Comment by fest

5 days ago

When I was doing stepper control on rp2040 I looked into using PIO but the 5 bit counters and 32 instruction limit made it too awkward to use. What worked better for my needs was dedicating 2nd core for just motion control and bit-banging the step/dir signals- simple to implement and good enough for the modest needs I had (just trapezoidal velocity profile for single axis motion).

3 comments

fest

topspin 4 days ago

> but the 5 bit counters

5 bit shift register, I believe you mean. Have a look at this. 16 instructions. Completely untested. I qualified my comment about RP2350 as "I believe," because I haven't actually done this: only investigated it via ChatGPT and Gemini. But it appears sound. Precision bit banging. PIO is pretty cool.

    ; PIO program for high-precision stepper profiles with a two-word data format.
    ; RP2350 clock is 150 MHz
    ; Word 1: [32-bit Low-Time Delay]. A value of 0 is the sentinel to stop.
    ; Word 2: [32-bit Repetition Count] (N-1).

    .program stepper_pwm
    .define HIGH_PULSE_CYCLES 2998 ; 20us @ 150MHz. Total delay is (CONSTANT + 2) cycles. 3000 - 2 = 2998.
    .side_set 1 opt

    entry_point:
        pull block              ; Pull Word 1 (Low-Time Delay) from DMA.
        mov x, osr              ; Copy to X for sent sentinel test
        jmp !x, process_step    ; fall through to the stop_sequence when sentinel == 0

    stop_sequence:
        nop side 0              ; Force the output pin LOW.
        irq set 5               ; Signal the core that the profile has finished.

    halt_loop:
        jmp halt_loop           ; Halt state machine.

    process_step:
        mov y, x                ; Y now holds the 32-bit Low-Time Delay.
        mov isr, y              ; Save LOW delay in ISR for use in every repetition.
        pull block              ; Pull Word 2 (Repetition Count) from DMA.
        mov x, osr              ; X now holds the 32-bit Repetition Count.

    rep_loop:
        ; High pulse duration = (HIGH_PULSE_CYCLES + 2) clock cycles.
        mov y, HIGH_PULSE_CYCLES side 1

    high_loop:
        jmp y-- high_loop side 1
        ; Low pulse duration between reps = (LOW_DELAY_FROM_ISR + 3) clock cycles.
        mov y, isr side 0

    low_loop:
        jmp y-- low_loop side 0
        jmp x-- rep_loop        ; Decrement repetition counter and loop if not zero.
        jmp entry_point         ; Finished all reps for this step, get the next one.