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]

As first normal peaple dont waste capacity for learn every bits in ARM registers...

My tip only you dont setup mask seconds for alarm

    // 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);

try use LL STM32F439xx HAL User Manual: stm32f4xx_ll_rtc.h File Reference (upv.es)

#define 	LL_RTC_ALMA_MASK_NONE   0x00000000U
#define 	LL_RTC_ALMA_MASK_DATEWEEKDAY   RTC_ALRMAR_MSK4
#define 	LL_RTC_ALMA_MASK_HOURS   RTC_ALRMAR_MSK3
#define 	LL_RTC_ALMA_MASK_MINUTES   RTC_ALRMAR_MSK2
#define 	LL_RTC_ALMA_MASK_SECONDS   RTC_ALRMAR_MSK1
#define 	LL_RTC_ALMA_MASK_ALL   (RTC_ALRMAR_MSK4 | RTC_ALRMAR_MSK3 | RTC_ALRMAR_MSK2 | RTC_ALRMAR_MSK1)

usw

[Piranha]

It's better to use normal register names and definitions, not LL, because using LL doubles the amount of work. When reading reference manual and looking at registers with a debugger, one has to translate LL names to register names and back all the time. In the end it's just a useless waste of time and adds another layer of potential bugs and inefficiency...

[Piranha]

Rewrite the code using normal register defines.

And for a start - all modifications of registers RTC_ISR and EXTI_PR are broken.

[waclawek.jan]

> Rewrite the code using normal register defines.

+1

> And for a start - all modifications of registers RTC_ISR and EXTI_PR broken.

... in the interrupt (setting of INIT bit in RTC_ISR isOK), because status flags are not to be cleared by RMW operation.

> RTC->PRER |= ((2U << 7) - 1) << 16;

> RTC->PRER |= (2U << 8) - 1;

This is wrong, because

• the reset value of RTC_PRER is nonzero (OK in this particular case it does not matter, but generally it's wrong)
• this value results in RTC to run half as fast as it should, when using the standard 32.768kHz crystal - and this maybe is why you see LED blink once in 2.5sec rather than once in 5sec (I haven't checked rest of the code)

I wonder where do you have these values from, as I've seen them in other confused post, too.

JW

[Piranha]

RTC_ISR flag clearing code is broken because it will also clear other flags which are set by hardware (interrupts become pending) during the RMW operation.

[akoluacik]

@Community member​  I am using a book for these purposes(low-level coding), and it is best practice to use LSE clock, which has 32768 Hz frequency, or 32kHz. To have 1Hz signal, I adjusted the PRER value as in the book.

[akoluacik]

Why RTC_ISR and EXTI_PR broken? Because I set and reset the specific bits, and didn't make a change on any other bits?

[Tesla DeLorean]

Well doing a RMW action on EXTI_PR is apt to clear things unrelated to the bits you want.

Just write the mask for the bit(s) you actually own/manage at that point.

With register level code you likely need to own all the debugging, as very few want to spend hours walking thru others buggy code.

[Tesla DeLorean]

The LSI tends to be very unstable, and not suitable for powered down operation. Certainly in older STM32 the LSI was typically in the 37 KHz realm

A properly implemented LSE circuit with an appropriated loaded crystal should be a lot closer to 32.768 KHz and you'd pick the appropriate prescalers. The LSE should be testable via the export of the signal via a TAMPER or PA8/MCO pin, or via a TIM clocking via a different/better clock.