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Teensy 4.1 PlatformIO 2MHz Timer interrupt GPIO output

The following PlatformIO example uses TeensyTimerTool on the Teensy 4.1 to run a simple GPIO toggle interrupt at 4 MHz interrupt frequency (i.e. the interrupt is being run 4 million times each second), resulting in a 2 MHz GPIO output:

#include <Arduino.h>
#include <TeensyTimerTool.h>
using namespace TeensyTimerTool;

PeriodicTimer t1(GPT2);

void callback() // toggle the LED
{
    digitalWriteFast(33, !digitalReadFast(33));
}

void setup()
{
    t1.begin(callback, 250ns);

    pinMode(33, OUTPUT);
}

void loop()
{
}

platformio.ini:

[env:teensy41]
platform = teensy
board = teensy41
framework = arduino
lib_deps = luni64/TeensyTimerTool @ ^0.3.5

The 2MHz output looks like this on the oscilloscope: