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title	date	lastmod	draft	tags	categories	contentCopyright	hideHeaderAndFooter
STM32F0 with libopencm3 - Part 1: Simple Timer	2020-02-12	2020-02-12	true	libopencm3 stm32 tutorial	Tutorial	false	false

After having reviewed Part 0 of this series, we can now explore controlling GPIO with the hardware timers! Other tutorials have used the Systick timer as a good introduction to adding a delay for blinking an LED. However, it is my belief that this leads to confusion for beginners and only opens the door to misunderstandings. That being said, we will be using timers and their associated GPIO ports with Alternate Function modes.

Straight to the Chase

For those that want to cut to the chase and save time, here is the full source code with friendly names to get you started:

{{< admonition note "Source Code" true >}}

#include <libopencm3/stm32/gpio.h>
#include <libopencm3/stm32/rcc.h>
#include <libopencm3/stm32/timer.h>

#define LED_PORT    GPIOC
#define LED_PIN_BLU GPIO8
#define LED_PIN_GRN GPIO9
#define TIM_PSC_DIV 48000
#define SECONDS     1

volatile unsigned int i;

int main(void) {
    rcc_clock_setup_in_hsi_out_48mhz();
    rcc_periph_clock_enable(RCC_GPIOC);
    rcc_periph_clock_enable(RCC_TIM3);

    gpio_mode_setup(LED_PORT, GPIO_MODE_AF, GPIO_PUPD_NONE, LED_PIN_BLU | LED_PIN_GRN);
    gpio_set_output_options(LED_PORT, GPIO_OTYPE_PP, GPIO_OSPEED_HIGH, LED_PIN_BLU | LED_PIN_GRN);
    gpio_set_af(LED_PORT, GPIO_AF0, LED_PIN_BLU | LED_PIN_GRN);

    timer_set_mode(TIM3, TIM_CR1_CKD_CK_INT, TIM_CR1_CMS_CENTER_1, TIM_CR1_DIR_UP);

    // The math for seconds isn't quite right here
    timer_set_prescaler(TIM3, (rcc_apb1_frequency/TIM_PSC_DIV)/2*SECONDS);
    timer_disable_preload(TIM3);
    timer_continuous_mode(TIM3);
    timer_set_period(TIM3, TIM_PSC_DIV);

    timer_set_oc_mode(TIM3, TIM_OC3, TIM_OCM_PWM1);
    timer_set_oc_mode(TIM3, TIM_OC4, TIM_OCM_PWM2);

    int tim_oc_ids[2] = { TIM_OC3, TIM_OC4 };

    for (i = 0; i < (sizeof(tim_oc_ids)/sizeof(tim_oc_ids[0])); ++i) {
        timer_set_oc_value(TIM3, tim_oc_ids[i], (TIM_PSC_DIV/2));
    }

    timer_enable_oc_output(TIM3, TIM_OC3);
    timer_enable_oc_output(TIM3, TIM_OC4);

    timer_enable_counter(TIM3);

    while (1) {
        ;
    }

    return 0;
}

{{< /admonition >}}

Set up the GPIO

Assuming the reader is either familiar with GPIO setup for the STM32F0, or has reviewed Part 0 of this series we will set up the GPIO pins tied to the LEDs (port C, pins 8 and 9) in the Alternate Function mode.

Alternate Functions

The STM32 microcontroller's GPIO has a hardware feature allowing you to tie certain port's pins to a different register as part of the output or input control: {{< img src="/static/img/stm32-examples/part1/stm32-af-diagram.png" sub="GPIO Alternate Function Diagram" >}}

For accomplishing this, a few things need to happen:

1. The desired GPIO pins need to be set to GPIO_MODE_AF in gpio_mode_setup()
2. The alternate function mode number GPIO_AFx has to be set for the pins using gpio_set_af()

{{< admonition warning "Note for Different STM32Fx Microcontrollers" >}} Review the datasheet for the specific STM32Fx microcontroller being programmed, as the Alternate Function mappings may be significantly different! {{< /admonition >}}

GPIO Alternate Function Setup

For the STM32F0 we are using in this series, the Alternate Function selection number desired is GPIO_AF0 for use with TIM3_CH3 (timer 3, channel 3) and TIM3_CH4 (timer 3, channel 4): {{< img src="/static/img/stm32-examples/part1/stm32-af-gpiomap.png" sub="STM32F051 Alternate Function Mapping" >}}

Ultimately, the code with libopencm3 becomes the following for our use case:

gpio_mode_setup(GPIOC, GPIO_MODE_AF, GPIO_PUPD_NONE, GPIO8 | GPIO9);
gpio_set_output_options(GPIOC, GPIO_OTYPE_PP, GPIO_OSPEED_HIGH, GPIO8 | GPIO9);
gpio_set_af(GPIOC, GPIO_AF0, GPIO8 | GPIO9);