How to enable Teensy 4.x GPT timer in free running mode

In this example, we’ll use direct register access to enable the GPT1 timer module at 8 MHz counter frequency in free-running mode. Free-running mode means that the timer will just roll over once it has reached 0xFFFFFFFF (maximum 32 bit value).

How to configure the timer

CCM_CCGR1 |= CCM_CCGR1_GPT1_BUS(CCM_CCGR_ON); // Enable clock to GPT1 module
GPT1_CR = 0; // Disable for configuration
GPT1_PR = 3 - 1; // Prescale 24 MHz clock by 3 => 8 MHz
GPT1_CR = GPT_CR_EN /* Enable timer */
  | GPT_CR_CLKSRC(1) /* 24 MHz peripheral clock as clock source */
  | GPT_CR_FRR /* Free-Run, do not reset */;

Full example

This example works in PlatformIO without any external libraries, but you need to set monitor_speed = 115200 in platformio.ini so the serial port is read at the correct speed.

#include <Arduino.h>

void setup()
{
    // Setup USB serial port so we can print the timer value
    Serial.begin(115200);

    // Enable timer    
    CCM_CCGR1 |= CCM_CCGR1_GPT1_BUS(CCM_CCGR_ON); // Enable clock to GPT1 module
    GPT1_CR = 0; // Disable for configuration
    GPT1_PR = 3 - 1; // Prescale 24 MHz clock by 3 => 8 MHz
    GPT1_CR = GPT_CR_EN /* Enable timer */
      | GPT_CR_CLKSRC(1) /* 24 MHz peripheral clock as clock source */
      | GPT_CR_FRR /* Free-Run, do not reset */;
}

void loop()
{
    // Print the timer count every ~100 ms
    Serial.println(GPT1_CNT);
    delay(100);
}