MAX11800–MAX11803
Low-Power, Ultra-Small Resistive Touch-Screen
Controllers with I2C/SPI Interface
______________________________________________________________________________________
19
Pressure Measurements
Z1 and Z2 measurements determine the resistance
between the two planes within the panel sensor during
a touch (RTOUCH). Depending on the known physical
properties of the panel, one of two equations extract the
value of RTOUCH, providing information about the pres-
sure and area of the touch applied to the panel. Allow
adequate time for the panel to settle when switching
between position and pressure measurements. Figure 5
shows the physical setup of the panel when performing
pressure measurements.
Z1 and Z2 measurements allow observation of the volt-
age on either side of the effective RTOUCH resistance.
With both Z1 and Z2 measurements available, compute
RTOUCH as follows:
If only a Z1 measurement is available, compute
RTOUCH as follows:
The power applied across the panel during pressure
measurements is greatly dependent on RTOUCH and
the physical position of the touch. The maximum power
dissipation in the panel during a pressure measurement
is approximately PZ = VDD2/RTOUCH. This maximum
R
RX
TOUCH
XPLATE POSITION
NBITX
NBI
=
2
T
TZ
YPLATE
POSITION
NBIT
Z
R
Y
1
11
2
Y
RR
XZ
Z
TOUCH
XPLATE
POSITION
NBITX
=
2
1
2
1
OPERATION
MODE
PSU
PMC CMC TDM
LPM
AVG
FIFO
APER
PUR
PUF
CONT
MTAG
ETAG
X, Y,
Z1, Z2
EINT
CINT
COR
INT
DCM
MAX11800–
MAX11803
Yes
No
Yes
No
Yes
2
Yes
2
Yes
No
ACM
MAX11800/
MAX11801
Yes
1
Yes
1
Yes
1
Yes
No
Yes
No
Yes
AUX
No
Yes
3
No
—
Yes
3
No
Table 2. Operating Modes, Conditions, and Options
1In ACM, the choices are limited to X and Y scan, or X and Y and Z1 scan, or X and Y and Z1 and Z2 scan.
2In DCM, MTAG is always used. For DCM with CONT = 0, the following ETAGs are used: 00 = touch present (data valid), 10 = no
touch present (data may be invalid), 11 = measurement in progress (data invalid). For DCM with CONT = 1, the panel cannot be
scanned for a touch because panel setup switches are configured in a measurement mode; therefore, ETAG = 00 is used if a mea-
surement is not in progress, or ETAG = 11 if a measurement is in progress.
3A separate configuration register for delay time, sampling time, averaging, and ADC resolution settings configures the AUX input.
RTOUCH
X+
Y+
Y-
X-
Z1 PRESSURE MEASUREMENT
ADC
INPUT
PANEL
RTOUCH
VDD
X+
Y+
Y-
X-
Z2 PRESSURE MEASUREMENT
ADC
INPUT
PANEL
VDD
Figure 5. Pressure Measurements
RTOUCH
VDD
X+
Y+
Y-
X-
X POSITION MEASUREMENT
ADC
INPUT
PANEL
RTOUCH
VDD
X+
Y+
Y-
X-
Y POSITION MEASUREMENT
ADC
INPUT
PANEL
Figure 4. Position Measurements