pwm_blink.c

/*
 * Blink an LED on push-pull PA8 using TIM1 PWM mode
 */
#include "rcc.h"
#include "gpio.h"
#include "init.h"
#include "tim.h"

#define STOP \
    do {                \
        asm ("wfi");    \
    } while (1)

int main(void)
{
    volatile struct tim *tim = TIM1;

    /* Basic init */
    init();

    /*
     * Enable clocks
     */
    /* Enable APB2 clock to I/O port A and TIM1 */
    RCC->apb2enr |= RCC_APB2ENR_IOPAEN_MASK | RCC_APB2ENR_TIM1EN_MASK;

    /*
     * Enable PWM output
     */
    /* Set PA8 to alternate function push-pull output, max speed 2MHz */
    GPIO_A->crh = (GPIO_A->crh & (~GPIO_CRH_MODE8_MASK) & (~GPIO_CRH_CNF8_MASK)) |
                  (GPIO_MODE_OUTPUT_2MHZ << GPIO_CRH_MODE8_LSB) |
                  (GPIO_CNF_OUTPUT_AF_PUSH_PULL << GPIO_CRH_CNF8_LSB);

    /*
     * Set up the timer
     */
    /*
     * Set PWM mode 2 (active = CNT > CCR1), leave the default of output mode,
     * enable output compare preload
     */
    tim->ccmr1 = (tim->ccmr1 & (~TIM_CCMR1_OC1M_MASK)) |
                 (TIM_CCMRX_OCYM_VAL_PWM_MODE2 << TIM_CCMR1_OC1M_LSB) |
                 TIM_CCMR1_OC1PE_MASK;
    /* Enable auto-reload preload, leave the default of upcounting */
    tim->cr1 |= TIM_CR1_ARPE_MASK;
    /* Enable first channel (OC1), leave the default of active high */
    tim->ccer |= TIM_CCER_CC1E_MASK;
    /* Enable main output */
    tim->bdtr |= TIM_BDTR_MOE_MASK;
    /* Set prescaler to get CK_CNT = 2KHz = 72MHz(APB2) / 36000 */
    tim->psc = 36000;
    /* Set auto-reload register to have period of 1 second */
    tim->arr = 2000;
    /* Set capture/compare register to have 50% duty */
    tim->ccr1 = 1000;
    /* Generate an update event to transfer data to shadow registers */
    tim->egr |= TIM_EGR_UG_MASK;
    /* Enable counter */
    tim->cr1 |= TIM_CR1_CEN_MASK;

    STOP;
}