Touchscreen Low-Level Driver

You are provided with a low-level driver for accessing the touchscreen. The functions are available as part of the display library:

#include "display.h"

Programming Guide

There are only three functions in the display.h file that are related to the touch-screen:

// Returns whether the touchscreen display is currently being touched.
bool display_isTouched(void);

// Returns the touched x-y coordinate point and the pressure (z).
void display_getTouchedPoint(int16_t *x, int16_t *y, uint8_t *z);

// Throws away all previous touch data.
void display_clearOldTouchData();

To use the touch-screen do the following:

  • You detect a touch by checking the display_isTouched() function.
  • Once you have detected a touch, invoke display_clearOldTouchData() to clear all of the old data out of the touch-controller.
  • The touched location cannot be read instantaneously after the user touches the screen. You must wait at least 50 ms for the Analog to Digital Converters (ADC) settle.
  • After waiting at least 50 ms, invoke display_getTouchedPoint() to determine the coordinates of the touched region.

Return Using Pointers

Since the display_getTouchedPoint() needs to provide three values back to the caller, it cannot simply use the function return value. Instead, the function takes three pointers to variables allocated by the caller, and will populate the values into these variables.

This is a common approach used in C to return multiple values.

Your code will look something like this (the & operator gets the pointer to/address of a variable):

    int16_t x, y;
    uint8_t z;
    display_getTouchedPoint(&x, &y, &z);

Even though you will ignore the z value, you must still provide a valid pointer.


Touch-screen coordinates are in the same space as the LCD coordinates to make the programming a lot easier.