Clock setup

[dbrgn]

Use the external clock, if it makes sense.

[dbrgn]

The LSE (Low Speed External) clock source is only used for certain use cases like low power timers, the RTC, etc. It can't be used as sysclock, it's only a secondary clock.

Right now we're using the HSI (High Speed Internal) clock at 16 MHz. I tried switching to MSI (Multi Speed Internal):

let mut rcc = dp.RCC.freeze(
    hal::rcc::Config::msi(hal::rcc::MSIRange::Range5) // ~2.097 MHz
);

This is also the default configuration after reset. However, with that config the timer (TIM6) could not be initialized anymore (it hangs). We should probably investigate what the problem is.

@rnestler what are tradeoffs of faster/slower clock speed? Slower speeds use less energy but take longer to get things done, right? How can we decide whether HSI or MSI with a slower clock is better?

Also, when does one use the PLL instead?

[dbrgn]

@rnestler ping!