I used to write driver for touch panel and I would like to share my experience on touch panel.
The following is its mechanism.
1. When the touch panel (TP) is tounched, an interrupt is generated. Disable the interrupt for YP, but it depends on hardware design. If the interrupt is generated by falling/raising edge, it may be not neccessary. If it is generated by low/high, we need to disable the interrupt.
2. The driver has to control the IO to provide a voltage across the Y (can be X if step 3 is Y)panel and start the AD to measure the voltage. It will need to wait for sometime because the mechanism of AD. It is used for get Y coordinate.
3. The driver has to control the IO to provide a voltage across the X (can be Y if step 2 is X) panel and start the AD to measure the voltage. Again,it will need to wait for sometime because the mechanism of AD.It is used for get X coordinate.
4. The step 2 and 3 may be repeated a couple times for taking average. (It is because hardware may not be reliable.)
5. Set the I/O to check if the TP is pressed.
6. If the TP is pressed, the driver has to wait a default period and repeat checking again to generate PEN_MOVE message.
7. If the TP is released, set the interrupt works again.
Factors which eats CPU cycles.
1. If timer is avialable in step 2 and 3, it will save your CPU cycle because the driver designer may use interrupt for checking the reading of AD.
2. If the speed of AD is fast, the driver don't need to spend much time on getting reading.
3. Some driver designer, however, simple use polling to checking the reading. Then your CPU cycle will be wasted.
4. If the TP is pressed, the frequency of checking it by driver is also important.