apa102: add support for APA102 SPI controlled LEDs

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

apa102/apa102.go

@@ -0,0 +1,86 @@
+// Package apa102 implements a driver for the APA102 SPI LED.
+//
+// Datasheet: https://cdn-shop.adafruit.com/product-files/2343/APA102C.pdf
+package apa102
+
+import (
+	"image/color"
+	"machine"
+)
+
+const (
+	// BGR aka "Blue Green Red" is the current APA102 LED color order.
+	BGR = iota
+
+	// BRG aka "Blue Red Green" is the typical APA102 color order from 2015-2017.
+	BRG
+
+	// GRB aka "Green Red Blue" is the typical APA102 color order from pre-2015.
+	GRB
+)
+
+// Device wraps APA102 SPI LEDs.
+type Device struct {
+	bus   machine.SPI
+	Order int
+}
+
+// New returns a new APA102 driver. Pass in a fully configured SPI bus.
+func New(b machine.SPI) Device {
+	return Device{bus: b, Order: BGR}
+}
+
+// WriteColors writes the given RGBA color slice out using the APA102 protocol.
+// The A value (Alpha channel) is used for brightness, set to 0xff (255) for maximum.
+func (d Device) WriteColors(cs []color.RGBA) (n int, err error) {
+	d.startFrame()
+
+	// write data
+	for _, c := range cs {
+		// brightness is scaled to 5 bit value
+		d.bus.Tx([]byte{0xe0 | (c.A >> 3)}, nil)
+
+		// set the colors
+		switch d.Order {
+		case BRG:
+			d.bus.Tx([]byte{c.B}, nil)
+			d.bus.Tx([]byte{c.R}, nil)
+			d.bus.Tx([]byte{c.G}, nil)
+		case GRB:
+			d.bus.Tx([]byte{c.G}, nil)
+			d.bus.Tx([]byte{c.R}, nil)
+			d.bus.Tx([]byte{c.B}, nil)
+		case BGR:
+			d.bus.Tx([]byte{c.B}, nil)
+			d.bus.Tx([]byte{c.G}, nil)
+			d.bus.Tx([]byte{c.R}, nil)
+		}
+	}
+
+	d.endFrame(len(cs))
+
+	return len(cs), nil
+}
+
+// Write the raw bytes using the APA102 protocol.
+func (d Device) Write(buf []byte) (n int, err error) {
+	d.startFrame()
+	d.bus.Tx(buf, nil)
+	d.endFrame(len(buf) / 4)
+
+	return len(buf), nil
+}
+
+// startFrame sends the start bytes for a strand of LEDs.
+func (d Device) startFrame() {
+	d.bus.Tx([]byte{0x00, 0x00, 0x00, 0x00}, nil)
+}
+
+// endFrame sends the end frame marker with one extra bit per LED so
+// long strands of LEDs receive the necessary termination for updates.
+// See https://cpldcpu.wordpress.com/2014/11/30/understanding-the-apa102-superled/
+func (d Device) endFrame(count int) {
+	for i := 0; i < count/16; i++ {
+		d.bus.Tx([]byte{0xff}, nil)
+	}
+}