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Confusion about the Crystal on Nucleo_F7xxx boards

Question asked by Kristof Mulier on Dec 14, 2016
Latest reply on Jan 23, 2017 by Kristof Mulier

The Nucleo_F767ZI board has 4 crystals onboard:

  • X1:     8 MHz      connected to the JTAG-chip on the STLink
  • X2:     32 kHz    connected to STM32F767ZI for real time clock
  • X3:     Not soldered
  • X4:     25 MHz    connected to STM32F767ZI for HSE input clock

From this information, it is only logical to conclude that the 25 MHz crystal is connected to HSE input of the microcontroller. However, CubeMX fills in 8 MHz by default for the Nucleo board. It looks like that is plain wrong:

Defautl clock settings in CubeMX

 

I currently work with mbed-OS from ARM on this Nucleo board. I use the official mbed version that you can clone from github. I have noticed that this OS uses the ST HAL under the hood to access the chip hardware. So I don't use CubeMX to generate any code. Neverhteless, I discovered that the value of the crystal is also wrong in the HAL running under mbed-OS. Look at these files:

 system_stm32f7xx.c  ( mbed-os/targets/TARGET_STM/TARGET_STM32F7/

                       TARGET_NUCLEO_F767ZI/device/system_stm32f7xx.c )

#if !defined (HSE_VALUE)

    #define HSE_VALUE ((uint32_t)8000000) /*!< Default value of the External oscillator in Hz */
#endif /* HSE_VALUE */

 

 stm32f7xx_hal_conf.h  ( mbed-os/targets/TARGET_STM/TARGET_STM32F7/

                         TARGET_NUCLEO_F767ZI/device/stm32f7xx_hal_conf.h )

#if !defined (HSE_VALUE)

    #define HSE_VALUE 8000000U /*!< Value of the External oscillator in Hz */
#endif /* HSE_VALUE */

 

Naturally I fixed the bug by changing  HSE_VALUE  from 8 MHz to 25 MHz:

    #define HSE_VALUE ((uint32_t)25000000)

 

Unfortunately, this messes up just about everything. My SPI clocks fall down to a much lower frequency, and the mbed serial logger outputs nonsense characters. Probably because the baud rate is no longer correct.

 

So how can I correct this bug?

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