avr: simplify timer-based time

Simplify the interrupt-based timer code in a few ways:

  - Do not recalibrate the timer every 100ms. Instead, rely on the fact
    that the machine package will calbrate the timer if necessary if it
    makes changes to Timer0.
  - Do not configure Timer0 and then set nanosecondsInTick based on that
    value. Instead, use a fixed value.

These two changes together mean that in code that doesn't use PWM,
nanosecondsInTick will be constant which makes the TIMER0_OVF interrupt
handler a lot smaller.

Together this reduces the code size of AVR binaries by about 1200 bytes,
making it pretty close to the pre-timer code size (only about 250 bytes
larger).

It also somehow fixes a problem with
tinygo.org/x/drivers/examples/ws2812 on the Arduino Uno. I'm not quite
sure what was going wrong, but bisecting pointed towards the timer code
(https://github.com/tinygo-org/tinygo/pull/2428) and with this
simplification the bug appears to be gone.

src/runtime/runtime_avr.go

@@ -77,14 +77,6 @@ func nanosecondsToTicks(ns int64) timeUnit {
 // Sleep this number of ticks of nanoseconds.
 func sleepTicks(d timeUnit) {
 	waitTill := ticks() + d
-	// recalibrate if we have some time (>100ms) and it was a while when we did it last time.
-	if d > 100000 {
-		now := waitTill - d
-		if nextTimerRecalibrate < now {
-			nextTimerRecalibrate = now + timerRecalibrateInterval
-			adjustMonotonicTimer()
-		}
-	}
 	for {
 		// wait for interrupt
 		avr.Asm("sleep")
@@ -120,18 +112,17 @@ func abort() {
 	}
 }
 
-var ticksCount int64        // nanoseconds since start
-var nanosecondsInTick int64 // nanoseconds per each tick
+var ticksCount int64                // nanoseconds since start
+var nanosecondsInTick int64 = 16000 // nanoseconds per each tick
 
 func initMonotonicTimer() {
-	nanosecondsInTick = 0
 	ticksCount = 0
 
 	interrupt.New(avr.IRQ_TIMER0_OVF, func(i interrupt.Interrupt) {
 		// use volatile
-		increment := volatile.LoadUint64((*uint64)(unsafe.Pointer(&nanosecondsInTick)))
 		ticks := volatile.LoadUint64((*uint64)(unsafe.Pointer(&ticksCount)))
-		volatile.StoreUint64((*uint64)(unsafe.Pointer(&ticksCount)), ticks+increment)
+		ticks += uint64(nanosecondsInTick)
+		volatile.StoreUint64((*uint64)(unsafe.Pointer(&ticksCount)), ticks)
 	})
 
 	// initial initialization of the Timer0
@@ -146,8 +137,6 @@ func initMonotonicTimer() {
 	// - Set prescaler 1
 	avr.TCCR0B.Set(avr.TCCR0B_CS00)
 
-	adjustMonotonicTimer()
-
 	// - Unmask interrupt
 	avr.TIMSK0.SetBits(avr.TIMSK0_TOIE0)
 }
@@ -155,7 +144,9 @@ func initMonotonicTimer() {
 //go:linkname adjustMonotonicTimer machine.adjustMonotonicTimer
 func adjustMonotonicTimer() {
 	// adjust the nanosecondsInTick using volatile
+	mask := interrupt.Disable()
 	volatile.StoreUint64((*uint64)(unsafe.Pointer(&nanosecondsInTick)), uint64(currentNanosecondsInTick()))
+	interrupt.Restore(mask)
 }
 
 func currentNanosecondsInTick() int64 {