UM0851 datasheet(176/245 Pages) STMICROELECTRONICS

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Human interface device (HID) drivers

UM0851

176/245

Doc ID 16604 Rev 2

Figure 47.

State machine of touchscreen driver

The touchscreen driver is probed as input driver during Linux boot-up. The driver opens the

ADC and GPIO channels, and starts reading values from the ADC channels by polling in a

100 ms loop. The polling interval (currently 100 ms) of driver is important because it

determines the touchscreen response time.

50 initial readings are performed to estimate a threshold. The touchscreen is considered to

be touched (or pressed) only when the values go above the threshold. If the touchscreen is

pressed, the driver keeps sending coordinates until the touchscreen is untouched (or

released).

Touchscreen calibration

A CLCD panel touchscreen cannot initially return absolutely correct coordinates, it returns

values which need to be adjusted according to the CLCD panel dimensions. For this reason,

calibration is required to get the correct coordinates. The driver sends the raw (un-

calibrated) coordinates. The application layer is responsible for performing the calibration

which can be done using the 'tslib' standard utility 'ts_calibrate'.

Touchscreen driver interface to input layer

In the _init function, which is called either when the module is loaded or when booting the

kernel, the touchscreen driver grabs the required resources. Then it allocates a new input

device structure with input_allocate_device () and sets up input bitfields. In this way the

device driver tells the other parts of the input systems what it is and what events can be

generated or accepted by this input device.

The touchscreen driver enables the Touch and X-Y coordinate information to send to the

user application via the input device.

input_dev->evbit[0] = BIT_MASK(EV_KEY) | BIT_MASK(EV_ABS);

input_dev->keybit[BIT_WORD(BTN_TOUCH)] = BIT_MASK(BTN_TOUCH);

Then the driver registers the input device structure by calling input_register_device ().

struct input_dev

*input;

input_register_device( input );

Registration of the device adds the 'input' structure to the linked lists of the input driver and

calls the modules _connect functions in the device handler to tell them that a new input

IDLE

RELEASE

Initial default state

Returns to IDLE state from RELEASE state

after finger is removed

RELEASE state is entered when

finger is removed from panel.

Sends an event and changes

state to IDLE

PRESS

PRESS state is entered

when panel is

touched.

Remains in same

state when

touched and sends event

again and again

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