Query regarding STM32F070CBT6 internal clock source for USB

[Sarbjeet Singh]

Currently i am using USB peripheral for communication (CDC) with STM32F070CBT6 microcontroller. For generating 48MHz clock, i am using internal oscillator as a source for internal PLL circuit. I have to send certain data to STM32 (for save it to external EEPROM (on same PCB card on which STM32 is installed). The STM32 will then acknowledge it. Similarly, STM32 will send the data to PC when requested. So, data transfer is only request based. Practically, I have tested it several times and it is working.
But, now i have read the line from STM32F070CBT6 datasheet regarding USB as "It requires a precise 48 MHz clock which can be generated from the internal main PLL (the
clock source must use an HSE crystal oscillator)."
My Question is:- What consequences can occur if i continue with the internal source?

[STTwo-32]

Hello @Sarbjeet Singh 

The USB need a Very high accuracy clock which may be not guarantee using the HSI clock in some conditions (temperature,...). So, it is required to use the HSE crystal oscillator.

Best Regards.

STTwo-32 

[Tesla DeLorean]

>>Practically, I have tested it several times and it is working.

Yes, but get a few thousand devices out there, with all types of temperature and voltage variations, and equipment it is attached too, and you're likely to encounter some situations where it "doesn't work"

Costs of design/manufacture vs cost of support

You might want to make a value judgement based on testing you do on a selection of parts, and over the operational temperatures you want to warrant functionality.

[TDK]

Lots of other STM32 chips can do crystal-less USB, such as the STM32F0x2 series. Might be an option to use one of those chips instead.

https://www.st.com/resource/en/application_note/an4879-introduction-to-usb-hardware-and-pcb-guidelines-using-stm32-mcus-stmicroelectronics.pdf

 

[Sarbjeet Singh]

Thanks for the suggestion. It will be useful for my future designs.

[MKanc.1]

You can't continue using internal source as IDE does not permit it. If you modify source you will likely get disconnects, incorrect data etc.. on hardware. Who knows what you will get.

L4 series has internal clock as well as F042. I have some F042 but F070 might be cheaper overall. Buy a bag of crystals if PCB size does not matter. I personally bought both crystals and some cheap 20MHz osc from arrow.

Note F0 series are power-hungry. So it won't matter if oscillator consumes more power.

Crystal-less solutions uses host hardware signals to calibrate itself. I assume F07 does not have it.

[Sarbjeet Singh]

Thanks to all for your valuable reply. For, STM32F070CBT6 microcontroller, i will use the external oscillator.