AN2641

Touchscreen

Doc ID 14027 Rev 1

31/37

7.2

Linux input layer

The kernel's input subsystem layer was created to unify scattered drivers that handle

diverse classes of data-input devices such as keyboards, mice, and touch screens.

The kernel's input subsystem layer permits the uniform handling of functionally similar input

devices even when they are physically different. For example, all mice, such as PS/2, USB

or Bluetooth, are treated alike.

It provides an easy event interface for dispatching input reports to user applications. The

driver does not have to create and manage /dev nodes and related access methods.

Instead, it can simply invoke input APIs to send mouse movements, key presses, or touch

events upstream to the user layer. Applications such as X Windows work seamlessly over

the event interfaces exported by the input subsystem layer.

The name of the touchscreen driver is "drivers/input/touchscreen/spear_ts.c" (or

"drivers/input/touchscreen/spr_ts_st.c"). To integrate with the input subsystem layer, the

driver fills the structure 'struct input_dev *input_dev' and register through the call

input_register_device( ).

The sequence of events sent to the application layer is as illustrated in Figure 19:

Figure 19.

Sequence of events from touchscreen driver

7.3

Calibration process

Any CLCD panel touchscreen cannot return absolutely correct coordinates, rather it only

returns default values which need to be adjusted to the CLCD panel dimensions. This is the

purpose of a calibration tool.

The calibration tool works with a simple concept, it asks the user to touch four corner points

of the CLCD panel and stores these values. It then learns the offsets and scales used to

later calculate the correct coordinates. Calibration may vary for different CLCD panels so

before using the touchscreen, the CLCD panel has to be calibrated.

Once calibration is successfully performed, the calibration data is written in a file in a

particular format. It is up to the user application to use the information provided in the file to

calculate correct coordinates.

Figure 20 shows the calibration tool display after the calibration process is over.

PRESS EVENT (1)

X coordinate

Y coordinate

SYN

PRESS EVENT (1)

X coordinate

Y coordinate

SYN

(Press events may go multiple times )

.

.

.

RELEASE EVENT(0)

X coordinate

Y coordinate

SYN

(Release event will go only once)