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 V
CC
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.
02144-B-028
TOUCH
SCREEN
PENIRQ
ENABLE
EXTERNAL
PULL-UP
PENIRQ
ON
Y+
100kΩ
+V
CC
+V
CC
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 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 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.
02144-B-029
SA2A1A0 1 0
81 1 13 16
MODE
SER/
DFR
SCREEN
TOUCHED
HERE
INTERRUPT
PROCESSOR
NO RESPONSE TO TOUCH
t
PEN
PENIRQ
CS
DCLK
DIN
(START)
PD1 = 1, PD0 = 0, PENIRQ
ENABLED AGAIN
Figure 29.
PENIRQ
Timing Diagram
