How to implement 1MHz interrupt in PlatformIO / Arduino on STM32

In our previous post Minimal STM32 HardwareTimer PlatformIO / Arduino timer interrupt blink example we showed how to use HardwareTimer to blink the onboard LED of our STM32F407 board using a timer interrupt.

In this post, we’ll provide an example of how to use HardwareTimer and have a really fast interrupt which runs at 1 MHz - in other words: one million times per second.

#include <Arduino.h>

HardwareTimer timer(TIM1);
bool ledOn = false;

void OnTimer1Interrupt() {
    ledOn = !ledOn;
    digitalWrite(PC13, ledOn ? HIGH : LOW);

void setup() {
    pinMode(PC13, OUTPUT);
    // Configure timer
    timer.setPrescaleFactor(21); // Set prescaler to 21 => timer frequency = 168/21 = 8 MHz (from prediv'd by 1 clocksource of 168 MHz)
    timer.setOverflow(8); // Set ARR to 8 => timer frequency = 1 MHz
    timer.refresh(); // Make register changes take effect
    timer.resume(); // Start timre

void loop() {

Note that when running such a quick interrupt, you can’t do all too much within the interrupt before the next time the interrupt will run.