Support software SPI for APA102 (Itsy Bitsy M0 on-board "Dotstar" LED as example) (#86)

* Added implementation and example to support software-based SPI for APA102, for use with boards like Adafruit Itsy Bitsy M0 for instance.

examples/apa102/itsybitsy-m0/main.go

@@ -0,0 +1,84 @@
+// This example demostrates how to control the "Dotstar" (APA102) LED included
+// on the Adafruit Itsy Bitsy M0 board.  It implements a "rainbow effect" based
+// on the following example:
+// https://github.com/adafruit/Adafruit_Learning_System_Guides/blob/master/CircuitPython_Essentials/CircuitPython_Internal_RGB_LED_rainbow.py
+package main
+
+import (
+	"image/color"
+	"machine"
+	"time"
+
+	"tinygo.org/x/drivers/apa102"
+)
+
+var (
+	apa apa102.Device
+
+	led   = machine.PWM{machine.LED}
+	leds  = make([]color.RGBA, 1)
+	wheel = &Wheel{Brightness: 0x10}
+)
+
+func init() {
+
+	// APA102 on Itsy Bitsy is connected to pins that require a software-based
+	// SPI implementation.
+	apa = apa102.NewSoftwareSPI(machine.PA00, machine.PA01, 1)
+
+	// Configure the regular on-board LED for PWM fading
+	machine.InitPWM()
+	led.Configure()
+
+}
+
+func main() {
+
+	// We'll fade the on-board LED in a goroutine to show/ensure that the APA102
+	// works fine with the scheduler enabled.  Comment this out to test this code
+	// with the scheduler disabled.
+	go func() {
+		for i, brightening := uint8(0), false; ; i++ {
+			if i == 0 {
+				brightening = !brightening
+				continue
+			}
+			var brightness uint16 = uint16(i) << 8
+			if !brightening {
+				brightness = 0xFFFF - brightness
+			}
+			led.Set(brightness)
+			time.Sleep(5 * time.Millisecond)
+		}
+	}()
+
+	// Use the "wheel" function from Adafruit's example to cycle the APA102
+	for {
+		leds[0] = wheel.Next()
+		apa.WriteColors(leds)
+		time.Sleep(25 * time.Millisecond)
+	}
+
+}
+
+// Wheel is a port of Adafruit's Circuit Python example referenced above.
+type Wheel struct {
+	Brightness uint8
+	pos        uint8
+}
+
+// Next increments the internal state of the color and returns the new RGBA
+func (w *Wheel) Next() (c color.RGBA) {
+	pos := w.pos
+	if w.pos < 85 {
+		c = color.RGBA{R: 0xFF - pos*3, G: pos * 3, B: 0x0, A: w.Brightness}
+	} else if w.pos < 170 {
+		pos -= 85
+		c = color.RGBA{R: 0x0, G: 0xFF - pos*3, B: pos * 3, A: w.Brightness}
+	} else {
+		pos -= 170
+		c = color.RGBA{R: pos * 3, G: 0x0, B: 0xFF - pos*3, A: w.Brightness}
+	}
+	w.pos++
+	return
+}