update HC32 timer rates

[shadow578]

Description

As per the comment by @sjasonsmith in #26845 (comment), i've taken a closer look at the timer rates set in the HC32 HAL and updated them according to the comment.

1. F_CPU is now constexpr and matches HCLK, the rate of the main CPU core. The comment has been updated accordingly.
2. CYCLES_PER_MICROSECOND was removed to fallback to the default.
3. HAL_TIMER_RATE remains unchanged since it's used in the calculation for STEPPER_TIMER_RATE. The comment has been updated accordingly.

Requirements

HC32-based printer

Benefits

Timing on HC32 should now function closer to what Marlin actually expects.
While i haven't observed a difference in e.g. print quality, this should still be a good thing.

Configurations

N/A

Related Issues

N/A

[thinkyhead]

I added a compile-time warning about the hard-coded CPU clock, in prep for some future HC32 at a different clock rate.

Would it make sense to define sysclock this way?

constexpr uint32_t sysclock = F_CPU;

[shadow578]

Hi @thinkyhead ,

sadly some of the changes you've made may be incorrect:

1. constexpr uint32_t sysclock = F_CPU; is not correct: sysclock is the output of the PLL that HCLK (the actual F_CPU) is derived from. While both are set to 200 MHz, it may be confusing to set sysclk to F_CPU, since it's what defined sysclk.
2. same for HAL_TIMER_RATE, it too is derived from sysclock, not HCLK / F_CPU.
3. the warning "HC32 clock is assumed to be 200MHz. If this isn't the case for your board please submit a report so we can add support." doesn't really make sense in the context of HC32. On HC32, all clocks are derived from a PLL, with a fallback to a internal RC oscillator. Thus, it will never exist a board where the clock cannot be made to be 200 MHz.

Should another PR be created to update this ?

---

As an aside, i've attached a marked up block diagram of the HC32s clock circuitry (Section 4.2.1 of The Reference Manual).
Marked in yellow is the (happy-) path using a external 8 / 16 MHz crystal oscillator as PLL input. Magenta is the fallback PLL input using the HRC (16 MHz internal RC oscillator).

[thisiskeithb]

Should another PR be created to update this ?

Since this PR is already merged, another will need to be submitted with your corrections.