avr: add support for the digispark

Blinking the on-board LED works. Nothing else has been tested yet.

src/runtime/runtime_atmega.go

@@ -0,0 +1,35 @@
+// +build avr,atmega
+
+package runtime
+
+import (
+	"device/avr"
+)
+
+// 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.WDTCSR |= avr.WDTCSR_WDCE | avr.WDTCSR_WDE
+	// Enable WDT and set new timeout
+	*avr.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.SMCR = (0 << 1) | avr.SMCR_SE
+
+	// go to sleep
+	avr.Asm("sleep")
+
+	// disable sleep
+	*avr.SMCR = 0
+}