Cirrus Logic EP93 Series User Manual page 742

Analog Touch Screen Interface
EP93xx User's Guide
25
25.2.1 Touch Screen Scanning: Four-wire and Eight-wire Operation
Figure 25-2
TSDetect, TSDischarge, TSXSample, and TSYSample are derived from the switch positions
in the diagram. These values are listed in
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
input on a touch screen. The ARM Core must load all of the setup registers for the touch
25-4
Table 25-1. Switch Definitions and Logical Safeguards to Prevent Physical Damage (Continued)
Bit 18 sY-
Bit 19 X+
Bit 20 Y+
Bit 21 Y-
Bit 22 X+
Bit 23 Y+
Bit 24 sX+
Bit 25 sX-
Bit 26 sY+
Bit 27 sY-
TOUCH_PRES
Bit 28
S gate
Bit 29 DAC
Bit 30 VBAT ADC input
shows control for an 8-wire analog resistive touch screen. The register values
Figure 25-4
AGND
AVDD SW19 = HIGH and SW11 = LOW and SW0 = LOW
AVDD SW20 = HIGH and SW13 = LOW and SW2 = LOW
AVDD SW21 = HIGH and SW14 = LOW and SW3 = LOW
Pull-up to AVDD
(SW0 = LOW or the bus SW[7-0] = 0x01)
Pull-up to AVDD
(SW2 = LOW or the bus SW[7-0] = 0x04)
ADC REF+
ADC REF-
ADC REF+
ADC REF-
Touch_Press
NAND gate
ADC input
Load Resistor Also gates PRSTn for measuring battery with load
Table 25-2.
demonstrates the sample process used for determining touch
Copyright 2007 Cirrus Logic
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SW22 = HIGH and SW11 = LOW and
SW23 = HIGH and SW13 = LOW and
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-
-
Gates input to prevent oscillations
DAC to ADC feedback test switch
DS785UM1