avr: implement Get() function on AVR, and leave stubs for NRF and dummy machines

Signed-off-by: Ron Evans <ron@hybridgroup.com>

src/examples/button/button.go

@@ -0,0 +1,28 @@
+package main
+
+import (
+	"machine"
+	"runtime"
+)
+
+// This example assumes that the button is connected to pin 8. Change the value
+// below to use a different pin.
+const buttonPin = 8
+
+func main() {
+	led := machine.GPIO{machine.LED}
+	led.Configure(machine.GPIOConfig{Mode: machine.GPIO_OUTPUT})
+
+	button := machine.GPIO{buttonPin}
+	button.Configure(machine.GPIOConfig{Mode: machine.GPIO_INPUT})
+
+	for {
+		if button.Get() {
+			led.Low()
+		} else {
+			led.High()
+		}
+
+		runtime.Sleep(runtime.Millisecond * 10)
+	}
+}

src/machine/machine_avr.go

@@ -47,3 +47,14 @@ func (p GPIO) Set(value bool) {
 		}
 	}
 }
+
+// Get returns the current value of a GPIO pin.
+func (p GPIO) Get() bool {
+	if p.Pin < 8 {
+		val := *avr.PIND & (1 << p.Pin)
+		return (val > 0)
+	} else {
+		val := *avr.PINB & (1 << (p.Pin - 8))
+		return (val > 0)
+	}
+}

src/machine/machine_dummy.go

@@ -25,3 +25,7 @@ func (p GPIO) Configure(config GPIOConfig) {
 
 func (p GPIO) Set(value bool) {
 }
+
+func (p GPIO) Get() bool {
+	return false
+}

src/machine/machine_nrf.go

@@ -39,3 +39,9 @@ func (p GPIO) Set(high bool) {
 		nrf.P0.OUTCLR = 1 << p.Pin
 	}
 }
+
+// Get returns the current value of a GPIO pin.
+func (p GPIO) Get() bool {
+	// TODO: implement
+	return false
+}