Cirrus Logic EP93xx - Manual (Page 753)

DS785UM1 25-15
Copyright 2007 Cirrus Logic
Analog Touch Screen Interface
EP93xx User’s Guide
2
5
2
5
25
25.2.5 Measuring Touch Screen Resistance
The analog switch array can be configured to get an approximation of touch screen
resistance. This may be useful for advanced touch screen algorithms that either try to
determine how hard a touch screen is being pressed, or if it is being pressed in more than
one location. The touch screen controller built-in algorithm does not use this feature. More
advanced algorithms would need to perform either primary or additional scanning through the
APB interface. Please note that the initial A / D converter read should be viewed as a convert
command where the data provided is stale and should be discarded.
The register values for TSDirect can be derived from the switch positions in the diagram. A
“1” in the register bit position indicates that the switch is made or closed. Therefore, the
TSDirect value for 4- or 8-wire X-axis resistance measurement should be set to
0x0040_1601, the TSDirect value for 4- or 8-wire Y-axis resistance measurement should be
set to 0x0080_4604, and the TSDirect value for 5-wire resistance measurement should be
set to 0x0080_4604. See
Figure 25-9 .
Figure 25-9. Measure Resistance Switching Diagram
4/8 wire Measure X-AXIS
Resistance
4/8 wire Measure Y-AXIS
Resistance
X+
X-
Y+
Y-
SW12
SW14
SW13
SW20
A/D CONVERTER
IN
REF+
REF-
VDD
SW21
SW19
SW11
SW0
SW1
SW2
SW3
SW24
SW25
SW26
SW27
100K SW23
SW22 100K
SW9
SW10
SW6
SW7
SW4
SW5
SW15
SW16
SW17
SW18
X+
X-
Y+
Y-
SW12
SW14
SW13
SW20
A/D CONVERTER
IN
REF+
REF-
VDD
SW21
SW19
SW11
SW0
SW1
SW2
SW3
SW24
SW25
SW26
SW27
100K SW23
SW22 100K
SW9
SW10
SW6
SW7
SW4
SW5
SW15
SW16
SW17
SW18
Wiper
Not used
Not used
Not used
SW12
SW14
SW13
SW20
A/D CONVERTER
IN
REF+
REF-
VDD
SW21
SW19
SW11
SW0
SW1
SW2
SW3
SW24
SW25
SW26
SW27
100K SW23
SW22 100K
SW9
SW10
SW6
SW7
SW4
SW5
SW15
SW16
SW17
SW18
V+
V-
Z+/-
Z-/+
5 wire Measure Corner to
Corner Resistance