Graeme Winter

µPython for real time day 3

Day 3: interrupts - extend the multi-pio program from yesterday to also use interrupts to perform inter-pio communication. This is a trivial example of using irq(0) to indicate to another process, which is waiting on that IRQ, to switch on or off the built in LED. Because this is an additional instruction for the PIO the number of counts is now -3…

I don’t have a clock on this but should time how long the LED is on for… good measure of how “instant” the interrupt handling is.

import time

from machine import Pin
import rp2

p0 = Pin(0)
p25 = Pin(25, Pin.OUT)

# counter program using side-set to control output
@rp2.asm_pio(sideset_init=rp2.PIO.OUT_LOW)
def square():
    wrap_target()
    mov(x, osr).side(1)
    irq(0)
    label("high")
    jmp(x_dec, "high")
    mov(x, osr).side(0)
    irq(0)
    label("low")
    jmp(x_dec, "low")
    wrap()


# counter for how many ticks we are high
@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()


# program to side-set the LED based on PIO irq
@rp2.asm_pio(sideset_init=rp2.PIO.OUT_LOW)
def wait_irq():
    wrap_target()
    wait(1, irq, 0).side(1)
    irq(clear, 0)
    wait(1, irq, 0).side(0)
    irq(clear, 0)
    wrap()


sm0 = rp2.StateMachine(0, square, sideset_base=p0)
sm1 = rp2.StateMachine(1, count_high, jmp_pin=p0)
sm2 = rp2.StateMachine(2, wait_irq, sideset_base=p25)

# half a second - so should report 500,000,000 ns once things warm up
sm0.put(62500000 - 3)
sm0.exec("pull()")

sm2.active(1)
sm1.active(1)
sm0.active(1)

for j in range(10):
    high = 0xFFFFFFFF - sm1.get()
    print(f"{high * 16e-9:.3f} s / {2 * high} ticks")

sm0.active(0)
sm1.active(0)
sm2.active(0)


# tidy
p0.off()
p25.off()

Also “traditional” interrupts… these seem to work OK, can time how quickly they work by setting up an interrupt which will switch off a pin, then set it to trigger on rising edge: turns out is relatively simple provided that you don’t control the pin which the IRQ is watching with the PIO:

from machine import Pin
import rp2


p0 = Pin(0, Pin.OUT)
p0.irq(lambda pin: pin.off(), Pin.IRQ_RISING, hard=True)
p0.off()

# counter for how many ticks we are high
@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()


sm1 = rp2.StateMachine(1, count_high, jmp_pin=p0)

sm1.active(1)

for j in range(10):
    p0.on()
    high = 0xFFFFFFFF - sm1.get()
    print(f"{2 * high} cycles")

sm1.active(0)


# tidy
p0.off()

Turns out this is slow though, literally thousands of cycles to execute (like 4000-6000) unlike the C implementation which was down to … 25 🤔 that is not much competition.

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