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 addHSE_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 capacitor 8MHz
  (so PLL_M is set to 8 and HSE_VALUE set to 8000000) and
  F411 without external capacitor (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 </2>	APB2 CLOCK in MHz </1>	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