[sam] Do not run disabled interrupt handlers

On SAM:
 - all pins in a port share a single interrupt pin
 - the interrupt status flag for a pin is set even when it is disabled
 - The actual ISR does not look at the interrupt enable flags, it just
   runs the callback for every pin whos interrupt status flag is set.

This meant that if:
 - an interrupt was attached and later detached,
 - then the interrupt condition occured, setting the flag and
 - then another (still attached) interrupt on a different pin in the
   same port occured,
then the detached interrupt callback would also be called.

By masking off the Interrupts Status Register using the Interrupt Mask
Register, only actually enabled interrupt handlers are called.

The incorrect behaviour has been verified using the following test sketch:

	/*
	  To test:
	  - Upload this sketch to the Arduino Due
	  - Open the serial monitor at 115200
	  - Connect a jumper wire to ground and touch the other end to pins 25 and 26, alternating between them
	*/

	void int25() {
		Serial.println("INT25");
	}

	void int26() {
		Serial.println("INT26");
	}

	void setup() {
		Serial.begin(115200);
		pinMode(25, INPUT_PULLUP);
		pinMode(26, INPUT_PULLUP);
		attachInterrupt(25, int25, CHANGE);
		attachInterrupt(26, int26, CHANGE);
		detachInterrupt(26);
	}
	void loop() { }

Author: matthijskooijman

hardware/arduino/sam/cores/arduino/WInterrupts.c

@@ -134,7 +134,7 @@ extern "C" {
 #endif
 
 void PIOA_Handler(void) {
-	uint32_t isr = PIOA->PIO_ISR;
+	uint32_t isr = PIOA->PIO_ISR & PIOA->PIO_IMR;
 	uint32_t i;
 	for (i=0; i<32; i++, isr>>=1) {
 		if ((isr & 0x1) == 0)
@@ -145,7 +145,7 @@ void PIOA_Handler(void) {
 }
 
 void PIOB_Handler(void) {
-	uint32_t isr = PIOB->PIO_ISR;
+	uint32_t isr = PIOB->PIO_ISR & PIOB->PIO_IMR;
 	uint32_t i;
 	for (i=0; i<32; i++, isr>>=1) {
 		if ((isr & 0x1) == 0)
@@ -156,7 +156,7 @@ void PIOB_Handler(void) {
 }
 
 void PIOC_Handler(void) {
-	uint32_t isr = PIOC->PIO_ISR;
+	uint32_t isr = PIOC->PIO_ISR & PIOC->PIO_IMR;
 	uint32_t i;
 	for (i=0; i<32; i++, isr>>=1) {
 		if ((isr & 0x1) == 0)
@@ -167,7 +167,7 @@ void PIOC_Handler(void) {
 }
 
 void PIOD_Handler(void) {
-	uint32_t isr = PIOD->PIO_ISR;
+	uint32_t isr = PIOD->PIO_ISR & PIOD->PIO_IMR;
 	uint32_t i;
 	for (i=0; i<32; i++, isr>>=1) {
 		if ((isr & 0x1) == 0)