NeoPixel: Add invertedDIN option to use single transistor driver.

* Patch allows using single transistor (i.e. IRLML6344) to drive WS2812B.
* Transistor is required for MCU 3.3V signals to drive 5V WS2812B.
* Inverted DIN and bistream hi-to-lo only is compensated by timings.
* Please verify on your hardware.

How compensation timings were calculated:
```
    STREAM: 11001010
       BUS: 110 110 100 100 110 100 110 100
       LET: A B A B CD  CD  A B CD  A B CD
INV STREAM: 00110101
   INV BUS: 001 001 011 011 001 011 001 011
   INV LET: D C D C BA  BA  D C BA  D C BA
```

Signed-off-by: Tomasz 'CeDeROM' CEDRO <tomek@cedro.info>

drivers/neopixel/neopixel.py

@@ -1,26 +1,50 @@
 # NeoPixel driver for MicroPython
 # MIT license; Copyright (c) 2016 Damien P. George, 2021 Jim Mussared
+#
+# (C) 2021 CeDeROM Tomasz CEDRO.
+# Note: Inverted DIN using single transistor (i.e. IRLML6344) is experimental!
+# Below analysis of inverted DIN timings is presented. Works on WS2812B.
+#     STREAM: 11001010
+#        BUS: 110 110 100 100 110 100 110 100
+#        LET: A B A B CD  CD  A B CD  A B CD
+# INV STREAM: 00110101
+#    INV BUS: 001 001 011 011 001 011 001 011
+#    INV LET: D C D C BA  BA  D C BA  D C BA
+
 
 from machine import bitstream
 
+_BITSTREAM_TYPE_HIGH_LOW = const(0)
+_TIMING_WS2818_800 = (400, 850, 800, 450)
+_TIMING_WS2818_400 = (800, 1700, 1600, 900)
+_TIMING_WS2818_800_INV = (850, 400, 450, 800)
+_TIMING_WS2812_400_INV = (1700, 800, 900, 1600)
 
 class NeoPixel:
     # G R B W
     ORDER = (1, 0, 2, 3)
 
-    def __init__(self, pin, n, bpp=3, timing=1):
+    def __init__(self, pin, n, bpp=3, timing=1, invertedDIN=False):
         self.pin = pin
         self.n = n
         self.bpp = bpp
         self.buf = bytearray(n * bpp)
+        self.invertedDIN = invertedDIN
         self.pin.init(pin.OUT)
         # Timing arg can either be 1 for 800kHz or 0 for 400kHz,
         # or a user-specified timing ns tuple (high_0, low_0, high_1, low_1).
-        self.timing = (
-            ((400, 850, 800, 450) if timing else (800, 1700, 1600, 900))
-            if isinstance(timing, int)
-            else timing
-        )
+        if self.invertedDIN:
+            self.timing = (
+                (_TIMING_WS2818_800_INV if timing else _TIMING_WS2818_400_INV)
+                if isinstance(timing, int)
+                else timing
+            )
+        else:
+            self.timing = (
+                (_TIMING_WS2818_800 if timing else _TIMING_WS2818_400)
+                if isinstance(timing, int)
+                else timing
+            )
 
     def __len__(self):
         return self.n
@@ -46,5 +70,6 @@ def fill(self, v):
                 j += bpp
 
     def write(self):
-        # BITSTREAM_TYPE_HIGH_LOW = 0
-        bitstream(self.pin, 0, self.timing, self.buf)
+        if self.invertedDIN: pin.value(1)
+        bitstream(self.pin, _BITSTREAM_TYPE_HIGH_LOW, self.timing, self.buf)
+        if self.invertedDIN: pin.value(1)