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Does somebody know what is 'usart_ker_ckpres' in USART BRR description?


Hi all,

Just read RM0440, the section is USART Baudrate generator. There is the formula contatins input never met in all other document. (found 3 times, all of them are on the screenshot).



What happened with ST technical writers? I never met cases like that 10 years before.


ST Employee

Hello @remixoff_cr,

Thank you for pointing out this issue.

usart_ker_ckpres should be usart_ker_ck_pres, which is the peripheral input clock divided by a programmed prescaler.

I highlighted your request to the appropriate team in internal ticket number (ID 168184) for correction and improve this part.

(PS: internal ticket number (ID 168184) is not accessible or usable by customers).

Thanks for your contribution.


When your question is answered, please close this topic by clicking "Accept as Solution".

The F1 was simpler.

It is the instance clock, so nationally APB1, APB2, or the peripheral clock source routing depending on which the specific U(S)ART is plumbed too.

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There's also an additional prescaler within UART itself, so that's why usart_ker_ck (which comes from RCC) is not the same as usart_ker_ck_pres .

What's still wrong - and with increasing complexity gets worse - with the STM32 RMs is, that the individual peripherals chapters are written strictly from the perspective of separate IPs, and the Interconnect chapter - which should connect them all - is severely lacking. In this case, it's either RCC chapter, or the Interconnect chapter, or both, which is to explain, that uart_ker_ck is output from primary and derived clock sources of RCC through muxes controlled by RCC_CCIPR.[LP]U[S]ARTxSEL bitfields; and, ideally, also description of those bitfields in RCC chapter could contain references to usart_ker_ck.