AD7873
Data Sheet
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
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
and this CMOS logic open-drain output, the
output
PENIRQ
remains high normally. If
is enabled (see Table 8), when
PENIRQ
the touch screen connected to the AD7873 is touched by a pen
or finger, the output goes low, initiating an interrupt to
end of conversion and the
pin was still low. Once the
PENIRQ
PENIRQ
next start bit is detected by the AD7843, the
is again disabled.
function
PENIRQ
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.
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
function is
PENIRQ
enabled in the control register, it is disabled by the start bit
again before the end of the conversion is reached, so
Figure 34 assumes that the
function was enabled in
PENIRQ
the
function effectively cannot be used in this mode.
PENIRQ
However, as conversions are occurring continuously,
the function is not necessary and is therefore
the last write or that the part was just powered up so
enabled by default. Once the screen is touched, the
is
PENIRQ
PENIRQ
PENIRQ
redundant.
output goes low a time tPEN later. This delay is approximately
5 µs, assuming a 10 nF touch screen capacitance, and varies
+V
CC
with the touch screen resistance actually used. Once the START
bit is detected, the pen interrupt function is disabled and
EXTERNAL
PULL-UP
100kΩ
the
cannot respond to screen touches. The
PENIRQ
PENIRQ
PENIRQ
Y+
+V
CC
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
X+
TOUCH
SCREEN
PENIRQ
ENABLE
loaded to the control register. Regardless of whether
is
PENIRQ
Y–
ON
to be enabled again, the
output normally always idles
PENIRQ
is enabled again as shown in
high. Assuming the
PENIRQ
PENIRQ
Figure 33.
Functional Block Diagram
Figure 34, then once the conversion is complete, the
PENIRQ
SCREEN
TOUCHED
HERE
PD1 = 1, PD0 = 0, PENIRQ
ENABLED AGAIN
tPEN
NO RESPONSE TO TOUCH
PENIRQ
INTERRUPT
ROCESSOR
P
CS
1
8
1
13
16
DCLK
DIN
SER/
DFR
MODE
S
A2 A1 A0
1
0
(START)
PENIRQ
Figure 34.
Timing Diagram
Rev. F | Page 18 of 28
ADI [ ADI ]
