STM32L USB clock from HSI

brunoeagle · ‎2013-09-12

Posted on September 13, 2013 at 00:42

I'm a little bit confused with clocking scheme of the USB peripheral in STM32L151.

On the datasheet, we see this affirmation, below the general clocking scheme of the microcontroller:

''For the USB function to be available, both HSE and PLL must be enabled, with the CPU running at either 24 MHz or 32 MHz.''

But, on the user manual, there isn't the same advice, all that i found was the description of PLL on 5.2.4, which tell us that both HSI or HSE can supply the PLL, and that this is whom clock the usb.

My question is: Is it really necessary to have the HSE running to use USB peripheral, or HSI + PLL is enough?

Thanks in advance!

#usb-stm32l

Tesla DeLorean · ‎2013-09-12

Posted on September 13, 2013 at 00:58

The HSI does not have the stability/accuracy required to meet the USB spec.

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dimitri · ‎2015-04-02

Posted on April 02, 2015 at 10:27

Well, in the STM32L151xE STM32L152xE datasheet, paragraph 2.2, it clearly says ''USB crystal-less''. And the errata does not seem to mention that it is a mistake. So these documents should certainly be updated.