Electronic – STM32 RTC – how to update

We faced the same problem while working with RTC on my STM32F030 Controller. Our alarm interrupt triggered once, but it would never trigger again.

The problem was as follows:

1. We needed to put the system in low power mode when not in use
2. To enable the low power mode, we had to call this function RCC_APB1PeriphResetCmd(RCC_APB1Periph_PWR, ENABLE);
3. This function prevents write access to the DBP bit of register PWR_CR and it makes DBP bit = 0. This bit must be set before we can change any values of RTC registers (as mentioned in reference manual)

Reference manual RM0360 for STM32F030 and STM32F070 states that:

After system reset, the RTC registers are protected against parasitic write access by clearing the DBP bit in the PWR_CR register (refer to the power control section). DBP bit must be set in order to enable RTC registers write access. After RTC domain reset, all the RTC registers are write-protected. Writing to the RTC registers is enabled by writing a key into the Write Protection register, RTC_WPR. The following steps are required to unlock the write protection on all the RTC registers except for RTC_TAFCR and RTC_ISR[13:8].

1. Write ‘0xCA’ into the RTC_WPR register.

2. Write ‘0x53’ into the RTC_WPR register. Writing a wrong key reactivates the write protection. The protection mechanism is not affected by system reset.

We solved the problem by making following changes to the IRQ handler of RTC:

void RTC_IRQHandler(void)
{
   /*  Check line 17 has triggered the IT */
   if ((EXTI->PR & 0x20000) != 0)  
   {  
      /* Makes the DBP bit of PWR_CR register write accessible */
      RCC_APB1PeriphResetCmd(RCC_APB1Periph_PWR, DISABLE);

      /* Writes 1 in the DBP bit of PWR_CR register */
      PWR_BackupAccessCmd(ENABLE);



      /* Your code here ... */


      /* RTC registers are made write accessible */
      RTC_WriteProtectionCmd(DISABLE);

      /* Clear the RTC Alarm-A Flag */
      RTC_ClearFlag(RTC_FLAG_ALRAF);


      /* Makes the RTC registers read only */
      RTC_WriteProtectionCmd(ENABLE);

      /* Locks the DBP bit of PWR_CR register again */
      RCC_APB1PeriphResetCmd(RCC_APB1Periph_PWR, ENABLE);

      /* Clear the EXTI Line 17 Pending bit (Connected internally to RTC Alarm) */
      EXTI->PR |= 0x20000;
  }
}

This is actually a common technique for low power design. It is called DVFS, dynamic voltage and frequency scaling. However, I am not sure what all the implications are of changing the supply voltage while operating. You well have to make sure the switchover is glitch free at the least. Check the MCU specs for what frequencies are allowed at what voltages. So long as you choose one that works at both voltages, you should not have to change the clock settings. Everything will consume more power when running at the higher voltage, though.

What might be a better idea is to run everything at 1.8 volts and then have some power supply switching components to select the supply voltage for the radio, then use a level shifter chip to connect the radio to the rest of the circuitry. At those voltages, simple ORing diodes will have too much drop, so you will probably want back to back pairs of PMOS transistors or dedicated power supply switch chips. Don't forget that you can also control the enable pin on the regulator to turn that off as well.