Under the Hood: Arduino UNO R4 – Timers

I was wondering how to use the timers in the new Arduino UNO R4. Unfortunately I did not find any documentation, so I decided to document my findings here.

The UNO R4 has two timer peripherals: the General PWM Timer (GPT) and the Asynchronous General Purpose Timer (AGT).

There are only 2 16 bit AGT timers (but one is already used to provide the Arduino millis() and microseconds() methods) and there are 7 GPT timers which are also used by PWM. To increase the flexibility you can explicitly request a timer which has been reserved for PWM.

The Arduino API implements the FspTimer class which provides all the necessary functionality.

Here is a test sketch that is setting up a GPT timer with a corresponding interrupt which executes a callback method at a defined interval rate e.g. 8000 hz.

#include "FspTimer.h"

FspTimer audio_timer;
uint64_t count=0;
uint64_t start_time=0;

// callback method used by timer
void timer_callback(timer_callback_args_t __attribute((unused)) *p_args) {
  count++;
}

bool beginTimer(float rate) {
  uint8_t timer_type = GPT_TIMER;
  int8_t tindex = FspTimer::get_available_timer(timer_type);
  if (tindex < 0){
    tindex = FspTimer::get_available_timer(timer_type, true);
  }
  if (tindex < 0){
    return false;
  }

  FspTimer::force_use_of_pwm_reserved_timer();

  if(!audio_timer.begin(TIMER_MODE_PERIODIC, timer_type, tindex, rate, 0.0f, timer_callback)){
    return false;
  }

  if (!audio_timer.setup_overflow_irq()){
    return false;
  }

  if (!audio_timer.open()){
    return false;
  }

  if (!audio_timer.start()){
    return false;
  }
  return true;
}

void setup() {
  Serial.begin(115200);
  beginTimer(8000);
  start_time = millis();
}

void loop() {
  // calculate the effective frequency
  int freq = 1000 * count / (millis()-start_time);
  Serial.println(freq);
  count = 0;
  start_time = millis();
  delay(1000);
}

This is pretty straight forward!

The cool thing, which is no shown here, is that you can add a final parameter to the audio_timer.begin() method which will end up as parameter value which is passed to the timer_callback!