Electronic – How to give several classes access to the single RTC on the MCU

cembeddedrtcstm32

I'm writing my first large-ish C++ embedded program on a STM32F413. I've become a reasonably firm believer in object oriented coding but am not particularly knowledgeable. The STM32F413 has one RTC but almost all my classes need to time stamp events. I have a class for each of my sensors like GPS, pressure, pH, humidity and so on. Every time a new piece of data comes in from any sensor, it needs to get time stamped. I'd like to write a RTC class and give many other classes access to the RTC date and time through a single public "getDateTime" method. The only way I know how to do this is with a singleton. Is there a better way?

Thanks in advance

Best Answer

If you decide to use a class paradigm, then your choice is really between having a member (or instance) function of a singleton, vs. having a static (or class) method.

If you feel it makes sense to simply read the RTC hardware each time, and you don't need to track any useful RTC related state in software, then you can simply go with simplicity of a static or class method. Call it from anywhere, it grabs some kind of hardware lock (probably buried in a C-language HAL library), reads the hardware, releases the lock, and returns the value.

But maybe you conclude you do need to track state in an object instance. Or maybe you'd like that mutex to be owned by an instance - in this case, you can legitimately say "hey, there's a single instance of the RTC hardware, there should be a single instance of the class as well" and go with a singleton pattern.

Of course there's a third choice, too... you don't have to use an OOP paradigm for the RTC. Either in C++, or by calling an extern "C" method defined in a C source file it's perfectly legitimate to just have code which interacts with the hardware - if you use a vendor HAL, you're likely to end up with that under the covers, even if you put an OOP wrapper on top.

I believe you'll find that on larger systems, system time is usually either something that breaks the OOP model, or else in a language that is fundamentally object oriented, it tends to be a static or class method. I can't off the top of my head recall a singleton case; though there are examples of IPC based system services (in Android for example) where you access certain resources by first getting a unique instance of a class to do the local end of the IPC.

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Electronic – STM32 RTC alarm not configuring for the second time

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:

We needed to put the system in low power mode when not in use
To enable the low power mode, we had to call this function RCC_APB1PeriphResetCmd(RCC_APB1Periph_PWR, ENABLE);
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].

Write ‘0xCA’ into the RTC_WPR register.

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;
  }
}

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