Example of using software-defined touch buttons on the STM32F042.

tensigral_lamp touch.c

#include "touch.h"
#include "systems.h"

uint8_t touch_calib[3];
uint8_t done_startup;

//DO_PD means we use the internal pull-down resistors to measure the capacitance
//if we define this to be 0, then it will use the internal pull-up resistors. 
//I am still experimenting with both options to see which is better overall and
//in a lot of situations.

#define DO_PD 1


//The methodology is we are trying to measure the capacitance of a pad.  This is
//done by driving the pad high, then, releasing the pad.  If the capacitance is
//low, then, the pad's voltage will very quickly drop.  If the capacitance is high
//then the pad's voltage will drop much more slowly.  If your finger is not present
//then the voltage will cross the 0 threshhold very quicky.  If your finger is present
//it will take much longer, sometimes as long as a microsecond!
//
//          v Stop driving pin high
// V  ------
// O        |
// L         \
// T          \
// A            \
// G               \
// E  -------------------------------  Threshhold for measuring zero
//                         \
//                                  \
//           ^ ------- ^ The time we're measuring

void init_touch()
{
	//This demo shows the use of 3 touch buttons, but really, an unlimited number
	//of touch buttons can be used using this method as long as they're all on the
	//same IO section.  I.e. This demo uses Port A.

	//I use "systems.c" of which an example can be found in the ColorChord Embedded
	//codebase.  The rest of this specific project will be made public soon.  But
	//you can use the HAL if you absolutely need to.  This is just to turn on the 
	//port's AHB, and enable the IO as an IO push-pull port will pull-down or pull
	//up resistors.
#if DO_PD
	ConfigureGPIO( 0x00, INOUT_IN | PUPD_DOWN | DEFAULT_ON );	//T0
	ConfigureGPIO( 0x01, INOUT_IN | PUPD_DOWN | DEFAULT_ON );	//T1
	ConfigureGPIO( 0x03, INOUT_IN | PUPD_DOWN | DEFAULT_ON );	//T2
#else
	ConfigureGPIO( 0x00, INOUT_IN | PUPD_UP | DEFAULT_OFF );	//T0
	ConfigureGPIO( 0x01, INOUT_IN | PUPD_UP | DEFAULT_OFF );	//T1
	ConfigureGPIO( 0x03, INOUT_IN | PUPD_UP | DEFAULT_OFF );	//T2
#endif

	//We run the algorithm twice.  The very first time, something is always
	//askew with the pads, not sure why, but the second run is 100%
	run_touch( touch_calib );
	run_touch( touch_calib );
	done_startup = 1;
}


void run_touch( uint8_t * counts )
{
	counts[0] = counts[1] = counts[2] = 0;

	//For this tiny period of time, we disable interrupts because timing matters.
	__disable_irq();

	//We stop driving the pin here, and let it float down.
	GPIOOf(0)->MODER = (GPIOOf(0)->MODER & ~(0x000cf));
	uint32_t idrcheck[32];
	uint32_t *idrcheckmark = idrcheck;
	int i;

	//We read the value of IDR for the port.  This will make it so
	//we can read every IO on the port at exactly the same time. This
	//makes it very convenient to synchronously read many touch sensors.

	//You can think of this a little bit like running a logic analyzer
	//for a very short period of time over every pin on a given port.

	//TODO: Switch to fixed-indexed arrays.  It gives you better res
	// if you don't have your compiler optimizations cranked.
	// https://godbolt.org/z/z2xA8t
	*(idrcheckmark++) = GPIOOf(0)->IDR;
	*(idrcheckmark++) = GPIOOf(0)->IDR;
	*(idrcheckmark++) = GPIOOf(0)->IDR;
	*(idrcheckmark++) = GPIOOf(0)->IDR;
	*(idrcheckmark++) = GPIOOf(0)->IDR;
	*(idrcheckmark++) = GPIOOf(0)->IDR;
	*(idrcheckmark++) = GPIOOf(0)->IDR;
	*(idrcheckmark++) = GPIOOf(0)->IDR;
	*(idrcheckmark++) = GPIOOf(0)->IDR;
	*(idrcheckmark++) = GPIOOf(0)->IDR;
	*(idrcheckmark++) = GPIOOf(0)->IDR;
	*(idrcheckmark++) = GPIOOf(0)->IDR;
	*(idrcheckmark++) = GPIOOf(0)->IDR;
	*(idrcheckmark++) = GPIOOf(0)->IDR;
	*(idrcheckmark++) = GPIOOf(0)->IDR;
	*(idrcheckmark++) = GPIOOf(0)->IDR;
	*(idrcheckmark++) = GPIOOf(0)->IDR;
	*(idrcheckmark++) = GPIOOf(0)->IDR;
	*(idrcheckmark++) = GPIOOf(0)->IDR;
	*(idrcheckmark++) = GPIOOf(0)->IDR;
	*(idrcheckmark++) = GPIOOf(0)->IDR;
	*(idrcheckmark++) = GPIOOf(0)->IDR;
	*(idrcheckmark++) = GPIOOf(0)->IDR;
	*(idrcheckmark++) = GPIOOf(0)->IDR;
	*(idrcheckmark++) = GPIOOf(0)->IDR;
	*(idrcheckmark++) = GPIOOf(0)->IDR;
	*(idrcheckmark++) = GPIOOf(0)->IDR;
	*(idrcheckmark++) = GPIOOf(0)->IDR;
	*(idrcheckmark++) = GPIOOf(0)->IDR;
	*(idrcheckmark++) = GPIOOf(0)->IDR;
	*(idrcheckmark++) = GPIOOf(0)->IDR;
	*(idrcheckmark++) = GPIOOf(0)->IDR;

	//Once we've spent about 2 microseconds or so reading the ports, we
	//then go back to driving the ports high.
	GPIOOf(0)->MODER = (GPIOOf(0)->MODER | (0x00045));

	//Because we're messing with MODER here, we probably don't want to let
	//an interrupt fire before we set it back, otherwise we might corrupt
	//it's value.
	__enable_irq();
	
  
	//Now, that we have our data of what the 
	counts[0] = counts[1] = counts[2] = 0;
	for( i = 0; i < 32; i++ )
	{
#if DO_PD
    //The &1 selects PinA.0, &2 selects PinA.1 and &8 selects PinA.3
		if( ( idrcheck[i] & 1 ) ) counts[0]++;
		if( ( idrcheck[i] & 2 ) ) counts[1]++;
		if( ( idrcheck[i] & 8 ) ) counts[2]++;
#else
		if( !( idrcheck[i] & 1 ) ) counts[0]++;
		if( !( idrcheck[i] & 2 ) ) counts[1]++;
		if( !( idrcheck[i] & 8 ) ) counts[2]++;
#endif
	}
  
	//Potentially write calibration data, and/or subtract out
	//our calibration data as being our reference "zero" pressure.

	//If the read value every drops very percipitiously, then, it
	//may be because the user had pressed the button at boot.  This
	//will allow it to reset back down.

	//XXX TODO: Don't allow the calibration to reset down super fast
	//because there is theoretically (thouh I could not make it happen)
	//a possibility that ESD could make there be some sort of transient.
	for( i = 0; i < 3; i++ )
	{
		if( done_startup )
		{
			if( counts[i] < touch_calib[i] )
				touch_calib[i] = counts[i];
			counts[i] = counts[i] - touch_calib[i];
		}
		else
		{
			touch_calib[i] = counts[i];
		}
	}
}