Based on kernel version 2.6.38. Page generated on 2011-03-22 22:18 EST.
1 README on the ADC/Touchscreen Controller 2 ======================================== 3 4 The LH79524 and LH7A404 include a built-in Analog to Digital 5 controller (ADC) that is used to process input from a touchscreen. 6 The driver only implements a four-wire touch panel protocol. 7 8 The touchscreen driver is maintenance free except for the pen-down or 9 touch threshold. Some resistive displays and board combinations may 10 require tuning of this threshold. The driver exposes some of its 11 internal state in the sys filesystem. If the kernel is configured 12 with it, CONFIG_SYSFS, and sysfs is mounted at /sys, there will be a 13 directory 14 15 /sys/devices/platform/adc-lh7.0 16 17 containing these files. 18 19 -r--r--r-- 1 root root 4096 Jan 1 00:00 samples 20 -rw-r--r-- 1 root root 4096 Jan 1 00:00 threshold 21 -r--r--r-- 1 root root 4096 Jan 1 00:00 threshold_range 22 23 The threshold is the current touch threshold. It defaults to 750 on 24 most targets. 25 26 # cat threshold 27 750 28 29 The threshold_range contains the range of valid values for the 30 threshold. Values outside of this range will be silently ignored. 31 32 # cat threshold_range 33 0 1023 34 35 To change the threshold, write a value to the threshold file. 36 37 # echo 500 > threshold 38 # cat threshold 39 500 40 41 The samples file contains the most recently sampled values from the 42 ADC. There are 12. Below are typical of the last sampled values when 43 the pen has been released. The first two and last two samples are for 44 detecting whether or not the pen is down. The third through sixth are 45 X coordinate samples. The seventh through tenth are Y coordinate 46 samples. 47 48 # cat samples 49 1023 1023 0 0 0 0 530 529 530 529 1023 1023 50 51 To determine a reasonable threshold, press on the touch panel with an 52 appropriate stylus and read the values from samples. 53 54 # cat samples 55 1023 676 92 103 101 102 855 919 922 922 1023 679 56 57 The first and eleventh samples are discarded. Thus, the important 58 values are the second and twelfth which are used to determine if the 59 pen is down. When both are below the threshold, the driver registers 60 that the pen is down. When either is above the threshold, it 61 registers then pen is up.