Ensure HSE crystal oscillator operation on F030

[sw]

On the PY32F030, the RCC_ECSCR register has a field HSE_FREQ that needs to be set for the crystal oscillator to operate. That field is not present on the F072.

Being new to Rust, I'm not sure this is the right way to do it. I would like to use the Hertz type, but that doesn't seem to be possible in a range pattern. Feel free to suggest improvements or modify it yourself.

[sw]

Is there a way to make the code depend on the presence of set_hse_freq instead of using #[cfg?

[decaday]

Thanks for the contribution! The current implementation looks good.

If you've tested this on PY32F030, would you mind adding an example to demonstrate and help with testing?
(I don't have any F030 Board with a HSE, but I have F072, F403 boards with it)

[decaday]

Is there a way to make the code depend on the presence of set_hse_freq instead of using #[cfg?

We keep all clock control operations in the py32_hal::init() function to prevent peripheral clock errors that could occur if RCC operations happen after peripheral initialization. The use of #[cfg] is appropriate here I think.

[sw]

Rebased and added an example. I was planning to combine the MCO and HSE in an example, but you're right that it's probably best to keep the examples very simple.

[decaday]

Thanks!