ws2812: work-arounds to allow Digispark to control WS2812 LEDs

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

ws2812/ws2812_avr_digispark.go

@@ -0,0 +1,50 @@
+// +build digispark
+
+package ws2812
+
+// This file implements the WS2812 protocol for 16.5MHz Digispark AVR microcontrollers.
+// This is a slightly different implementation than the one for the atmega to work around a compiler bug.
+
+import (
+	"device/avr"
+)
+
+// Send a single byte using the WS2812 protocol.
+func (d Device) WriteByte(c byte) error {
+	// For the AVR at 16MHz
+	portSet, maskSet := d.Pin.PortMaskSet()
+	portClear, maskClear := d.Pin.PortMaskClear()
+
+	// See:
+	// https://wp.josh.com/2014/05/13/ws2812-neopixels-are-not-so-finicky-once-you-get-to-know-them/
+	// T0H: 4  cycles or 250ns
+	// T0L: 14 cycles or 875ns -> together 18 cycles or 1125ns
+	// T1H: 9  cycles or 562ns
+	// T1L: 8  cycles or 500ns -> together 17 cycles or 1062ns
+	avr.AsmFull(`
+	send_bit:
+		st    {portSet}, {maskSet}     ; [2]   set output high
+		lsl   {value}                  ; [1]   shift off the next bit, store it in C
+		brcs  skip_store               ; [1/2] branch if this bit is high (long pulse)
+		st    {portClear}, {maskClear} ; [2]   set output low (short pulse)
+	skip_store:
+		nop                            ; [4]   wait before changing the output again
+		nop
+		nop
+		nop
+		st    {portClear}, {maskClear} ; [2]   set output low (end of pulse)
+		nop                            ; [3]
+		nop
+		nop
+		subi  {i}, 1                   ; [1]   subtract one (for the loop)
+		brne  send_bit                 ; [1/2] send the next bit, if not at the end of the loop
+	`, map[string]interface{}{
+		"value":     c,
+		"i":         byte(8),
+		"maskSet":   maskSet,
+		"portSet":   portSet,
+		"maskClear": maskClear,
+		"portClear": portClear,
+	})
+	return nil
+}