runtime (atsamd51): allow interrupts to wake the scheduler

src/runtime/runtime_atsamd51.go

@@ -249,7 +249,9 @@ func sleepTicks(d timeUnit) {
 	for d != 0 {
 		ticks() // update timestamp
 		ticks := uint32(d)
-		timerSleep(ticks)
+		if !timerSleep(ticks) {
+			return
+		}
 		d -= timeUnit(ticks)
 	}
 }
@@ -266,7 +268,9 @@ func ticks() timeUnit {
 }
 
 // ticks are in microseconds
-func timerSleep(ticks uint32) {
+// Returns true if the timer completed.
+// Returns false if another interrupt occured which requires an early return to scheduler.
+func timerSleep(ticks uint32) bool {
 	timerWakeup.Set(0)
 	if ticks < 8 {
 		// due to delay waiting for the register value to sync, the minimum sleep value
@@ -287,8 +291,20 @@ func timerSleep(ticks uint32) {
 	// enable IRQ for CMP0 compare
 	sam.RTC_MODE0.INTENSET.SetBits(sam.RTC_MODE0_INTENSET_CMP0)
 
-	for timerWakeup.Get() == 0 {
-		arm.Asm("wfi")
+wait:
+	arm.Asm("wfe")
+	if timerWakeup.Get() != 0 {
+		return true
+	}
+	if hasScheduler {
+		// The interurpt may have awoken a goroutine, so bail out early.
+		// Disable IRQ for CMP0 compare.
+		sam.RTC_MODE0.INTENCLR.SetBits(sam.RTC_MODE0_INTENSET_CMP0)
+		return false
+	} else {
+		// This is running without a scheduler.
+		// The application expects this to sleep the whole time.
+		goto wait
 	}
 }