CH32Vxxx system.c overview

system_ch32vxxx.c

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The system.c file manages the system clock configuration.

• The system clock (SYSCLK) is the main frequency source for the CPU core.
• This clock can be derived from either an internal oscillator (HSI) or an external crystal oscillator (HSE).
• Often, a Phase-Locked Loop (PLL) is used to multiply the base clock frequency to reach higher speeds.
• Proper clock configuration ensures correct peripheral operation and timing accuracy.

Key Variables and Macros

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• SystemCoreClock A global variable holding the current core clock frequency (Hz). It is updated whenever the clock setup changes. Your code and HAL use it for delay calculations and peripheral timing.
• SYSCLK_FREQ_* macros Preprocessor definitions like SYSCLK_FREQ_96MHz_HSE select the target clock frequency and source. In your file, only one should be uncommented to choose the system clock configuration. Example:

#define SYSCLK_FREQ_96MHz_HSE  96000000

• HSI_VALUE, HSE_VALUE Constants defining the frequency of internal and external oscillators, typically 8 MHz or 32 MHz depending on the chip variant.

Functions

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SystemInit(void)

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• Called early during MCU reset/startup (usually from startup assembly).
• Resets RCC (clock control) registers to a known state.
• Enables the internal oscillator (HSI) as a safe default.
• Calls SetSysClock() to configure and switch the system clock based on selected macro.
• Prepares Flash memory interface and bus prescalers suitably.

SystemCoreClockUpdate(void)

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• Reads current clock configuration registers.
• Calculates the active core clock frequency based on oscillator source, PLL multiplier, and bus prescalers.
• Updates the global SystemCoreClock variable.
• Useful to call after runtime clock changes to refresh your software’s knowledge of CPU frequency.

SetSysClock(void) (static helper)

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• Dispatches to one of many SetSysClockTo...() functions depending on which frequency macro is active.
• Ensures the correct clock setup function is called based on your frequency choice (e.g., SetSysClockTo96_HSE()).

SetSysClockToXX_YYY() functions

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• There are many functions like SetSysClockTo96_HSE(), SetSysClockTo72_HSI() that configure:
  • Enabling and stabilizing the source oscillator (HSI or HSE).
  • Configuring and enabling the PLL with appropriate multipliers.
  • Setting bus prescalers for AHB, APB1, APB2 to safe clocks.
  • Switching SYSCLK source to PLL output.
  • Waiting for clock source switch to complete.
• Each function targets a specific frequency and clock source combination.

Summary Table

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Practical Notes

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• Only one SYSCLK_FREQ_* macro should be active at a time.
• If your board has an external crystal (HSE), prefer using it for better accuracy.
• PLL allows running at higher clock rates derived from base oscillators.
• Bus prescalers ensure peripherals run safely within their speed limits.
• If no macro is selected, MCU defaults to using the internal oscillator (HSI).
• Use SystemCoreClock variable in your code for timing functions.
• Call SystemCoreClockUpdate() if you change clock settings at runtime.

Example Usage

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At the very start of your program or in startup code, SystemInit() is invoked to configure your desired system clock frequency. Later, if clock settings change, SystemCoreClockUpdate() can be called to keep track of current CPU speed.

int main(void) {
    SystemInit();
    // Your application code here

    while(1){
        // Application loop
    }
}