machine/samd21: basic implementation for DAC (#1183)

* machine/samd21: basic DAC implementation

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

src/examples/dac/dac.go

@@ -0,0 +1,40 @@
+// Simplistic example using the DAC on the Circuit Playground Express.
+//
+// To actually use the DAC for producing complex waveforms or samples requires a DMA
+// timer-based playback mechanism which is beyond the scope of this example.
+package main
+
+import (
+	"machine"
+	"time"
+)
+
+func main() {
+	enable := machine.PA30
+	enable.Configure(machine.PinConfig{Mode: machine.PinOutput})
+	enable.Set(true)
+
+	speaker := machine.A0
+	speaker.Configure(machine.PinConfig{Mode: machine.PinOutput})
+
+	machine.DAC0.Configure(machine.DACConfig{})
+
+	data := []uint16{32768, 8192, 2048, 512, 0}
+
+	for {
+		for _, val := range data {
+			play(val)
+			time.Sleep(500 * time.Millisecond)
+		}
+	}
+}
+
+func play(val uint16) {
+	for i := 0; i < 100; i++ {
+		machine.DAC0.Set(val)
+		time.Sleep(2 * time.Millisecond)
+
+		machine.DAC0.Set(0)
+		time.Sleep(2 * time.Millisecond)
+	}
+}