STM32 Touch Sensors

Only some STM32 processors have touch sensor circuitry built in, most noticeable the STM32Lxx series.

Hardware Example

As part of another project I was using an STM32L432 MCU and while doing that design I decided to add 3 touch sensors.

The circuitry involved in the touch sensing is the C5 and the actual touch pads J2, J3 and J4 and their accompanying resistors.

Layout wise I did two different approaches:

The top and bottom touch pads are just big single sided SMD pads which is covered by soldermask. The center "hole" is a big through hole pad.

The gadget came out like this:

Software

STM32CubeMX Configuration

Messing around with this it turns out that the middle pad (the through hole one) is much to close to the neighbors, so in the software example I only deal with the two pads covered by soldermask.

Under Middleware/Touchsensing:

Code

The touchsensing middleware will do all the heavy lifting and only a few lines of code is needed to use the touch sensors.

First of all, in the main program loop:

  /* Infinite loop */
  /* USER CODE BEGIN WHILE */

	uint32_t then = 0, now = 0;

	for (;;) {

		now = HAL_GetTick();
		if (now % 50 == 0 && now != then) {

			tsl_user_Exec_IT();

			if (now % 1000 == 0) {

				printf("Tick %5lu B0: %2d / %2d B1: %2d / %2d\n", now / 1000,
						MyTKeys[0].p_Data->StateId, MyTKeys[0].p_ChD->Meas,
						MyTKeys[1].p_Data->StateId, MyTKeys[1].p_ChD->Meas);
			}

			then = now;
		}

    /* USER CODE END WHILE */

    /* USER CODE BEGIN 3 */

	}
  /* USER CODE END 3 */
}

Essentially this will call the "tsl_user_Exec_IT()" function every 50 ms (20 times per second) and each second it will print out the status of the keys.

The interrupt handler callback is defined in tsl_user.c

void HAL_TSC_ConvCpltCallback(TSC_HandleTypeDef* htsc)
{
/* USER CODE BEGIN HAL_TSC_ConvCpltCallback start*/

/* USER CODE END HAL_TSC_ConvCpltCallback start*/
  TSL_acq_BankGetResult(idx_bank_it, 0, 0);
  idx_bank_it++;
  if (idx_bank_it > TSLPRM_TOTAL_BANKS-1)
  {
    // End of all banks acquisition, restart bank 0 in while(1) loop (This is a choice)
    idx_bank_it=0;
    acq_done_it++;
  }else{
    // We restart next bank acquisition
    TSL_acq_BankConfig(idx_bank_it);
    TSL_acq_BankStartAcq_IT();
  }
/* USER CODE BEGIN HAL_TSC_ConvCpltCallback*/
  handle_touch();
/* USER CODE END HAL_TSC_ConvCpltCallback*/
}

All I do there is to call "handle_touch()" at the conclusion of each conversion.

The handle_touch() is defined like:

// Called after touch keys have been measured
void handle_touch() {
	uint32_t t = HAL_GetTick();
	for (int i = 0; i < 2; i++) {
		TSL_StateId_enum_T state = MyTKeys[i].p_Data->StateId;
		if (state != touch_state[i] && (state == TSL_STATEID_RELEASE || state == TSL_STATEID_DETECT)) {
			printf("At tick %d Key %d changed - now = %d\n", t, i, MyTKeys[i].p_Data->StateId);
			touch_state[i] = MyTKeys[i].p_Data->StateId;
		}
	}
}

Touching each of the touchpads result in this output:

Resources

• Getting started with touch sensing control on STM32 microcontrollers
• Developing applications on STM32Cube with STMTouch touch sensing library
• Tuning a touch sensing application on MCUs
• Design with surface sensors for touch sensing applications on MCUs
• Improve conducted noise robustness for touch sensing applications on MCUs
• Sampling capacitor selection guide for touch sensing applications on MCUs
• ESD considerations for touch sensing applications on MCUs

• Hardware Design for Capacitive Touch