RM0090: USART minimum attainable baudrate

[waclawek.jan]

Posted on June 29, 2017 at 17:15

The mantissa in USART_BRR is 12-bit i.e. its maximum value is 4095. That limits accordingly the minimum attainable baudrate for given APB frequency.

The following values in example tables are thus invalid and those baudrates (1200/2400Baud, quite common in various environments) can't be achieved at those particular APB-clock setups.

Found out the hard way - on a 16-sampling USART1 at the maximum APB2 the USART supposed to work at 1200 Baud was working at almost 19200 Baud, as the mantisa of baudrate divider was 0x117 instead of the required 0x1117.

Instead of those lengthy and quite useless tables, those limits should be highlighted in the UM, and formulas to calculate the limits and the errors should be given; and then a more detailed explanation should go to a well-crafted AN, which can then house the extensive tables, too, and even more. My 2 eurocents.

JW

[Khouloud GARSI]

Posted on June 30, 2017 at 11:58

Hi

Waclawek.Jan

‌,

Your feedback and suggestions were highlighted internally: The issue is confirmed and will be corrected on the next versions of the reference manual.

Thanks a lot for bringing this to our attention and sorry for any inconvenience this may have caused.

Khouloud.

[waclawek.jan · Accepted solution]

Posted on July 25, 2017 at 11:27

Fixed in RM0090 rev.15.

JW

[Khouloud GARSI]

Posted on July 26, 2017 at 16:23

Hi Jan,

Thank you very much for your continuous contribution to the enhancement of our STM32 resources.

We really appreciate this

Khouloud.