PWM not working - STM32F042F4

Question

I've been practicing bare-metal programming with a custom board that I bought from a friend.

My idea is to control a LED brightness, connected to the pin PA4, over pwm. After flashing the device (STM32F042F4), the LED doesn't change its brightness, it remains turned off.

I used the following configurations:

• General purpose timer 14 in PWM mode 1 and clocked by HSI source (8MHz);
• PA4 selected as alternate function 4 (TIM14 Channel 1) and clocked by HSI source (8MHz) as well;

The code is shown below. I used the example provided in the reference manual as a guide.

#include "stm32f042x6.h"


void ConfigurePA4asAF2()
{
    // enabling clock of GPIOA
    RCC->AHBENR |= RCC_AHBENR_GPIOAEN_Msk;

    GPIOA->MODER |= GPIO_MODER_MODER4_1; // alternate function
    GPIOA->OTYPER &= ~GPIO_OTYPER_OT_4; // push-pull
    GPIOA->OSPEEDR |= GPIO_OSPEEDR_OSPEEDR4_1; // low speed
    GPIOA->PUPDR |= GPIO_PUPDR_PUPDR4_1; // pull-down

    GPIOA->AFR[0] |= GPIO_AFRL_AFSEL4; //Alternate function 4, TIM14_CH1
}

void ConfigureTIM14()
{

    //Enabling TIM14 clock
    RCC->APB1ENR |= RCC_APB1ENR_TIM14EN;

    TIM14->PSC = 7; // Pre scaler (PSC + 1), PSC final value = 8, CC module clock = 1MHz 
    TIM14->ARR = 1000-1; // auto reload value, period of PWM
    TIM14->CCR1 = 500-1; // capture compare register, DutyCycle = 50%
    TIM14->CCMR1 |= TIM_CCMR1_OC1M_2 + TIM_CCMR1_OC1M_1 + TIM_CCMR1_OC1PE; // (PWM mode 1)
    TIM14->CCER |= TIM_CCER_CC1E; //capture compare enable

    TIM14->BDTR |= TIM_BDTR_MOE;

    TIM14->EGR |= TIM_EGR_UG;
    TIM14->CR1 |= TIM_CR1_CEN; //Enabling timer
}


int main(void)
{
    ConfigurePA4asAF2();
    ConfigureTIM14();

    while(1);

    return 0;
}

In the example provided by the documentation (A.9.8 Edge-aligned PWM configuration example), the bit TIM_BDTR_MOE is set on register BDTR, and the description of this line says: "(6) Enable output (MOE = 1)".

However, Timer 14 doesn't have the register BDTR, does it mean that it's impossible to output pwm signal over PA4?

PS: My board doesn't support debugging feature.

Answer 1

It seems there are some errors in your bit field access codes. Try these code snippets:

For configuring GPIO:

RCC->AHB1ENR |= RCC_AHB1ENR_GPIOAEN;
__DSB();
GPIOA->AFR[0] |= (4u << GPIO_AFRL_AFSEL4_Pos); // 4u means AF4
GPIOA->MODER |= (0b10u << GPIO_MODER_MODE4_Pos); // 0b10u means AF

For configuring TIM14:

RCC->APB1ENR |= RCC_APB1ENR_TIM14EN;
__DSB();
TIM14->PSC = 7;
TIM14->ARR = 1000 - 1;
TIM14->CCR1 = 500 - 1;
TIM14->CCMR1 |= (0b110u << TIM_CCMR1_OC1M_Pos) // 0b110u means PWM mode 1
             | TIM_CCMR1_OC1PE;
TIM14->CCER |= TIM_CCER_CC1E;
TIM14->EGR |= TIM_EGR_UG;
TIM14->CR1 |= TIM_CR1_CEN;

I haven't tested these code snippets. I guess your TIM code was OK, but the main problem was the wrong GPIO AF configuration.