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_xosc.go

@@ -0,0 +1,42 @@
+// +build rp2040
+
+package machine
+
+import (
+	"device/rp"
+	"runtime/volatile"
+	"unsafe"
+)
+
+type xoscType struct {
+	ctrl     volatile.Register32
+	status   volatile.Register32
+	dormant  volatile.Register32
+	startup  volatile.Register32
+	reserved [3]volatile.Register32
+	count    volatile.Register32
+}
+
+var xosc = (*xoscType)(unsafe.Pointer(rp.XOSC))
+
+// init initializes the crystal oscillator system.
+//
+// This function will block until the crystal oscillator has stabilised.
+func (osc *xoscType) init() {
+	// Assumes 1-15 MHz input
+	if xoscFreq > 15 {
+		panic("xosc frequency cannot be greater than 15MHz")
+	}
+	osc.ctrl.Set(rp.XOSC_CTRL_FREQ_RANGE_1_15MHZ)
+
+	// Set xosc startup delay
+	delay := (((xoscFreq * MHz) / 1000) + 128) / 256
+	osc.startup.Set(uint32(delay))
+
+	// Set the enable bit now that we have set freq range and startup delay
+	osc.ctrl.SetBits(rp.XOSC_CTRL_ENABLE_ENABLE << rp.XOSC_CTRL_ENABLE_Pos)
+
+	// Wait for xosc to be stable
+	for !osc.status.HasBits(rp.XOSC_STATUS_STABLE) {
+	}
+}