add rp2040, pico

adds preliminary support (just enough to run blinky1) for the Raspberry Pi Pico board along with the rp2040 mcu.

src/machine/machine_rp2040_timer.go

@@ -0,0 +1,51 @@
+// +build rp2040
+
+package machine
+
+import (
+	"device/rp"
+	"runtime/volatile"
+	"unsafe"
+)
+
+const numTimers = 4
+
+type timerType struct {
+	timeHW   volatile.Register32
+	timeLW   volatile.Register32
+	timeHR   volatile.Register32
+	timeLR   volatile.Register32
+	alarm    [numTimers]volatile.Register32
+	armed    volatile.Register32
+	timeRawH volatile.Register32
+	timeRawL volatile.Register32
+	dbgPause volatile.Register32
+	pause    volatile.Register32
+	intR     volatile.Register32
+	intE     volatile.Register32
+	intF     volatile.Register32
+	intS     volatile.Register32
+}
+
+var timer = (*timerType)(unsafe.Pointer(rp.TIMER))
+
+// TimeElapsed returns time elapsed since power up, in microseconds.
+func (tmr *timerType) timeElapsed() (us uint64) {
+	// Need to make sure that the upper 32 bits of the timer
+	// don't change, so read that first
+	hi := tmr.timeRawH.Get()
+	var lo, nextHi uint32
+	for {
+		// Read the lower 32 bits
+		lo = tmr.timeRawL.Get()
+		// Now read the upper 32 bits again and
+		// check that it hasn't incremented. If it has, loop around
+		// and read the lower 32 bits again to get an accurate value
+		nextHi = tmr.timeRawH.Get()
+		if hi == nextHi {
+			break
+		}
+		hi = nextHi
+	}
+	return uint64(hi)<<32 | uint64(lo)
+}