Library 40- Output clocks from STM32F4

STM32F4 has capability to output different clocks on specific 2 pins. You are able to output clocks in original frequency (no prescaler) and also with prescaler values 2, 3, 4 and 5.

2 pins are used to output different frequencies. They are not able to output the same clocks. You can output:

  • MCO1
    • HSI: High Speed Internal oscillator, 16MHz RC
    • HSE: High Speed External oscillator, or External User clock
    • LSE: Low speed, 32768Hz oscillator or ceramic resonator
    • PLLCLK: Output clock from PLL
      • Differ between settings for PLL
  • MCO2
    • SYSCLK: System core clock output
    • PLLI2SCLK: Accurate clock for high-quality audio performance in I2S and SAI interfaces
    • HSE: High Speed External oscillator, or External User clock
    • PLLCLK: Output clock from PLL
      • Differ between settings for PLL

Each clock can be output with no prescaler, or value between 2 – 5.



  • Output different system clocks from STM32F4
  • 2 output pins supported


    • STM32F4xx
    • STM32F4xx RCC
    • STM32F4xx GPIO
  • TM
    • TM GPIO
    • defines.h
STM32F4xx MCOx Description
PA8 MCO1 MCO1 output pin
PC9 MCO2 MCO2 output pin

Functions and enumerations


Example below outputs:

  • HSI clock on pin MCO1 (PA8) with prescaler 2 = 16MHz / 2 = 8MHz
  • SYSCLK clock on pin MCO2 (PC9) with prescaler 4
    • On STM32F4-Discovery = 168MHz / 4 = 42MHz
    • On STM32F429-Discovery = 180MHz / 4 = 45MHz
    • On Nucleo F401RE = 84MHz / 4 = 21MHz

Project is available on Github, download library below.


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

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