Datasheet

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 +V
CC
and this CMOS logic open-drain output, the
PENIRQ
output
remains high normally. If
PENIRQ
is enabled (see Tabl e 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 t
PEN
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–
ON
X+
TOUCH
SCREEN
EXTERNAL
PULL-UP
PENIRQ
ENABLE
+V
CC
+V
CC
PENIRQ
100kΩ
02164-033
S A2 A1 A0 1 0
81 1 13 16
MODE
SER/
DFR
SCREEN
TOUCHED
HERE
NO RESPONSE TO TOUCH
t
PEN
PENIRQ
CS
DCLK
DIN
(START)
PD1 = 1, PD0 = 0, PENIRQ
ENABLED AGAIN
INTERRUPT
P
ROCESSOR
02164-034