Fix bug in analogWrite when accessing 16 bit register (#94)

Fix bug in analogWrite when accessing 16 bit register

Author: landret

Diff for: cores/arduino/wiring_analog.c

@@ -145,6 +145,8 @@ void analogWrite(uint8_t pin, int val)
 		uint16_t* timer_cmp_out;
 		TCB_t *timer_B;
 
+		uint8_t savedSREG;
+
 		/* Find out Port and Pin to correctly handle port mux, and timer. */
 		switch (digital_pin_timer) {
 
@@ -153,7 +155,10 @@ void analogWrite(uint8_t pin, int val)
 				timer_cmp_out = ((uint16_t*) (&TCA0.SINGLE.CMP0BUF)) + bit_pos;
 
 				/* Configure duty cycle for correct compare channel */
-				(*timer_cmp_out) = (val);
+				savedSREG = SREG;
+				cli();
+				(*timer_cmp_out) = (val); // non-atomic 16-bit write operation
+				SREG = savedSREG;
 
 				/* Enable output on pin */
 				TCA0.SINGLE.CTRLB |= (1 << (TCA_SINGLE_CMP0EN_bp + bit_pos));
@@ -170,7 +175,12 @@ void analogWrite(uint8_t pin, int val)
 				timer_B = ((TCB_t *)&TCB0 + (digital_pin_timer - TIMERB0));
 
 				/* set duty cycle */
-				timer_B->CCMPH = val;
+				// (16-bit read/write operation are non-atomic and use a temporary register)
+				savedSREG = SREG;		
+				cli();							
+				timer_B->CCMPL = timer_B->CCMPL;  // copy CCMPL into temporary register
+				timer_B->CCMPH = val;             // set CCMPH value + copy temporary register content into CCMPL
+				SREG = savedSREG;				
 
 				/* Enable Timer Output	*/
 				timer_B->CTRLB |= (TCB_CCMPEN_bm);