Support fewer than 8 parallel strips

This requires slightly different PIO code, because a variable number
of bits have to be thrown away

Author: jepler

adafruit_neopxl8.py

@@ -19,7 +19,7 @@
 import adafruit_pixelbuf
 import rp2pio
 
-_PROGRAM = """
+_PROGRAM8 = """
 .program piopixl8
 top:
     mov pins, null      ; always-low part (last cycle is the 'pull ifempty' after wrap)
@@ -46,7 +46,33 @@
     jmp top
 """
 
-_ASSEMBLED = adafruit_pioasm.assemble(_PROGRAM)
+_PROGRAMN = """
+.program piopixl8
+top:
+    mov pins, null      ; always-low part (last cycle is the 'pull ifempty' after wrap)
+    pull block          ; wait for fresh data
+    out y, 32           ; get count of NeoPixel bits
+
+; NeoPixels are 800khz bit streams. We are choosing zeros as <312ns hi, 936 lo>
+; and ones as <700 ns hi, 546 ns lo> and a clock of 16*800kHz, so the always-high
+; time is 4 cycles, the variable time is 5 cycles, and the always-low time is 7 cycles
+bitloop:
+    pull ifempty [1]     ; don't start outputting HIGH unless data is available (always-low part)
+    mov pins, ~ null [3] ; always-high part
+    out pins, {} [3]     ; variable part
+    out x, {}            ; variable part
+    mov pins, null       ; always-low part (last cycle is the 'pull ifempty' after wrap)
+
+    jmp y--, bitloop     ; always-low part
+
+; A minimum delay is required so that the next pixel starts refreshing the front of the strands
+    pull block
+    out y, 32
+
+wait_reset:
+    jmp y--, wait_reset
+    jmp top
+"""
 
 # Pixel color order constants
 RGB = "RGB"
@@ -135,11 +161,19 @@ def __init__(
 
         self._num_strands = num_strands
 
+        if num_strands == 8:
+            assembled = adafruit_pioasm.assemble(_PROGRAM8)
+        else:
+            program = _PROGRAMN.format(num_strands, 8 - num_strands)
+            print(program)
+            assembled = adafruit_pioasm.assemble(program)
+
+        print("num strands is", num_strands)
         self._sm = rp2pio.StateMachine(
-            _ASSEMBLED,
+            assembled,
             frequency=800_000 * 16,
             first_out_pin=data0,
-            out_pin_count=8,
+            out_pin_count=num_strands,
             first_set_pin=data0,
             auto_pull=False,
             out_shift_right=True,