usb: add support for midi

src/examples/usb-midi/main.go

@@ -0,0 +1,54 @@
+package main
+
+import (
+	"fmt"
+	"machine"
+	"machine/usb/midi"
+	"time"
+)
+
+func main() {
+	led := machine.LED
+	led.Configure(machine.PinConfig{Mode: machine.PinOutput})
+
+	button := machine.BUTTON
+	button.Configure(machine.PinConfig{Mode: machine.PinInputPullup})
+
+	m := midi.New()
+	m.SetCallback(func(b []byte) {
+		led.Set(!led.Get())
+		fmt.Printf("% X\r\n", b)
+		m.Write(b)
+	})
+
+	prev := true
+	chords := []struct {
+		name string
+		keys []byte
+	}{
+		{name: "C ", keys: []byte{60, 64, 67}},
+		{name: "G ", keys: []byte{55, 59, 62}},
+		{name: "Am", keys: []byte{57, 60, 64}},
+		{name: "F ", keys: []byte{53, 57, 60}},
+	}
+	index := 0
+
+	for {
+		current := button.Get()
+		if prev != current {
+			led.Set(current)
+			if current {
+				for _, c := range chords[index].keys {
+					m.Write([]byte{0x08, 0x80, c, 0x40})
+				}
+				index = (index + 1) % len(chords)
+			} else {
+				for _, c := range chords[index].keys {
+					m.Write([]byte{0x09, 0x90, c, 0x40})
+				}
+			}
+			prev = current
+		}
+		time.Sleep(10 * time.Millisecond)
+	}
+}