Cirrus Logic EP93xx - Manual (Page 739)

DS785UM1 25-1
Copyright 2007 Cirrus Logic
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Chapter 25
25 Analog Touch Screen Interface
25.1 Introduction
Note: The EP9301 and EP9302 processors each support a general 5-bit ADC, but no touch
screen .
Note: The EP9307, EP9312, and EP9315 processors each support up to an 8-wire touch
screen or a general 12-bit ADC.
The touch screen controller is a hardware engine that controls scanning for 4, 5, 7, or 8-wire
analog resistive touch screens (TS). The engine performs all sampling, averaging, and range
checking for analog-to-digital converter values. The hardware engine also controls an analog
switch array for controlling the different scan modes.
Through the APB interface, it is possible to disable the touch screen hardware engine and
directly control the switch matrix and the analog-to-digital converter. This allows for the
implementation of more complex software scanning algorithms if desired. However, the
hardware engine provides all features necessary to implement a standard interface and
eliminates almost all ARM Core intervention in the scanning process.
For both X and Y axes, the touch screen controller engine does the following:
• Takes 4, 8, 16, or 32 sample sets.
• Checks to see if the deviation in the sample set is too great.
• Averages the samples.
• Checks the average to see if the move is too great to be realistic.
• Checks to see if the move is large enough to bother the ARM Core.
Many touch screen controllers send a continuous information stream to the processor when
the touch screen is in use. This controller, in contrast, only interrupts the ARM Core when the
stimulus change is significant.
25.2 Touch Screen Controller Operation
To understand the controller operation, it is first important to understand the electrical
implementation of touch screen technologies. The schematics for various resistive touch
screen technologies are shown in
Figure 25-1 . When the front and back indium-tin-oxide
layers are pressed together, a resistive contact is made.