SDLC in Pi 2040 PIO

rp2040_sdlc.pio

; This is a first pass attempt at making a Pi Zero PIO module handle zero-bit stuffing and flag transmission
; It will need cycle delays adding, etc. to turn it into usable code.
; I got half-way through it, then realised it wasn't going to work for my use case (NABU Adaptor packet generation) because
; the NABU Adaptor has a LFSR-based bitstream randomizer (scrambler) after the SDLC block.
;
; Set up autopull, and a bit length of 9 (if such a thing is possible, I haven't checked).
; The first bit shifted is a 0 for normal data, or a 1 for 'raw' sending, which is used to send flags (0x17E).

.program sdlc
.side_set 1

; Pin assignments:
;  - SCK  is side-set pin 0
;  - DATA is OUT pin 0 and SET pin 0

public entry_point:
	; autopull on, shiftcount 9

byteloop:
	out x					side 0	; Get first bit of our 9-bit word
	jmp !x cooked					; If bit low then do cooked write

	; RAW WRITE - used for FLAGs. Resets one-counter (yreg) to zero.
	set y, 0						; Assume we're sending a flag, which ends with a zero
	set x, 7						; Preload bit counter (8 bits)
raw_bitloop:
	out pins, 1				side 0
	jmp x-- raw_bitloop		side 1
	jmp byteloop
	

	; COOKED WRITE -- inserts zeroes as needed
cooked:
	set x, 7						; Preload bit counter (8 bits)
cooked_bitloop:
	out y, 1				side 0	; Shift 1 bit out and write it to Y, sideset the clock
	jmp x-- cooked_bitloop	side 1	; sideset the clock and go around