the PENIRQ function is enabled in the

last write or that the part has just been 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 has been clocked in, at which

point it returns high as soon as possible, regardless 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 or not, the PENIRQ

output normally always idles high. Assuming that the PENIRQ

is enabled again as shown in Figure 29, once the conversion is

complete, the PENIRQ output responds to a screen touch again.

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 could

occur if the interrupt request line on the microprocessor/DSP

was unmasked during or toward the end of conversion with the

PENIRQ pin still low. Once the next START bit is detected by

the AD7843, the PENIRQ function is disabled again.

If the control register write operation overlaps with the data

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

conversion. This means that even if the PENIRQ function has

been enabled in the control register, it is disabled by the START

bit again before the end of the conversion is reached; therefore

the PENIRQ function effectively cannot be used in this mode.

However, as conversions are occurring continuously, the

PENIRQ function is not necessary and, therefore, redundant.

GROUNDING AND LAYOUT

For information on grounding and layout considerations for the

AD7843, refer to Application Note AN-577, Layout and

Grounding Recommendations for Touch Screen Digitizers.

SA2

MODE

SER/

DFR

SCREEN

TOUCHED

HERE

INTERRUPT

PROCESSOR

NO RESPONSE TO TOUCH

tPEN

PENIRQ

DCLK

DIN

(START)

PD1 = 1, PD0 = 0, PENIRQ

ENABLED AGAIN

Figure 29. PENIRQ Timing Diagram

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