homepage/docroot/posts/2018-04-30.01/article.pag

<!-- { "title": "Yet Another Debouncing Method" } -->

<h1>#title#</h1>
<p>
You can find several approaches for debouncing mechanical switches on the Internet, some work better, some not so good.
</p>

<p>
One common approach is to ignore events in an ISR when they come too fast:<br/>

<pre><code class="C">
void count() {
	static uint32_t lastEvent = 0;
	uint32_t currentEvent = micros();
	if (currentEvent &gt; (lastEvent + configBlock.debounce)) {
		lastEvent = currentEvent;
		cnt++;
	}
}

void setup() {
        pinMode(REED_PIN, INPUT_PULLUP);
        attachInterrupt(REED_PIN, count, FALLING);
}
</code>
</pre>

This works very good when only the tipping of a switch is relevant.
</p>

<p>
When also the time the button was pressed is relevant and when it is especially necessary to distinguish between a short and a long press this approach doesn't work anymore.<br/>

Since I couldn't remember the approaches I read about earlier I've sketched this state machine:<br/>

<img src="files/20180430110848869_0001.jpg" width="500"/>

<br/>
(The double-lined states are action-states which send out the related information.)<br>

At least for me, this approach is working very reliable so far, I'm quite happy with it.<br>

<pre><code class="C">
enum tPressedState { psHIGH, psLOW, psACCEPTED_LOW, psLONG_START, psLONG_CONT, psLONG_CONT_SEND, psLONG_END, psSHORT, psINVALID };

typedef struct {
	uint8_t index;
	uint8_t buttonPin;
	tPressedState pressedState;
	tPressedState oldPressedState;
	uint32_t lastStateChange;
} tButton;

tButton buttons[] = {
					  { 1, SWITCH_1, psHIGH, psINVALID, 0 },
					  { 2, SWITCH_2, psHIGH, psINVALID, 0 },
					  { 3, SWITCH_3, psHIGH, psINVALID, 0 },
					  { 0, 0, psINVALID, psINVALID, 0 } // END MARKER
};

static void buttonHandler(tButton *button) {
	uint32_t currentMicros = micros();
	uint8_t buttonState = digitalRead(button-&gt;buttonPin);

#ifdef DEBUG
	if (button-&gt;oldPressedState != button-&gt;pressedState) {
		Serial.print("Index ");
		Serial.print(button-&gt;index);
		Serial.print(", state changed from ");
		Serial.print(button-&gt;oldPressedState);
		Serial.print(" to ");
		Serial.print(button-&gt;pressedState);
		Serial.println();
		button-&gt;oldPressedState = button-&gt;pressedState;
	}
#endif

	switch (button-&gt;pressedState) {
	case psHIGH:
		if (buttonState == LOW) {
			button-&gt;pressedState = psLOW;
			button-&gt;lastStateChange = currentMicros;
		}
		break;
	case psLOW:
		if (buttonState == HIGH) {
			button-&gt;pressedState = psHIGH;
			button-&gt;lastStateChange = currentMicros;
		} else {
			if (currentMicros &gt; (button-&gt;lastStateChange + configBlock.debounce)) {
				button-&gt;pressedState = psACCEPTED_LOW;
				button-&gt;lastStateChange = currentMicros;
			}
		}
		break;
	case psACCEPTED_LOW:
		if (buttonState == HIGH) {
			button-&gt;pressedState = psSHORT;
			button-&gt;lastStateChange = currentMicros;
		}
		if (currentMicros &gt; (button-&gt;lastStateChange + (configBlock.longPress * 1000))) {
			button-&gt;pressedState = psLONG_START;
			button-&gt;lastStateChange = currentMicros;
		}
		break;
	case psSHORT:
		sendMsg(button-&gt;index, "PRESS_SHORT");
		button-&gt;pressedState = psHIGH;
		button-&gt;lastStateChange = currentMicros;
		break;
	case psLONG_START:
		sendMsg(button-&gt;index, "PRESS_LONG_START");
		button-&gt;pressedState = psLONG_CONT;
		button-&gt;lastStateChange = currentMicros;
		break;
	case psLONG_CONT:
		if (buttonState == HIGH) {
			button-&gt;pressedState = psLONG_END;
			button-&gt;lastStateChange = currentMicros;
		}
		if (currentMicros &gt; (button-&gt;lastStateChange + (configBlock.longPressRepeat * 1000))) {
			button-&gt;pressedState = psLONG_CONT_SEND;
			button-&gt;lastStateChange = currentMicros;
		}
		break;
	case psLONG_CONT_SEND:
		sendMsg(button-&gt;index, "PRESS_LONG_CONT");
		button-&gt;pressedState = psLONG_CONT;
		button-&gt;lastStateChange = currentMicros;
		break;
	case psLONG_END:
		sendMsg(button-&gt;index, "PRESS_LONG_END");
		button-&gt;pressedState = psHIGH;
		button-&gt;lastStateChange = currentMicros;
		break;
	default:
		button-&gt;pressedState = psHIGH;
		button-&gt;lastStateChange = currentMicros;
	}
}
</code>
</pre>
</p>

<p>
Find it embedded in the code of a small ESP8266-based switch thing I'm using in 
my home automation setup (home grown control code (<a href="https://gitlab.com/wolutator/dispatcher_ng">on gitlab</a>), 
<a href="https://homegear.eu/">homegear</a> for device integration and <a href="https://www.openhab.org/">openhab</a>
as user interface) <a href="https://gitlab.com/wolutator/MySwitch">here on gitlab</a>.
</p>