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±15kV ESD-Protected Touch-Screen
Controllers Include DAC and Keypad Controller
+AV DD
V REF
1M Ω
SW1
SW2
SW255
S1
OUT
TOUCH
SCREEN
X+
TOUCH-SCREEN
DETECTOR
PENIRQ
Y+
Figure 5. DAC Current-Steering Topology
Output Buffer
The DAC voltage output is an internally buffered unity-
gain follower that slews at up to ±0.4V/μs. The output
can swing from zero to full scale. With a 1/4FS to 3/4FS
output transition, the amplifier output typically settles to
1/2LSB in less than 5μs when loaded with 10k Ω in par-
allel with 50pF. The buffer amplifier is stable with any
combination of resistive loads >10k Ω and capacitive
loads <50pF.
Power-On Reset
All registers of the MAX1233/MAX1234 power up at a
default zero state, except the DAC data register, which
is set to 10000000, so the output is at midscale.
Keypad Controller and GPIO
The keypad controller is designed to interface a matrix-
type 4 rows × 4 columns (16 keys or fewer) keypad to a
host controller. The KEY control register controls keypad
interrupt, keypad scan, and keypad debounce times.
The KeyMask and ColumnMask registers enable mask-
ing of a particular key or an entire column of the keypad
when they are not in use. The MAX1233/MAX1234 offer
two keypad data registers. KPData1 is the pending reg-
ister. KPData2 holds keypad scan results of only the
unmasked keys. If 12 or fewer keys are being monitored,
one or more of the row/column pins of the
MAX1233/MAX1234 can be software programmed as
GPIO pins.
Touch-Screen Detection
Touch-screen detection can be enabled or disabled by
writing to the ADC control register as shown in Table 4.
Touch-screen detection is disabled at initial power-up.
Once touch-screen detection is enabled, the Y- driver
is on and the Y- pin is connected to GND. The X+ pin is
internally pulled to AV DD through a 1M Ω resistor as
X-
Y-
S2
Figure 6. Touch-Screen Detection Block Diagram
shown in Figure 6. When the screen is touched, the X+
pin is pulled to GND through the touch screen and a
touch is detected.
When the 1M Ω pullup resistor is first connected, the X+
pin can be floating near ground. To prevent false touch
detection in this case, the X+ pin is precharged high for
0.1μs using the 7 Ω PMOS driver before touch detection
begins.
Key-Press Detection
Key-press detection can be enabled or disabled by
writing to the keypad control register as shown in Table
17. Key-press detection is disabled at initial power-up.
Once key-press detection is enabled, the C_ pins are
internally connected to DV DD and the R_ pins are inter-
nally pulled to GND through a 16k Ω resistor. When a
key is pressed, the associated row pin is pulled to
DV DD and the key press is detected. Figure 7 shows
the key-press detection circuitry.
Interrupts
PEN Interrupt Request ( PENIRQ )
The PENIRQ output can be used to alert the host con-
troller of a screen touch. The PENIRQ output is normally
high and goes low after a screen touch is detected.
16
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