sk6812.pio

.program sk6812

.side_set 1

; 12 "cycles" in total, for each bit transmitted
; to transmit L bit:
;   111000000000
; to transmit H bit:
;   111111000000
;

.wrap_target
bitloop:
    ; always start with low pin, in case we stall this gets interpreted as reset/latch
    out x, 1        side 0  [0]

    jmp !x do_zero  side 1  [2]             ; 3 cycle
do_one:
    nop             side 1  [2]             ; 3 cycles
    jmp  bitloop    side 0  [4]             ; 5 cycles (+1 for the out at top)
do_zero:
    nop             side 0  [7]             ; 8 cycles (+1 for the out at top)
.wrap

% c-sdk {
#include "hardware/clocks.h"

static inline void sk6812_program_init(PIO pio, uint sm, uint offset, uint pin, int freq) {

    pio_gpio_init(pio, pin);
    pio_sm_set_consecutive_pindirs(pio, sm, pin, 1, true);

    pio_sm_config c = sk6812_program_get_default_config(offset);

    // we use side-set to actually manipulate the data pin.
    sm_config_set_sideset_pins(&c, pin);

    // after having pushed out 24 bits (grb), auto-pull from the 32-bit tx fifo.
    // we are feeding grbx pixels from the cpu side, and convert that into grb on the fly here.
    sm_config_set_out_shift(&c, false, true, 24);

    // there's no input, we only output. so use all fifo space for output.
    sm_config_set_fifo_join(&c, PIO_FIFO_JOIN_TX);

    // our SM needs to run at a freq that is high enough to push out 12 "bits" within one period of 800 kHz.
    // every payload bit is encoded as 12 carrier "bits".
    int cycles_per_bit = 12;
    float div = (float) clock_get_hz(clk_sys) / (float) (freq * cycles_per_bit);
    sm_config_set_clkdiv(&c, div);

    pio_sm_init(pio, sm, offset, &c);
    pio_sm_set_enabled(pio, sm, true);
}
%}