AN2641 datasheet(30/37 Pages) STMICROELECTRONICS

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Touchscreen

AN2641

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Doc ID 14027 Rev 1

Touchscreen

7.1

Theory of operation

SPEAr uses a combination of ADC and GPIO for touchscreen operation. By outputting

GPIO high the device-driver reads the X-coordinate from ADC channel number 5 and again

by outputting GPIO low it reads the Y-coordinate from ADC channel number 6. There is no

dedicated touchscreen controller in SPEAr.

The touchscreen driver has a state machine which is called every 100 milliseconds and

reads each ADC channel. The state machine has 3 states and works as shown in Figure 18.

Figure 18.

Touchscreen state machine

The touchscreen driver is probed as an input driver during Linux boot-up. At this time it only

opens the ADC. The State machine starts only after the CLCD is activated (at user level this

is typically done by an FB_ACTIVATE_FORCE call).

Just after Activation of the CLCD the ADC channels are read to get the idle values when

touchscreen is not touched. This gives an estimated threshold value.

Once the state machine is started, it reads the values from ADC channels (by polling in a

100 millisecond loop) and once it gets values above threshold, the touchscreen is assumed

to be touched (or pressed).

Once the touchscreen is pressed, the driver keeps sending coordinates unless the values

from the ADC channels go lower. This event means that the touchscreen is untouched (or

released).

The polling time (currently 100 milliseconds) of the driver is an important parameter

because it determines the touchscreen driver response time.

The touchscreen driver can be selected from menu config as follows:

Device Drivers

→ Input device support → Touchscreens →

[*] Touchscreen for SPEAr based platforms

●

The name of the touchscreen device is '/dev/input/ts' with the major number 13 and

minor number 64.

- PRESS state comes when panel is

touched.

- It remains in the same state while touched

and send event again and again

IDLE

PRESS

RELEASE

- This state comes when finger is

removed from panel

- Just send one event and change

state to IDLE

- Initial default state

- Comes back to IDLE state from

RELEASE after finger is removed

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