sam: add support for Atmel SAMD21 based ItsyBitsy M0

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

src/machine/machine_atsamd21g18.go

@@ -0,0 +1,45 @@
+// +build sam,atsamd21g18a
+
+// Peripheral abstraction layer for the atsamd21g18.
+//
+// Datasheet:
+// http://ww1.microchip.com/downloads/en/DeviceDoc/SAMD21-Family-DataSheet-DS40001882D.pdf
+//
+package machine
+
+import (
+	"device/sam"
+)
+
+const CPU_FREQUENCY = 48000000
+
+type GPIOMode uint8
+
+const (
+	GPIO_INPUT  = 0
+	GPIO_OUTPUT = 1
+)
+
+// Configure this pin with the given configuration.
+func (p GPIO) Configure(config GPIOConfig) {
+	if config.Mode == GPIO_OUTPUT { // set output bit
+		sam.PORT.DIRSET0 = (1 << p.Pin)
+	} else {
+		sam.PORT.DIRCLR0 = (1 << p.Pin)
+	}
+}
+
+// Get returns the current value of a GPIO pin.
+func (p GPIO) Get() bool {
+	return (sam.PORT.IN0>>p.Pin)&1 > 0
+}
+
+// Set the pin to high or low.
+// Warning: only use this on an output pin!
+func (p GPIO) Set(high bool) {
+	if high {
+		sam.PORT.OUTSET0 = (1 << p.Pin)
+	} else {
+		sam.PORT.OUTCLR0 = (1 << p.Pin)
+	}
+}