avr: clean up ATtiny definitions

Add definitions for constants like PB0.

src/machine/machine_attiny85.go

@@ -0,0 +1,36 @@
+// +build attiny85
+
+package machine
+
+import (
+	"device/avr"
+	"runtime/volatile"
+)
+
+const (
+	PB0 Pin = iota
+	PB1
+	PB2
+	PB3
+	PB4
+	PB5
+)
+
+// Configure sets the pin to input or output.
+func (p Pin) Configure(config PinConfig) {
+	if config.Mode == PinOutput { // set output bit
+		avr.DDRB.SetBits(1 << uint8(p))
+	} else { // configure input: clear output bit
+		avr.DDRB.ClearBits(1 << uint8(p))
+	}
+}
+
+func (p Pin) getPortMask() (*volatile.Register8, uint8) {
+	return avr.PORTB, 1 << uint8(p)
+}
+
+// Get returns the current value of a GPIO pin.
+func (p Pin) Get() bool {
+	val := avr.PINB.Get() & (1 << uint8(p))
+	return (val > 0)
+}