How can I toggle an LED at 5th second of each minute with RTC Alarm?

[akoluacik]

I am working STM32F746-Disco Board, and trying to implement an Alarm in register level. I am trying to implement a system such that it toggles the user led at 5th second of each minute. However, my program toggles differentially. For now, I can notice that it toggles for 2 seconds, but I don't know at which second, or minute?

My codes are below:

RTC_Utils.c

/*
 * RTC_Utils.c
 *
 *  Created on: Sep 8, 2022
 */
 
 
#include "RTC_utils.h"
#include "stm32f746xx.h"
 
 
void RTC_Init(void) {
 
    uint32_t readBit;
 
//  __HAL_RCC_RTC_ENABLE();
    // Enable write access to the backup domain
    if(((RCC->APB1ENR) & (1U << 28)) == 0) {
        // Enable power interface clock
        RCC->APB1ENR |= (1U << 28);
 
        readBit = RCC->APB1ENR;
        //UNUSED(readBit);
    }
 
    // Select LSE as RTC clock source
    if(((PWR->CR1) & (1U << 8)) == 0) {
        PWR->CR1 |= (1U << 8);
        // Wait until the backup domain write protection has been disabled
        while(((PWR->CR1) & (1U << 8)) == 0);
    }
 
    // Reset LSEON and LSEBY before configuring
    RCC->BDCR &= ~((1U << 0) | (1U << 2));
 
    RCC->BDCR |= (1U << 16);
    RCC->BDCR &= ~(1U << 16);
 
    // Wait until LSE clock is ready
//  while(((RCC->BDCR) & (1U << 1)) == 0);
 
    RCC->BDCR |= (1U << 0);
 
    // Select LSE as RTC clock source
    RCC->BDCR |= 1U << 8;
    RCC->BDCR &= ~(1U << 9);
 
    // Disable power interface clock
    RCC->APB1ENR &= ~(1U << 28);
 
    RCC->BDCR |= 1U << 15;
 
    // Remove Write Protection
    RTC->WPR = 0xCA;
    RTC->WPR = 0x53;
 
    // Set INIT bit to 1 in the RTC_ISR register to enter initialization mode
    RTC->ISR |= 1U << 7;
 
    // Poll INITF bit of in the RTC_ISR register
    while((RTC->ISR & (1U << 6)) == 0);
 
    RTC->CR &= (1U << 6);
 
    // program both the prescaler factors to generate 1 Hz clock
    RTC->PRER |= ((2U << 7) - 1) << 16;
    RTC->PRER |= (2U << 8) - 1;
 
    // Load the initial time and date values in the shadow registers
    RTC->TR |= (1U << 22) | (1U << 21) | (8U << 16) | (4U << 12) | (5U << 8);
    RTC->DR |= (2U << 20) | (2U << 16) | (5U << 13) | (0U << 12) | (9U << 8) | (0U << 4) || (8U << 0);
 
    // configure the time format (12 or 24 hours)
    RTC->CR &= ~(1U << 6);
 
    // Disable Initialization
    RTC->ISR &= ~(1U << 7);
 
    // Enable WPR
    RTC->WPR = 0xFF;
}
 
void RTC_Set_Alarm(void) {
 
    RTC->WPR = 0xCA;
    RTC->WPR = 0x53;
 
    // Clear ALRAE in RTC_CR to disable Alarm A
    RTC->CR &= ~(1U << 8);
 
    // Set off alarm A if the second is 5
    RTC->ALRMAR |= 0x5;
 
    // Program the Alarm A registers (RTC_ALRMASSR/RTC_ALRMAR).
    // Mask every second option
    RTC->ALRMAR |= (1U << 23) | (1U << 15) | (1U << 31);
    RTC->ALRMAR &= ~(1U << 7);
 
    RTC->CR |= (1U << 8);
    RTC->CR |= 1U << 12;
 
    RTC->WPR = 0xFF;
}
 
void RTC_Alarm_Enable(void) {
    RTC_Init();
    RTC_Set_Alarm();
    // Configure EXTI17
    // Select Triggering Edge
    EXTI -> RTSR |= EXTI_RTSR_TR17;
 
    // Interrupt Mask Register
    EXTI -> IMR |= EXTI_IMR_IM17;
 
    // Event Mask Register
    EXTI -> EMR |= EXTI_EMR_EM17;
 
    // Interrupt Pending Register
    EXTI -> PR |= EXTI_PR_PR17;
 
    // Set It Priority Most Urgent
    NVIC_EnableIRQ(RTC_Alarm_IRQn);
    NVIC_SetPriority(RTC_Alarm_IRQn, 0);
}
 
void RTC_Alarm_IRQHandler(void) {
    if (RTC->ISR & (1U << 8)) {
        GPIOI->ODR ^= (1U << 1);
        RTC->ISR &= ~(1U << 8); // Clear the flag
    }
    EXTI -> PR |= EXTI_PR_PR17; // Clear pending interrupt flag
}

What is wrong with my code and what to do to toggle led at the 5th second of each minute? Furthermore, I have some doubts about RTC->TR and RTC->DR registers. What happened if I don't make any adjustments on them and how do they work? When and why we should use them? Thx in advance for your response.

[MM..1]

Maybe better is ask, what you need? 5s toggle LED or RTC Alarm.

5s toggle LED can be more effective handled with PWM TIM = complete MCU offload = no one instruction after start required.

[waclawek.jan]

2U << 7 = 256

2U << 8 = 512

32768Hz / (256 * 512) = 0.25Hz

I don't think this is what you wanted.

It's in fact 0.5Hz, because the asynchronous prescaler has only 7 bits.

JW