| Marc Singer | 51cb21a | 2006-05-16 11:41:43 +0100 | [diff] [blame] | 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 it's |
| 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. |