all: rewrite sleep function

time.Sleep now compiles on all systems, so lets use that.
Additionally, do a few improvements in time unit handling for the
scheduler. This should lead to somewhat longer sleep durations without
wrapping (on some platforms).

Some examples got smaller, some got bigger. In particular, code using
the scheduler got bigger and the blinky1 example got smaller (especially
on Arduino: 380 -> 314 bytes).

src/runtime/runtime_nrf.go

@@ -7,7 +7,9 @@ import (
 	"device/nrf"
 )
 
-const Microsecond = 1
+type timeUnit int64
+
+const tickMicros = 1024 * 32
 
 //go:export _start
 func _start() {
@@ -48,31 +50,35 @@ func putchar(c byte) {
 	nrf.UART0.EVENTS_TXDRDY = 0
 }
 
-func sleep(d Duration) {
-	ticks64 := d / 32
-	for ticks64 != 0 {
-		monotime()                          // update timestamp
-		ticks := uint32(ticks64) & 0x7fffff // 23 bits (to be on the safe side)
-		rtc_sleep(ticks)                    // TODO: not accurate (must be d / 30.5175...)
-		ticks64 -= Duration(ticks)
+//go:linkname sleep time.Sleep
+func sleep(d timeUnit) {
+	sleepTicks(d / tickMicros)
+}
+
+func sleepTicks(d timeUnit) {
+	for d != 0 {
+		ticks()                       // update timestamp
+		ticks := uint32(d) & 0x7fffff // 23 bits (to be on the safe side)
+		rtc_sleep(ticks)              // TODO: not accurate (must be d / 30.5175...)
+		d -= timeUnit(ticks)
 	}
 }
 
 var (
-	timestamp      uint64 // microseconds since boottime
-	rtcLastCounter uint32 // 24 bits ticks
+	timestamp      timeUnit // nanoseconds since boottime
+	rtcLastCounter uint32   // 24 bits ticks
 )
 
-// Monotonically increasing numer of microseconds since start.
+// Monotonically increasing numer of ticks since start.
 //
 // Note: very long pauses between measurements (more than 8 minutes) may
 // overflow the counter, leading to incorrect results. This might be fixed by
 // handling the overflow event.
-func monotime() uint64 {
+func ticks() timeUnit {
 	rtcCounter := uint32(nrf.RTC0.COUNTER)
 	offset := (rtcCounter - rtcLastCounter) % 0xffffff // change since last measurement
 	rtcLastCounter = rtcCounter
-	timestamp += uint64(offset * 32) // TODO: not precise
+	timestamp += timeUnit(offset) // TODO: not precise
 	return timestamp
 }