How to properly set clock speed for STM32F4xx devices

I see that you have problems with your devices when you don’t know even (and you don’t even ask) on which speed your device is actually running. Speed of your device depends on PLL settings or clock source you have selected for system core clock.

In most cases system core clock’s source is PLL output. To get proper value, you check ALWAYS first these settings when something is not working. Because I’m tired of questions that something is not working but then your MCU clock is totally failed.

STM32F4xx series MCUs have ALL 16MHz RC oscillator inside which can be used for PLL input clock. It is not so accurate but it works just well if you don’t have connected external clock.

External clock is probably your crystal you have connected. In this example we will talk in case that you have external crystal connected.

If your clock is not properly set, then you have problems with everything. USB not working, USART baudrate failed, timer’s clock not OK…

This content has only sense, if you use Standard Peripheral Drivers and not HAL drivers from ST!

To be able to get your MCU to full speed, you have to do the following changes:

  • In file stm32F4xx.h set HSE_VALUE to match your crystal frequency (In Hz)
    • In case of any STM32F4xx Discovery board, select HSE_VALUE = 8000000
    • In other case, you can set this to your crystal
    • If you have an option for global defines in your compiler, then add HSE_VALUE=8000000 to your defines.
    • If you don’t use external crystal or clock, then you don’t need to change anything
  • In file system_stm32f4xx.c set PLL_M value to match your crystal frequency (In MHz)
    • In case of any STM32F4xx Discovery board, select PLL_M = 8
    • This will divide input clock with 8 to get 1MHz on the input for PLL
    • If you don’t use external clock, then this value MUST be set to 16, because internal RC will be used for PLL
  • Now you can expect top speed for your device.

Some informations about Nucleo boards:

  • Nucleo boards don’t use external clock by default, but they have pins for adding external crystal.
  • In case you don’t add external crystal or clock, then set PLL_M value to 16
  • I have at home set F401 Nucleo board with external crystal 8MHz (so PLL_M is set to 8 and HSE_VALUE set to 8000000) and F411 without external crystal (PLL_M set to 16)

STM32F4xx Clocks

Proper PLL settings to get maximum clocks (except of PLL_M parameter everything should be already defined in system_stm32f4xx.c file).

SystemCoreClock = ((INPUT_CLOCK (HSE_OR_HSI_IN_HZ) / PLL_M) * PLL_N) / PLL_P

STM32F4xx MAX CLOCK in MHz APB1 CLOCK in MHz APB2 CLOCK in MHz PLL_M PLL_N PLL_P
STM32F401 84 42 84 Must match input frequency in MHz (HSE or HSI) 336 4
STM32F411 100 50 100 Must match input frequency in MHz (HSE or HSI) 400 4
STM32F405 168 42 84 Must match input frequency in MHz (HSE or HSI) 336 2
STM32F407 168 42 84 Must match input frequency in MHz (HSE or HSI) 336 2
STM32F415 168 42 84 Must match input frequency in MHz (HSE or HSI) 336 2
STM32F417 168 42 84 Must match input frequency in MHz (HSE or HSI) 336 2
STM32F427 180 45 90 Must match input frequency in MHz (HSE or HSI) 360 2
STM32F429 180 45 90 Must match input frequency in MHz (HSE or HSI) 360 2
STM32F437 180 45 90 Must match input frequency in MHz (HSE or HSI) 360 2
STM32F439 180 45 90 Must match input frequency in MHz (HSE or HSI) 360 2
STM32F446 180 45 90 Must match input frequency in MHz (HSE or HSI) 360 2

Settings for STM32F4xx boards

This settings should be used in case of any STM32F4xx boards used.

STM32F4xx MAX CLOCK in MHz APB1 CLOCK in MHz APB2 CLOCK in MHz DEFAULT PLL INPUT CLOCK PLL_M PLL_N PLL_P
STM32F401-Discovery 84 42 84 HSE 8MHz 8 336 4
STM32F411-Discovery 100 50 100 HSE 8MHz 8 400 4
STM32F4-Discovery 168 42 84 HSE 8MHz 8 336 2
STM32F429-Discovery 180 45 90 HSE 8MHz 8 360 2
Nucleo F401 84 42 84 HSI 16MHz HSE_VALUE / 1MHz in case of external crystal or 16 in case of internal HSI RC oscillator 336 4
Nucleo F411 100 50 100 HSI 16MHz HSE_VALUE / 1MHz in case of external crystal or 16 in case of internal HSI RC oscillator 400 4

 

tilz0R

Owner of this site. Application engineer, currently employed by STMicroelectronics. Exploring latest technologies and owner of different libraries posted on Github.

You may also like...

Read before commenting!

Before you make a new comment, make sure you agree with things listed below:

  • - Read post to make sure if it is already posted what you are asking for,
  • - Make sure you have the latest version of libraries used in your project,
  • - Make a clean and grammatically correct written message,
  • - Report as many details as possible, including what have you done so far,
  • - Do NOT post any code here. Use Pastebin,
  • - Do NOT post any error codes here. Use Pastebin,
  • - Specify STM32Fxxx family and used Discovery/EVAL/Nucleo or custom made board,
  • - Make sure your clock is set correct for PLL,
  • - If you are using my HAL drivers, please check this post how to start.
Comment will be deleted on breaking these rules without notification!