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Code Examples – Debounce a mechanical button (or switch) – made easy...

Debounce can sometimes be a difficult task to get your head around. I am going to make this task a bit easier for you – I have provided 3 code examples below.

All 3 examples use a simple serial sampling technique. This is a very robust, compact and fast approach.

It has 2 parts – A Timer Interrupt, and a Foreground Task (usually in main()). These 2 parts are connected via a global volatile boolean as shown in the examples.

The Timer is usually in the range of 1 to 10mS, but you can tailor it to your specific requirements. IMPORTANT, the sample variable (static uint button_samples) needs to sized to suit the number of samples – 100mS debounce with a 1mS timer will require at least 100 bits… It is usually best to use a Timer interval so the number of samples fits into your CPU’s native integer size – 8, 16 or 32 bit.

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Example 1 is just a simple debounce process with a button_down flag. --Show code-- Copy code

        
/*Example 1 - design spec:-
	Button down = pin high.
	Minimum debounce (for both press and release) = 60mS.
	Using a 5mS timer interrupt.

	Fixed sampling timers, like the one below, have an iteration tolerance of +0, -1. So,
	if your debounce time needs to be no less than 60mS, you will need to add 1 to the iteration count.
	A 5mS timer makes it 13 (60-65mS debounce), a 10mS timer makes it 7 (30-40mS debounce)...
*/

volatile bool button_down = false;

#define DEBOUNCEDNMASK 0b0001111111111111     /// = 13 bit mask = 2^((60 \ 5) + 1) - 1
#define DEBOUNCEUPMASK DEBOUNCEDNMASK

void timer_5mS_interrupt(void) {

	static uint button_samples = 0;

	button_samples <<= 1;
	if ((my_button_port->input_data_register & my_button_bit) != 0) {      // for active low button - change !=0 to ==0
		button_samples++;
	}

	if ((button_samples & DEBOUNCEUPMASK) == 0) {
		button_down = false;
	}
	else if ((button_samples & DEBOUNCEDNMASK) == DEBOUNCEDNMASK) {
		button_down = true;
	}
}

------ meanwhile, back at main.c -------

extern volatile bool button_down;

main() {
	...
	if (button_down) {
		... handle button down
	}
	...
}
      

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Example 2 uses a different Timer and Debounce than Example 1. It has a button_pressed flag which must be acknowledged in the Foreground task. --Show code-- Copy code

        
/*Example 2 - design spec:-
	This is similar to Example 1, with a transition state as 'button_press'
	Button down = pin high.
	Minimum debounce (for both press and release) = 30mS.
	Using a 5mS timer interrupt.

	Fixed sampling timers, like the one below, have an iteration tolerance of +0, -1. So,
	if your debounce time needs to be no less than 30mS, you will need to add 1 to the iteration count.
	A 5mS timer makes it 7 (30-35mS debounce).
*/

volatile bool button_pressed = false;

#define DEBOUNCEDNMASK 0b01111111     /// = 7 bit mask = 2^((30 \ 5) + 1) - 1
#define DEBOUNCEUPMASK DEBOUNCEDNMASK

void timer_5mS_interrupt(void) {

	static uint button_samples = 0;
	static bool button_down = false;

	button_samples <<= 1;
	if ((my_button_port->input_data_register & my_button_bit) != 0) {      // for active low button - change !=0 to ==0
		button_samples++;
	}

	if ((button_samples & DEBOUNCEUPMASK) == 0) {
		button_down = false;
	}
	else if ((button_samples & DEBOUNCEDNMASK) == DEBOUNCEDNMASK) {
		if (!button_down) {
			button_down = true;
			button_pressed = true;
		}
	}
}

------ meanwhile, back at main.c -------

extern volatile bool button_pressed;

main() {
	...
	if (button_pressed) {
		button_pressed = false;   /// acknowledge button_pressed
		... handle button press
	}
	...
}
      

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Example 3 extends Example 2 with key-repeat function. --Show code-- Copy code

        
/*Example 3 design spec:-
	Using Example 2 above
	Following the debounce, if the button is held down for > 1 second, it repeats 5 times per second.
*/

volatile bool button_pressed = false;

#define DEBOUNCEDNMASK 0b01111111         /// = 7 bit mask = 2^((30 \ 5) + 1) - 1
#define DEBOUNCEUPMASK DEBOUNCEDNMASK
#define INITBUTTONRPT 200               /// initial button press threshold (1 second / 5mS = 200)
#define BUTTONRPT INITBUTTONRPT-40      /// repeat interval (1 second / 5 repeats = 200mS, 200mS / 5mS = 40)

void timer_5mS_interrupt(void) {

	static uint button_samples = 0;
	static bool button_down = false;
	static int button_repeat_cnt = 0;

	button_samples <<= 1;
	if ((my_button_port->input_data_register & my_button_bit) != 0) {      // for active low button - change !=0 to ==0
		button_samples++;
	}

	if ((button_samples & DEBOUNCEUPMASK) == 0) {
        button_down = false;
	}
	else if ((button_samples & DEBOUNCEDNMASK) == DEBOUNCEDNMASK) {
		if (!button_down) {
			button_down = true;
			button_pressed = true;
			button_repeat_cnt = 0;
		}
		else {
			if (++button_repeat_cnt >= INITBUTTONRPT) {
				button_pressed = true;
				button_repeat_cnt = BUTTONRPT;
			}
		}
	}
}

------ meanwhile, back at main.c -------

extern volatile bool button_pressed;

main() {

	...
	if (button_pressed) {
		button_pressed = false;   /// acknowledge button_pressed
		... handle button press
	}
	...
}
      

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Please feel free to contact me if you have any questions.