Real Time Clock in STM32 | VIDEO 40 

Tldr: sadly there is no replacement for a dedicated external crystal with a tight tolerance. That's the nature of internal rc based oscillators. I have seen the difference between internal rtc and external crystal. I think that in that case the internal one was gaining millisecond values, reading it with its wakeup interrupt for highest precision. Using external crystal basically stalled this rising and left values very still even in millisecond precision. I do not know of a feature that would mitigate this problem in rtc peripheral. If you find one I would be grateful for a comment of a solution. Hope this helps.

Hi. What I would do is keep the rtc running and just read the time on request. Let's say for a stopwatch. I would implement a pushbutton interrupt and when it's pressed I would read the time and store it as "start". I would also save the current "stopwatch state as "start" or "begin". Then on the next press I would save the time into "stop" and calculate the difference. Also give that interrupt higher priority so that from the time of pressing the button and reading the current time takes as little as possible. Should not be that hard to implement. Also, dont forget to read the date as well, even if you won't use it, just call the read function on a struct. The peripheral has an internal locking mechanism that prevents further date and time updates in the registers before both time and date were read. Good idea, but people miss that so the time is never updated again after first read and you might think that you messed something up. Hope this helps.

Thank you, I have fixed "sDate.Date = 22" and added weekday "sDate.WeekDay = RTC_WEEKDAY_MONDAY" for example.

I already have some similar parts to build a good electronic load, I'll make a video on the project in the future.

Also, I connected serial port via PuTTy but when I run the code, the putty is not showing anything. Did I miss something?

In the video you can see only external system clock configured. This one is not used for rtc peripheral. What I failed to mention is the clock tree and the rtc is by default connected to internal clock generator. For best results use an external 32k oscillator. You can configure it in the RCC tab. To use serial port, make sure to connect and configure the correct u(s)art peripheral. On the f4 discovery board, uart is not connected to the built in debugger, but some boards do. Make sure to read the board manual. In case of this discovery board I used external usb to serial converter attached to pins seen in the CubeMx window. Let me know if this answered your question.

I would use rtc for time keeping. Maybe use an interrupt triggered by rtc every 1s to decrement the time. You can have a global flag that can disable the countdown when needed. Driver for 7 segment display should not be that difficult to implement, you would just need to use more of them. However I would use (SN)7446 driver that decodes bcd code on 4 bits to 7 segment output. That way you just apply the right combination of bcd pins as they translate to number position and you're set. You will however need at least 4 gpio outputs for each 7 segment display, but should be straightforward to implement. Best of success!

@ I think this is correct, would you be able to make a tutorial? I had thought of a ds3231 for rtc

I salvaged one from a rtc module that was defective, the circuit is just an inverter with two resistors and two capacitors, look it up online