62 lines
2.1 KiB
Plaintext
62 lines
2.1 KiB
Plaintext
README on the ADC/Touchscreen Controller
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========================================
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The LH79524 and LH7A404 include a built-in Analog to Digital
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controller (ADC) that is used to process input from a touchscreen.
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The driver only implements a four-wire touch panel protocol.
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The touchscreen driver is maintenance free except for the pen-down or
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touch threshold. Some resistive displays and board combinations may
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require tuning of this threshold. The driver exposes some of it's
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internal state in the sys filesystem. If the kernel is configured
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with it, CONFIG_SYSFS, and sysfs is mounted at /sys, there will be a
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directory
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/sys/devices/platform/adc-lh7.0
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containing these files.
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-r--r--r-- 1 root root 4096 Jan 1 00:00 samples
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-rw-r--r-- 1 root root 4096 Jan 1 00:00 threshold
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-r--r--r-- 1 root root 4096 Jan 1 00:00 threshold_range
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The threshold is the current touch threshold. It defaults to 750 on
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most targets.
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# cat threshold
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750
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The threshold_range contains the range of valid values for the
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threshold. Values outside of this range will be silently ignored.
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# cat threshold_range
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0 1023
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To change the threshold, write a value to the threshold file.
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# echo 500 > threshold
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# cat threshold
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500
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The samples file contains the most recently sampled values from the
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ADC. There are 12. Below are typical of the last sampled values when
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the pen has been released. The first two and last two samples are for
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detecting whether or not the pen is down. The third through sixth are
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X coordinate samples. The seventh through tenth are Y coordinate
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samples.
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# cat samples
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1023 1023 0 0 0 0 530 529 530 529 1023 1023
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To determine a reasonable threshold, press on the touch panel with an
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appropriate stylus and read the values from samples.
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# cat samples
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1023 676 92 103 101 102 855 919 922 922 1023 679
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The first and eleventh samples are discarded. Thus, the important
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values are the second and twelfth which are used to determine if the
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pen is down. When both are below the threshold, the driver registers
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that the pen is down. When either is above the threshold, it
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registers then pen is up.
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