digitalPinToInterrupt(21) returns 255 with ATmega328PB

[macdonaldtomw]

I'm using Visual Micro along with Atmel Studio, using mEDBG debugger on an Xplained Mini 328PB (using internal 8MHz oscillator)

I'm trying to get the following sketch to work:

const byte ledPin = 13;
const byte interruptPin = 21;
volatile byte state = LOW;

void setup() {
	pinMode(ledPin, OUTPUT);
	pinMode(interruptPin, INPUT_PULLUP);
	attachInterrupt(digitalPinToInterrupt(interruptPin), blink, CHANGE);
}

void loop() {
	digitalWrite(ledPin, state);
}

void blink() {
	state = !state;
}

If I build a debugging version of this sketch, and step through it, it appears that the call to digitalPinToInterrupt(21) returns 255, which makes the attachInterrupt() call silently fail.

See video for step-debugging:

https://youtu.be/QOlM6OYCwww

I think ultimately this is a problem with the digitalPinToInterrupt() function in pins_arduino.h which uses the following implementation of digitalPinToInterrupt():

#elif defined(__AVR_ATmega328PB__)
#define digitalPinToPCICR(p)     (((p) >= 0 && (p) <= 26) ? (&PCICR) : ((uint8_t *)0))
#define digitalPinToPCICRbit(p)  (((p) <= 7) ? 2 : (((p) <= 13 || (p) == 20 || (p) == 21) ? 0 : (((p) <= 22) ? 1 : 2)))
#define digitalPinToPCMSK(p)     (((p) <= 7) ? (&PCMSK2) : (((p) <= 13 || (p) == 20 || (p) == 21) ? (&PCMSK0) : (((p) <= 22) ? (&PCMSK1) : (((p) <= 26) ? (&PCMSK3) : ((uint8_t *)0)))))
#define digitalPinToPCMSKbit(p)  (((p) <= 7) ? (p) : (((p) <= 13) ? ((p) - 8) : ((p) <= 21) ? ((p) - 14) : ((p) == 22) ? ((p) - 16) : ((p) - 23)))
#define digitalPinToInterrupt(p) ((p) == 2 ? 0 : ((p) == 3 ? 1 : NOT_AN_INTERRUPT))
#endif

[MCUdude]

The arduino API doesn't really support PCINTs out of the box. That means digitalPinToInterrupt() will only work if you insert the number 2 or 3 in it.

NOT_AN_INTERRUPT is defined as -1, which will become 255 if you store the value to an unsigned variable.

Try a google search for Arduino PCINT library

[macdonaldtomw]

Okay, thanks. I'm just rolling up my sleeves and doing it in Atmel Studio with avrlibc.

Thanks again!