Add AVR support

This requires support in LLVM, as AVR support is still experimental. For
example, in bindings/go/build.sh, add
-DLLVM_EXPERIMENTAL_TARGETS_TO_BUILD=AVR to cmake_flags.

src/runtime/runtime_avr.go

@@ -0,0 +1,109 @@
+
+// +build avr
+
+package runtime
+
+import (
+	"device/avr"
+)
+
+const BOARD = "arduino"
+
+const Microsecond = 1
+
+var currentTime uint64
+
+func init() {
+	currentTime = 0
+}
+
+// Watchdog timer periods. These can be off by a large margin (hence the jump
+// between 64ms and 125ms which is not an exact double), so don't rely on this
+// for accurate time keeping.
+const (
+	WDT_PERIOD_16MS = iota
+	WDT_PERIOD_32MS
+	WDT_PERIOD_64MS
+	WDT_PERIOD_125MS
+	WDT_PERIOD_250MS
+	WDT_PERIOD_500MS
+	WDT_PERIOD_1S
+	WDT_PERIOD_2S
+)
+
+func init() {
+	initUART()
+}
+
+func initUART() {
+	// Initialize UART at 115200 baud when running at 16MHz.
+	avr.USART.UBRR0H = 0
+	avr.USART.UBRR0L = 8
+	avr.USART.UCSR0B = avr.UCSR0B_RXEN0 | avr.UCSR0B_TXEN0 // enable RX and TX
+	avr.USART.UCSR0C = avr.UCSR0C_UCSZ0                    // 8-bits data
+}
+
+func putchar(c byte) {
+	for (avr.USART.UCSR0A & avr.UCSR0A_UDRE0) == 0 {
+		// Wait until previous char has been sent.
+	}
+	avr.USART.UDR0 = avr.RegValue(c) // send char
+}
+
+// Sleep by the given amount.
+//
+// TODO: not very accurate. Improve accuracy by calibrating on startup and every
+// once in a while.
+func sleep(d Duration) {
+	currentTime += uint64(d)
+	d /= 16384 // about 16ms
+	for d != 0 {
+		sleepWDT(WDT_PERIOD_16MS)
+		d -= 1
+	}
+}
+
+// Sleep for a given period. The period is defined by the WDT peripheral, and is
+// on most chips (at least) 3 bits wide, in powers of two from 16ms to 2s
+// (0=16ms, 1=32ms, 2=64ms...). Note that the WDT is not very accurate: it can
+// be off by a large margin depending on temperature and supply voltage.
+//
+// TODO: disable more peripherals etc. to reduce sleep current.
+func sleepWDT(period uint8) {
+	// Configure WDT
+	avr.Asm("cli")
+	avr.Asm("wdr")
+	// Start timed sequence.
+	avr.WDT.WDTCSR |= avr.WDTCSR_WDCE | avr.WDTCSR_WDE
+	// Enable WDT and set new timeout (0.5s)
+	avr.WDT.WDTCSR = avr.WDTCSR_WDIE | avr.RegValue(period)
+	avr.Asm("sei")
+
+	// Set sleep mode to idle and enable sleep mode.
+	// Note: when using something other than idle, the UART won't work
+	// correctly. This needs to be fixed, though, so we can truly sleep.
+	avr.CPU.SMCR = (0 << 1) | avr.SMCR_SE
+
+	// go to sleep
+	avr.Asm("sleep")
+
+	// disable sleep
+	avr.CPU.SMCR = 0
+}
+
+func monotime() uint64 {
+	return currentTime
+}
+
+func abort() {
+	avr.Asm("cli")
+	for {
+		sleepWDT(WDT_PERIOD_2S)
+	}
+}
+
+// Align on a word boundary.
+func align(ptr uintptr) uintptr {
+	// No alignment necessary on the AVR.
+	return ptr
+}