Support for RTC

[dirkbr]

It would be nice to have support for the internal RTC of the STM32F103 series.

[salkinium]

What features would the implementation require? What is your use-case for using the RTC?

[salkinium]

Ideally the API would implement the libc and libc++ interfaces for time and date, right? So the interface is kinda already defined, which is nice.

[dirkbr]

A libc and libc++ API would be perfect! One use-case is a mobile data logger, that doesn't have a permanent power supply.

[salkinium]

Note that modm is not designed for doing anything fancy with power (neither sleeping, nor purely interrupt based operations, nor non-volatile storage, nor partial power operations), so check if you can meet your power budget before relying on modm too much.

[StevenMacias]

I am interested on contributing to this area. My use case is to set up DateTime in the STM32 using a communication protocol like RS422 and then being able to retrieve a timestamp with millisecond resolution. This would be handy when storing data logs in flash.

I was thinking on using two separate variables for this purpose:

• A 32 bit variable for storing a Unix timestamp
• A 32 bit variable for storing milliseconds calculated from the SubSeconds register.

Similar to the timeval in here: https://www.daemon-systems.org/man/timespec.3.html

I am currently working on the G4 family but probably the driver can be used for other families too. How and where should I start this journey? I have contributed before with examples and fixes but this would be my first time with an actual driver contribution at modm. Any advice and recommendation is more than welcome. Thanks!

[salkinium]

You would create a new peripheral in src/modm/platform/rtc/stm32 with a module.lb file that only enables the G4 family for now (we can always extend it later), then add you C++ implementation there.
I would not worry too much about the interface, you can make it STM32 specific, the use-cases that consume the RTC data are probably so diverse that they would need a translation layer anyways.

[salkinium]

You'll probably also need to look into the RCC implementation since the low-speed clock inputs are not regularly used, so there may be some code missing to connect it to the RTC peripheral.

[StevenMacias]

Thank you so much for the insights! I will start digging into it soon.