Analysis of 5 clock sources of STM32 microcontroller

one. Explanation of clock system block diagram

1. 5 clock sources of STM32

a. High-speed internal clock HSI, about 8MHz

b. High-speed external clock HSE, the crystal oscillator value range is 4M--16M, generally 8M crystal oscillator is used

c. Low-speed external clock LSE, 32.768k crystal oscillator, can be used as the clock source of RTC

d. Low-speed internal clock LSI, about 40KHz

e. PLL phase locked loop clock

2. System Clock

3 sources of system clock SYSCLK

a. HSI

b. HSE

c. PLL

3. RTC clock source

a. LSE

b. LSI

c. 128 frequency division of LSE

4. Independent watchdog IWDGCLK clock source

LSI

5. The source of the USB clock USBCLK

The maximum frequency is 48MHz

The PLL is divided by 1 or 1.5 through the prescaler

6. The internal clock output MCO, which is a pin PA8 of the STM32 chip

a. SYSCLK

b. HSI

c. HSE

d. PLL clock divided by 2

7. APB1 bus clock, up to 36M

8. APB2 bus clock, up to 72M

APB, APB1, APB2 provide clock for peripherals

All clocks have clock enable and peripherals can only be used when the clock is on.

Configuration of the clock

See "STM32 Chinese Reference Manual"

three. System clock system initialization important functions

SystemInit();

Its function is to initialize the system clock and initialize the system clock to a default state. This function will be called automatically after the system is started.

In the startup_stm32f10x_hd.s file:

Reset_Handler PROC

EXPORT Reset_Handler [WEAK]

IMPORT __main

IMPORT SystemInit

LDR R0, =SystemInit //Execute the SystemInit() function first

BLX R0

LDR R0, =__main //execute the main() function again

BX R0

ENDP

This function defines the system clock as 72MHz by default

#define SYSCLK_FREQ_72MHz 72000000

The state of the system after initialization:

SYSCLK 72MHz

AHB 72MHz

PCLK1 36MHz

PCLK2 72MHz

PLL 72MHz

Four. System clock configuration call function

RCC_HSE_ConfiguraTIon();

Note: When enabling any clock, wait for the clock to stabilize before continuing.

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