Why is my Pico W picking daiseys sometimes, and why is it so slow?

[frohro]

I am trying to write an application that is time sensitive, and so I like to have code do what I expect. See my example code below. I have this very simple program to toggle GPIO18 as fast as I can. I haven't looked at assembly language for this, but I expected it to be significantly less than 100 clock cycles to to toggle a GPIO with the CPU. but it doesn't seem that way. The fastest I can do is 507 kHz. But the really big thing is every once in a while there will be a 12 us pause. See the scope photo.

What is happening?

#include <Arduino.h>

const int LED_PIN = 18;

void setup() {
    pinMode(LED_PIN, OUTPUT);
}

void loop() {
    digitalWrite(LED_PIN, HIGH);
    digitalWrite(LED_PIN, LOW);
}

[earlephilhower]

USB interrupts, possibly. Or some other periodic IRQ. Use setup1 and loop1 for tight loops since there's nothing running on that core unless you put it there.

As for the digitalWrite speed, it's doing a lot of work besides toggling a pin since it's a generic function and needs to check parameters/etc. Also, after loop returns and before it's called again there's other housekeeping being done in the core.

For highest speed set the GPIO pin directly from inside while(1) loop on the 2nd core so that it never exits.