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LPC4088DM Using Touch

Table of Contents

  1. Using Touch
  2. Multitouch
  3. Calibration

All display modules have displays with touch controllers. The touch controllers could be either resistive or capacitive depending on module.

The DMSupport library handles both capacitive and resistive touch controllers with the same interface:

Import library
DMSupport - TouchPanel Class Reference

Public Member Functions

virtual TouchError read (touch_coordinate_t &coord)=0
Read coordinates from the touch panel.
virtual TouchError read (touch_coordinate_t *coord, int num)=0
Read up to num coordinates from the touch panel.
virtual TouchError info (bool *resistive, int *maxPoints, bool *calibrated)=0
Returns information about the touch panel.
virtual TouchError calibrateStart ()=0
Start to calibrate the display.
virtual TouchError getNextCalibratePoint (uint16_t *x, uint16_t *y, bool *last=NULL)=0
Get the next calibration point.
virtual TouchError waitForCalibratePoint (bool *morePoints, uint32_t timeout)=0
Wait for a calibration point to have been pressed and recored.

Using Touch

To use touch it must first be enabled in the dm_board_config.h file:

#define DM_BOARD_USE_TOUCH


The initialization of the touch controller is done in the init() function in DMBoard.

The interface can be used directly by repeatedly calling the read function (i.e. polling), but the preferred way is to listen for changes like this:

// A selected event flag. Can be any that the sender and receiver agrees upon
#define MY_TOUCH_FLAG  (1<<0)
EventFlags event_flags;

static void onTouchEvent() {
  event_flags.set(MY_TOUCH_FLAG);
}

void doStuff() {
  TouchPanel* touch = DMBoard::instance().touchPanel();
  touch->setListener(callback(onTouchEvent));

  // wait for touch events
  touch_coordinate_t coord;
  while (true) {
    event_flags.wait_any(MY_TOUCH_FLAG);
    if (touch->read(coord) != TouchPanel::TouchError_Ok) {
      // failed to read the coordinates
      continue;
    }
    
    // use coord.x, coord.y, coord.z
  }
}

Multitouch

Capacitive touch controllers normally supports multiple fingers at the same time. To test how many fingers are supported by your controller call the info() function:

TouchPanel* touch = DMBoard::instance().touchPanel();
bool isResistive;
int maxFingers;
bool canCalibrate;
if (touch->info(&isResistive, &maxFingers, &canCalibrate) == TouchPanel::TouchError_Ok) {
  DMBoard::instance().logger()->printf("The touch controller supports up to %d fingers\n", maxFingers);
}


Reading multiple fingers at the same time requires only small changes to the program. 

// A selected signal ID. Can be any 2^x that the sender and receiver agrees upon
#define MY_TOUCH_FLAG  (1<<0)
EventFlags event_flags;

static void onTouchEvent() {
  event_flags.set(MY_TOUCH_FLAG);
}

void doStuff() {

  TouchPanel* touch = DMBoard::instance().touchPanel();
  touch->setListener(callback(onTouchEvent));

  // wait for touch events
  bool dontCare;
  int fingers;
  if (touch->info(&dontcare, &fingers, &dontCare) != TouchPanel::TouchError_Ok) {
    // something is wrong
  }  
  if (fingers > 5) {
    fingers = 5;
  }
  touch_coordinate_t coords[5];
  while (true) {
    event_flags.wait_any(MY_TOUCH_FLAG);
    if (touch->read(coords, fingers) != TouchPanel::TouchError_Ok) {
      // failed to read the coordinates
      continue;
    }
    
    // use coord[0].x, coord[0].y, coord[0].z, coord[1].x, ...
  }
}


For controllers that doesn't support multitouch the number of fingers will be 1, so the same setup as for multitouch can be used. The opposite is also true, a multitouch controller can be called with the read(coord) function to only get the coordinate of the first finger.

Calibration

Some touch controllers can be calibrated for a better result. Use the info() function as shown above to determine if the touch controller supports calibration. The procedure of calibrating consists of:

  1. Call TouchPanel::calibrateStart()
  2. Call TouchPanel::getNextCalibratePoint() to get X and Y coordinates
  3. Draw a marker on the display at X, Y
  4. Call TouchPanel::waitForCalibratePoint() to block until the user has clicked at X, Y
  5. While the #morePoints# parameter to waitForCalibratePoint() is true, repeat from step 2 above

This procedure is implemented in the AppTouchCalibration app which is part of the DMBasicGUI library.

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