Cirrus Logic EP93xx - Manual (Page 742)

25-4 DS785UM1
Copyright 2007 Cirrus Logic
Analog Touch Screen Interface
EP93xx User’s Guide
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25.2.1 Touch Screen Scanning: Four-wire and Eight-wire Operation
Figure 25-2 shows control for an 8-wire analog resistive touch screen. The register values
TSDetect, TSDischarge, TSXSample, and TSYSample are derived from the switch positions
in the diagram. These values are listed in
Table 25-2 .
The left most part of the diagram shows how the switches are driven to detect a touch press.
This configuration is controlled by the TSDetect register.
The second part of the diagram shows all lines of the touch screen switched to ground for
discharging as controlled by the TSDischarge register.
The third example shows a voltage driven across the X-axis with the A / D sample input
connected to both Y-axis lines and the A / D referenced to the SX feedback lines as
controlled by the TSXSample register.
The right most part of the diagram shows a voltage driven across the Y-axis with the A / D
sample input connected to both X-axis lines and the A / D referenced to the SY feedback
lines for digitization as controlled by the TSYSample register.
Each time the analog switching configuration is changed, an appropriate time interval must
be provided for the circuit to stabilize due to the touch screen capacitance, and any EMI
filtering provided on the signals. This is controlled by the DLY field in the TSSetup register.
The analog touch screen interface circuitry is internally connected to a signed 12-bit analog-
to-digital converter. The 12-bit digital result is held stable until another sample is requested.
The controller reads the digital value by issuing a read pulse. This read pulse signal from the
controller is also used as a convert command for the analog-to-digital converter to begin
conversion of its next sample. The output of the ADC is a signed value. For the comparison
functions of the touch controller to work correctly, the TSSetup2.SIGND bit should be set.
The flow chart in Figure 25-4 demonstrates the sample process used for determining touch
input on a touch screen. The ARM Core must load all of the setup registers for the touch
Bit 18 sY- AGND -
Bit 19 X+ AVDD SW19 = HIGH and SW11 = LOW and SW0 = LOW
Bit 20 Y+ AVDD SW20 = HIGH and SW13 = LOW and SW2 = LOW
Bit 21 Y- AVDD SW21 = HIGH and SW14 = LOW and SW3 = LOW
Bit 22 X+ Pull-up to AVDD
SW22 = HIGH and SW11 = LOW and
(SW0 = LOW or the bus SW[7-0] = 0x01)
Bit 23 Y+ Pull-up to AVDD
SW23 = HIGH and SW13 = LOW and
(SW2 = LOW or the bus SW[7-0] = 0x04)
Bit 24 sX+ ADC REF+ -
Bit 25 sX- ADC REF- -
Bit 26 sY+ ADC REF+ -
Bit 27 sY- ADC REF- -
Bit 28
TOUCH_PRES
S gate
Touch_Press
NAND gate
Gates input to prevent oscillations
Bit 29 DAC ADC input DAC to ADC feedback test switch
Bit 30 VBAT ADC input Load Resistor Also gates PRSTn for measuring battery with load
Table 25-1. Switch Definitions and Logical Safeguards to Prevent Physical Damage (Continued)