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

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

Makefile

@@ -99,7 +99,9 @@ smoke-test:
 	@md5sum ./build/test.hex
 	tinygo build -size short -o ./build/test.hex -target=arduino-nano33 ./examples/wifinina/webclient/main.go
 	@md5sum ./build/test.hex
-	tinygo build -size short -o ./build/test.hex -target=circuitplay-express ./examples/ws2812/main.go
+	tinygo build -size short -o ./build/test.hex -target=circuitplay-express ./examples/ws2812
+	@md5sum ./build/test.hex
+	tinygo build -size short -o ./build/test.hex -target=digispark ./examples/ws2812
 	@md5sum ./build/test.hex
 	tinygo build -size short -o ./build/test.hex -target=trinket-m0 ./examples/bme280/main.go
 	@md5sum ./build/test.hex

examples/ws2812/digispark.go

@@ -0,0 +1,10 @@
+// +build digispark
+
+package main
+
+import "machine"
+
+// This is the pin assignment for the Digispark only.
+// Replace neo in the code below to match the pin
+// that you are using if different.
+var neo machine.Pin = 0

examples/ws2812/main.go

@@ -1,8 +1,7 @@
-// Connects to an WS2812 RGB LED strip with 10 LEDS, such as
-// on an Adafruit Circuit Playground Express board.
+// Connects to an WS2812 RGB LED strip with 10 LEDS.
 //
-// Replace machine.NEOPIXELS in the code below to match the pin
-// that you are using, if you have a different board.
+// See either the others.go or digispark.go files in this directory
+// for the neopixels pin assignments.
 package main
 
 import (
@@ -13,12 +12,15 @@ import (
 	"tinygo.org/x/drivers/ws2812"
 )
 
+var leds [10]color.RGBA
+
 func main() {
-	neo := machine.NEOPIXELS
+	led := machine.LED
+	led.Configure(machine.PinConfig{Mode: machine.PinOutput})
+
 	neo.Configure(machine.PinConfig{Mode: machine.PinOutput})
 
 	ws := ws2812.New(neo)
-	leds := make([]color.RGBA, 10)
 	rg := false
 
 	for {
@@ -33,7 +35,8 @@ func main() {
 			}
 		}
 
-		ws.WriteColors(leds)
+		ws.WriteColors(leds[:])
+		led.Set(rg)
 		time.Sleep(100 * time.Millisecond)
 	}
 }

examples/ws2812/others.go

@@ -0,0 +1,9 @@
+// +build !digispark
+
+package main
+
+import "machine"
+
+// Replace neo in the code below to match the pin
+// that you are using if different.
+var neo machine.Pin = machine.NEOPIXELS

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
+}