Mbed Default Crystal HSE or HSI?


When selecting a board in the compiler and for example loading a simple blinky program, is the board using the internal oscillator HSI or the external oscillator HSE?

Hello Muhammed,

By default, NUCLEO boards use the HSE clock signal coming from the STLink programmer/debugger attached to the board.
For any target board you can find the related info in the https://github.com/ARMmbed/mbed-os/blob/master/targets/targets.json file:

  • Search for you board and then for “clock_source”. You should find something like:
            "clock_source": {
                "help": "Mask value : USE_PLL_HSE_EXTC | USE_PLL_HSE_XTAL (need HW patch) | USE_PLL_HSI",
                "value": "USE_PLL_HSE_EXTC|USE_PLL_HSI",
                "macro_name": "CLOCK_SOURCE"
  • If no “clock_source” is defined for the given target board then it is using the “inherited” value. Check what value is defined for the “inherited” target.

Best regards, Zoltan