Calculate STM32 clocks from the roots

[TomSaw]

There are hardcoded clock constants in each of **/board.hpp althought the required information to calculate the truth is available!

[TomSaw]

Request for a 1th review @salkinium.

[TomSaw]

You are now empowered to control the clocks from PLL factors and prescalers for blue-pill and f411ve-disco.
Before diving deeper, please confirm my design decisions. Further improvements may be added to the Rcc namespace but let's be happy with pll's and prescalers for now.

Rcc::AhbPrescaler does not behave as expected

When selecting anything but Div1 or Div2 for Rcc::AhbPrescaler, the F103-blue-bill refuses to work. f411-disco keeps working but the Timer frequencies (scoped in hardware) don't scale like expected. I've matched the various intermediate clocks using static_assert with what is shown in CubeMX for the same configuration and seems to be all fine 🤨.

Propably, another side effect enters the scene when going beyond Div1, Div2 with Rcc::AhbPrescaler.

Because all the boards use an uninspired Div1 the glitch shall be tolerated for now.
I'll add a comment at least.

[TomSaw]

To associate prescaler values to their registers, i've borrowed this construct from modm::platform::GeneralPurposeTimer::signalToChannel<...>(...) but using a switch operator.

[salkinium]

Sure, but I'd rather use a normal constexpr function like uint8_t Rcc::Apb1Div(Rcc::Apb1Prescaler pre) {...}, then it can also be used at runtime if necessary.

[TomSaw]

of course!

[salkinium]

Hm, so what I would want is to move the common pattern of

// Enable clock source
Rcc::enableExternalCrystal();
// Enable PLL
Rcc::enablePll(Rcc::PllSource::ExternalCrystal, pllFactors);
Rcc::setFlashLatency<Frequency>();
Rcc::enableSystemClock(Rcc::SystemClockSource::Pll);
// Set Bus Prescaler
Rcc::setAhbPrescaler(Rcc::AhbPrescaler::Div1);
Rcc::setApb1Prescaler(Rcc::Apb1Prescaler::Div2);
Rcc::setApb2Prescaler(Rcc::Apb2Prescaler::Div1);

into a helper class defined in modm and configured by the user:

template< OneConfigStruct? >
struct BaseSystemClock
{
// Computes the basic frequencies with static_assert < max freq

// Adds all the peripheral frequencies via modm-devices?

// has a basic enable() function that does the above always correctly
};

The BSP would then inherit from that for their SystemClock and perhaps overwrite the one of the other thing for customization.

// simple case
using SystemClock = BaseSystemClock< ConfigStruct >;
// advances case
struct SystemClock : public BaseSystemClock< ConfigStruct >;
{
// customize
};

This would probably cover 90% of use-cases.
Maaaany (8) years ago I tried something like that before, but I did it way too complicated. So please do it simpler if you're interested in that.

[TomSaw]

That't some good guidiance! You're way more convenient with the STM architecture than me, so i blindly follow your suggestions in this topic.

// simple case
using SystemClock = BaseSystemClock< ConfigStruct >;
// advances case
struct SystemClock : public BaseSystemClock< ConfigStruct >;
{
// customize
};

Thats simple and users can opt in to vary their clocks per application.
I go for this.