TinyTouchLib

This library provides functions for touch-button sensing on ATtiny microcontrollers with integrated ADC. It implements a charge sharing approach similar to Atmels Qtouch ADC that does not require additonal external components. The library performs automatic bias adjustment to the touch button capacitance and should therefore be immune to drift.

You can find an example app here.

Supported Devices

Device	Support	No Support	Comment
ATtiny 4/9		X	No ADC
ATtiny 5/10	X
ATtiny 13A	X
ATtiny 25/45/85	X
ATtiny 24/44/84		X	may be supported later
ATtiny 261/461/861		X	may be supported later

Library Functions

initialization

void  tinytouch_init(void);

Call this once to initialize the library functions and the ADC converter.

Sensing

uint8_t tinytouch_sense(void);

Possible return values are:
  tt_off=0    No touch sensed
  tt_on       Touch button is active and touch is sensed.
  tt_push     Touch button is pushed. Use this to initiate one time events.
  tt_release  Touch button is released. Use this to initiate one time events.
  tt_timeout  Touch button has been active too long and internal bias was	reset. 

This function performs touch pad sensing and internal housekeeping. It should be called at least 30 times per second. Please note that each call performs 32 analog to digital conversions with active waiting. This may take several ms.

Library Configuration

//////////////////////////////////////////////////////////////////////////
//
// User definable settings
//
//////////////////////////////////////////////////////////////////////////

// Define upper and lower threshold for the touch sensing. You may have to
// change these depending on the geometry of your touch button.
// Setting the "on" value lower will make the touch button more sensitive.
// Setting the "off" value higher will make the touch button less likely
// to be "stuck". Too high values can lead to oscillations.

#define touch_threshold_on 60
#define touch_threshold_off 20

// If the touch button is pressed, the bias value is not updated
// to prevent detection of multiple touches. In some cases this may
// lead to a "stuck button" situation. Therefore an update of the bias
// is forced after a certain time unless this function is deactivated.
//
// The value of timeout corresponds to the number of calls to tinytouch_sense().
// The maximum is 255. 
// Setting timeout to 0 will turn the functionality off.

#define touch_timeout 255
//#define touch_timeout 0  	// turn off timeout functionality


// Define pins to use for the reference input and the touch button
// The reference pin is used to charge or discharge the internal
// sample&hold capacitor. This pin is used in output mode and should
// not be shorted to VCC or GND externally.
// The sense pin is connected to the touch-button. To improve noise immunity
// a series resistor can be used.

// The pin number must match the corresponding analog input number ADCx. 
// Default port is PORTB. (ATtiny 5/10/13/25/45/85)

#define tt_refpin 2		// Use PB2 as reference pin
#define tt_refadc 1		// Use ADC1 as reference ADC input
#define tt_sensepin 3	// Use PB3 as sense pin
#define tt_senseadc 3	// Use ADC3 as sense ADC input

Example

This example implements a touch button on PB3 to toggle a LED connected to PB4.

#include <avr/io.h>
#include <util/delay.h>
#include "tinytouchlib.h"

int main(void)
{
  CLKPR=_BV(CLKPCE);
	CLKPR=0;			// set clock prescaler to 1 (attiny 25/45/85/24/44/84/13/13A)		

	DDRB=_BV(PB4);		// Enable LED output pin
	
	tinytouch_init();
	
	while(1) {
		if (tinytouch_sense()==tt_push) {PORTB^=_BV(PB4);}  // Toggle LED on touch event		
		_delay_ms(10);	
	}	
}