Graeme Winter

µPython for real time

Decided I should see what you can do with µPython and real-time programming on the PIO… turns out it works fine but obviously only useful for very limited values of real-time.

Implemented simple time counter on PIO: wait until high, start, while high decrement, return once falls to low: takes two instructions / cycle so 16ns with default config, confirmed yesterday on oscilloscope.

Example code for this (other programs will appear): todo assess how “noisy” a busy loop with sleep() is…

Like this then:

import time

from machine import Pin
import rp2

p0 = Pin(0, Pin.OUT)


@rp2.asm_pio()
def count_high():
    mov(x, invert(null))
    wait(1, pin, 0)
    label("high")
    jmp(x_dec, "next")
    label("next")
    jmp(pin, "high")
    mov(isr, x)
    push()


sm = rp2.StateMachine(0, count_high, jmp_pin=p0)

sm.active(1)

# target time for sleeping, ms
delta_ms = 50

for j in range(100):
    p0.on()
    time.sleep_ms(delta_ms)
    p0.off()
    delta = (16 * (0xFFFFFFFF - sm.get())) - delta_ms * 1_000_000
    print(f"∂ = {delta} ns")

sm.active(0)

Shows that the interaction with µPython gives noisy and measurable overhead: not surprising as p0.on() and p0.off() will take some finite time to execute (should do the same in C…) - results:

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