Understanding the Clock System of STM32F4

Oct 13, 2025 by wbrum.org 143 views

Introduction

When working with microcontrollers, such as the STM32F4 series, configuring the clock system is a crucial step in ensuring proper operation. The STM32F4 series uses a complex clock system, which can be challenging to understand and configure. In this article, we will discuss the frequency of APB1 on STM32F4 and whether it must always be set to 42 MHz.

Understanding the Clock System of STM32F4

The STM32F4 series uses a multi-layered clock system, which consists of several clock domains. The clock domains are:

SYSCLK: The system clock, which is the main clock that drives the entire system.
HCLK: The bus clock, which is the clock that drives the peripherals.
PCLK: The peripheral clock, which is the clock that drives the peripherals.
APB1: The APB1 clock, which is one of the two APB (Advanced Peripheral Bus) clocks.

The clock system of STM32F4 is configured using the RCC (Reset and Clock Control) module. The RCC module provides several registers that can be used to configure the clock system.

Configuring the Clock System of STM32F4

To configure the clock system of STM32F4, you need to follow these steps:

Select the SYSCLK source: The SYSCLK source can be one of the following:
- HSE (High Speed External Oscillator)
- HRI (High Speed Internal Oscillator)
- LSI (Low Speed Internal Oscillator)
- PLL (Phase-Locked Loop)
Configure the PLL: The PLL is used to generate the SYSCLK from the HSE, HRI, or LSI clock. The PLL can be configured to generate a clock frequency that is a multiple of the input clock frequency.
Configure the HCLK: The HCLK is the bus clock that drives the peripherals. The HCLK can be configured to be a multiple of the SYSCLK.
Configure the PCLK: The PCLK is the peripheral clock that drives the peripherals. The PCLK can be configured to be a multiple of the HCLK.
Configure the APB1: The APB1 is one of the two APB clocks. The APB1 can be configured to be a multiple of the PCLK.

Does the Frequency of APB1 on STM32F4 Have to Stay at 42 MHz?

The frequency of APB1 on STM32F4 does not have to stay at 42 MHz. The APB1 frequency can be configured to be a multiple of the PCLK. However, the APB1 frequency must be a multiple of the PCLK, and the PCLK must be a multiple of the HCLK.

Why Does the APB1 Frequency Have to be a Multiple of the PCLK?

The APB1 frequency has to be a multiple of the PCLK because the APB1 clock is used to drive the peripherals that are connected to the APB1 bus. The peripherals that are connected to the APB1 bus require a clock frequency that is a multiple of the PCLK.

Why Does the PCLK Have to be a Multiple of the HCLK?

The PCLK has to be a multiple of the HCLK because the HCLK is the bus clock that drives the peripherals. The peripherals that are connected to the HCLK bus require a clock frequency that is a multiple of the HCLK.

Why Does the HCLK Have to be a Multiple of the SYSCLK?

The HCLK has to be a multiple of the SYSCLK because the SYSCLK is the main clock that drives the entire system. The peripherals that are connected to the HCLK bus require a clock frequency that is a multiple of the SYSCLK.

Conclusion

In conclusion, the frequency of APB1 on STM32F4 does not have to stay at 42 MHz. The APB1 frequency can be configured to be a multiple of the PCLK, and the PCLK must be a multiple of the HCLK, and the HCLK must be a multiple of the SYSCLK.

Troubleshooting UART Issues

If you are experiencing issues with the UART, it may be due to the APB1 frequency not being a multiple of the PCLK. To troubleshoot this issue, you can try the following:

Check the APB1 frequency: Make sure that the APB1 frequency is a multiple of the PCLK.
Check the PCLK frequency: Make sure that the PCLK frequency is a multiple of the HCLK.
Check the HCLK frequency: Make sure that the HCLK frequency is a multiple of the SYSCLK.
Check the UART configuration: Make sure that the UART is properly configured.

Example Code

Here is an example code that demonstrates how to configure the clock system of STM32F4:

#include "stm32f4xx.h"
void RCC_Configuration(void)
{
// Select the SYSCLK source
RCC->CFGR = RCC_CFGR_SW_HSE;
// Configure the PLL
RCC-&gt;CFGR = RCC_CFGR_PLLSRC_HSE | RCC_CFGR_PLLMUL_4 | RCC_CFGR_PLLDIV_2;

// Enable the PLL
RCC-&gt;CR |= RCC_CR_PLLON;

// Wait for the PLL to lock
while (!(RCC-&gt;CR &amp; RCC_CR_PLLRDY));

// Select the PLL as the SYSCLK source
RCC-&gt;CFGR = RCC_CFGR_SW_PLL;

// Configure the HCLK
RCC-&gt;CFGR = RCC_CFGR_HPRE_DIV_1;

// Configure the PCLK
RCC-&gt;CFGR = RCC_CFGR_PPRE1_DIV_2;

// Configure the APB1
RCC-&gt;CFGR = RCC_CFGR_PPRE2_DIV_2;

}
int main(void)
{
// Configure the clock system
RCC_Configuration();
// Configure the UART
// ...

// Main loop
while (1)
{
    // ...
}

}

Note that this is just an example code, and you may need to modify it to suit your specific requirements.

Conclusion

Q: What is the default frequency of APB1 on STM32F4?

A: The default frequency of APB1 on STM32F4 is 42 MHz.

Q: Why does the APB1 frequency have to be a multiple of the PCLK?

A: The APB1 frequency has to be a multiple of the PCLK because the APB1 clock is used to drive the peripherals that are connected to the APB1 bus. The peripherals that are connected to the APB1 bus require a clock frequency that is a multiple of the PCLK.

Q: Why does the PCLK have to be a multiple of the HCLK?

A: The PCLK has to be a multiple of the HCLK because the HCLK is the bus clock that drives the peripherals. The peripherals that are connected to the HCLK bus require a clock frequency that is a multiple of the HCLK.

Q: Why does the HCLK have to be a multiple of the SYSCLK?

A: The HCLK has to be a multiple of the SYSCLK because the SYSCLK is the main clock that drives the entire system. The peripherals that are connected to the HCLK bus require a clock frequency that is a multiple of the SYSCLK.

Q: Can I change the frequency of APB1 on STM32F4?

A: Yes, you can change the frequency of APB1 on STM32F4. However, the APB1 frequency must be a multiple of the PCLK, and the PCLK must be a multiple of the HCLK, and the HCLK must be a multiple of the SYSCLK.

Q: How do I configure the clock system of STM32F4?

A: To configure the clock system of STM32F4, you need to follow these steps:

Select the SYSCLK source: The SYSCLK source can be one of the following:
- HSE (High Speed External Oscillator)
- HRI (High Speed Internal Oscillator)
- LSI (Low Speed Internal Oscillator)
- PLL (Phase-Locked Loop)
Configure the PLL: The PLL is used to generate the SYSCLK from the HSE, HRI, or LSI clock. The PLL can be configured to generate a clock frequency that is a multiple of the input clock frequency.
Configure the HCLK: The HCLK is the bus clock that drives the peripherals. The HCLK can be configured to be a multiple of the SYSCLK.
Configure the PCLK: The PCLK is the peripheral clock that drives the peripherals. The PCLK can be configured to be a multiple of the HCLK.
Configure the APB1: The APB1 is one of the two APB clocks. The APB1 can be configured to be a multiple of the PCLK.

Q: What are the benefits of configuring the clock system of STM32F4?

A: Configuring the clock system of STM32F4 provides several benefits, including:

Improved performance: Configuring the clock system of STM32F4 can improve the performance of the microcontroller by allowing it to operate at a higher clock frequency.
Reduced power consumption: Configuring the clock system of STM32F4 can reduce power consumption by allowing the microcontroller to operate at a lower clock frequency.
Improved reliability: Configuring the clock system of STM32F4 can improve reliability by reducing the risk of clock-related errors.

Q: What are the common mistakes to avoid when configuring the clock system of STM32F4?

A: Some common mistakes to avoid when configuring the clock system of STM32F4 include:

Not selecting the correct SYSCLK source: Failing to select the correct SYSCLK source can result in incorrect clock frequencies and errors.
Not configuring the PLL correctly: Failing to configure the PLL correctly can result in incorrect clock frequencies and errors.
Not configuring the HCLK correctly: Failing to configure the HCLK correctly can result in incorrect clock frequencies and errors.
Not configuring the PCLK correctly: Failing to configure the PCLK correctly can result in incorrect clock frequencies and errors.

Q: How do I troubleshoot issues with the clock system of STM32F4?

A: To troubleshoot issues with the clock system of STM32F4, you can follow these steps:

Check the clock configuration: Check the clock configuration to ensure that it is correct.
Check the clock frequencies: Check the clock frequencies to ensure that they are correct.
Check the PLL configuration: Check the PLL configuration to ensure that it is correct.
Check the HCLK configuration: Check the HCLK configuration to ensure that it is correct.
Check the PCLK configuration: Check the PCLK configuration to ensure that it is correct.

Q: What are the best practices for configuring the clock system of STM32F4?

A: Some best practices for configuring the clock system of STM32F4 include:

Use a consistent clock configuration: Use a consistent clock configuration throughout the project.
Use a clock configuration that is easy to understand: Use a clock configuration that is easy to understand and maintain.
Use a clock configuration that is flexible: Use a clock configuration that is flexible and can be easily modified.
Use a clock configuration that is reliable: Use a clock configuration that is reliable and can be trusted.