avr: add support for the digispark

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

src/machine/machine_attiny.go

@@ -0,0 +1,48 @@
+// +build avr,attiny
+
+package machine
+
+import (
+	"device/avr"
+)
+
+// Configure sets the pin to input or output.
+func (p GPIO) Configure(config GPIOConfig) {
+	if config.Mode == GPIO_OUTPUT { // set output bit
+		*avr.DDRB |= 1 << p.Pin
+	} else { // configure input: clear output bit
+		*avr.DDRB &^= 1 << p.Pin
+	}
+}
+
+// Set changes the value of the GPIO pin. The pin must be configured as output.
+func (p GPIO) Set(value bool) {
+	if value { // set bits
+		*avr.PORTB |= 1 << p.Pin
+	} else { // clear bits
+		*avr.PORTB &^= 1 << p.Pin
+	}
+}
+
+// Get returns the current value of a GPIO pin.
+func (p GPIO) Get() bool {
+	val := *avr.PINB & (1 << p.Pin)
+	return (val > 0)
+}
+
+// Configure is a dummy implementation. UART has not been implemented for ATtiny
+// devices.
+func (uart UART) Configure(config UARTConfig) {
+}
+
+// WriteByte is a dummy implementation. UART has not been implemented for ATtiny
+// devices.
+func (uart UART) WriteByte(c byte) error {
+	return nil
+}
+
+// Tx is a dummy implementation. I2C has not been implemented for ATtiny
+// devices.
+func (i2c I2C) Tx(addr uint16, w, r []byte) error {
+	return nil
+}