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How to drive the GPIO pins without using the CPU.

It's all very well to use the CPU to turn on and off an LED connected to a GPIO pin, but it is wasteful of resources. Having the CPU sit in a tight delay-loop wasting time until it's time to toggle the GPIO pin wastes processing time that could be doing something much more useful.

The STM32F4 device features a host of peripherals that, for the most part, can run independently of the CPU. In this example we'll configure a timer to toggle the GPIO pin the LED is connected to on the Nucleo-F401RE board.


Checking the schematic for the Nucleo board and the datasheet for the CPU, we discover that the LED is connected to pin A5, and that this is shared with Timer_2, (actually TIM2_CH1 / TIM2_ETR alternate functions using register AF01).

With some digging in the datasheets and header files for the STM32F401, I was able to figure out that we can map the GPIO status to the status of Timer_2. So as the timer reaches a set value, the GPIO pin will toggle - entirely independently of the CPU.

I actually proved the CPU does nothing by accidentally writing some buggy code that set the CPU to a hard fault (crash), but the LED kept on blinking.

Some information on Timer_2

The timer is clocked from the main system clock, I think on the Nucleo board that this is 84MHz, but I may be wrong - I'm still learning. The timer has a programmable pre-scaler between it and the main clock, this lets us clock the timer at a more manageable rate (we can slow it down) if the full clock rate is too fast.

When enabled, the timer counts up from zero until it reaches its maximum value (216) then it rolls over back to zero. We can set an auto-reload value that will instead be the value the timer rolls over at when reached, this allows more flexibility in how we use the timer.

There are several possible ways to detect when the timer overflows or auto reloads, the timer will set a flag we can poll for in code, or it can issue an interrupt, or as in this case, we can have it directly interact with another peripheral in the STM32F401 device.

In this example, we set up a hardware comparator to compare the current value of the timer and a pre-set value, on a match this toggles the GPIO pin. The timer is set to auto-reload at the same value as the comparator, but this isn't a requirement. The only requirement is that the auto-reload value is greater or equal to the comparison value (if it isn't, the comparison will ever be met, as the timer will never reach the comparison value).

The code

The first thing to do is up the GPIO port and enables clocks to the GPIO peripheral and the timer.

    RCC->AHB1RSTR |= RCC_AHB1RSTR_GPIOARST;              // Reset GPIOA
    RCC->AHB1RSTR = 0;                                                      // Exit reset state
    RCC->AHB1ENR |= RCC_AHB1ENR_GPIOAEN;                   // Enable GPIOA clock
    RCC->APB1ENR |= RCC_APB1ENR_TIM2EN;                      // Enable Timer Clock

Then we can set up the Alternate Function of the GPIO pin, there didn't seem to be any pre-calculated values in the header file, so I had to hit the datasheet for the correct value to set the AFR[0] register. If you change GPIO pins, this value will need re-calculating.

    GPIOA->MODER |= GPIO_MODER_MODER5_1;                 // Set GPIOA.5 to Alternate function (Took some digging to find this!)
    GPIOA->AFR[0]|= 0x100000;                            // Set Alternate function AF01 on pin 5 (defines seem to be missing?)

Now we can set up the timer peripheral. The value of the pre-scaler was just an initial guess, it worked so I didn't change it. So too were the auto-reload and comparison values. I expected to have to watch the GPIO toggle with an oscilloscope as it would probably be too fast for the LED, but I guessed pretty good values to start with.

TIM2->PSC = 29999;                                   // I think the counter clock is 84Mhz, divide it down a bit. Prescale = PSC +1
    TIM2->ARR = 500;                                     // Counter rolls back to zero when it reaches 500
    TIM2->CCR1 = 500;                                    // Value to compare with, 500 seems a nice number...

The last thing to do it to set the timer to toggle the GPIO on a comparison match and enable the timer.

TIM2->CCMR1 = TIM_CCMR1_OC1M_0 |TIM_CCMR1_OC1M_1;    // Output changes on channel 1 compare match. Also took some finding!
    TIM2->CCER = TIM_CCER_CC1E;                          // Enable compare
    TIM2->CR1 |= TIM_CR1_CEN;                            // Start timer

Result and full code

I've put a full project on github, this builds with PlatformIO and includes a QTCreator project file.

After building the code, you can flash it to the Nucleo board with platfomio, the STM32F401 device will reset and the LED will begin blinking at approximately 1 Hz