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I’m working with an STM32F042F4Px microcontroller and want to implement a USB full-speed (12 Mbps) device.

According to the datasheet, the chip offers:

  • HSI48: internal 48 MHz RC oscillator with USB SOF auto-trim
  • HSE: optional external crystal

Can the STM32F042F4Px’s internal HSI48 oscillator (with USB SOF auto-trim) meet this requirement and be selected as the USB clock in STM32CubeMX/HAL to drive the USB interface, or is an external crystal mandatory to comply with the USB specification?

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1 Answer 1

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Quotes from website of the STM32F042F4 MCU:

  • One Full-Speed device (crystal-less)
  • USB 2.0 full-speed interface, able to run from internal 48 MHz oscillator

Datasheet chapter on USB confirms one of the clock sources can be the internal oscillator in automatic trimming mode, using SOF signaling for synchronization which allows crystal-less operation.

Safe to say the USB will work just fine with the internal oscillator.

Another thing is, what else you might need to do and if you need to do it without USB and without crystal, but USB does not need crystal.

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