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tomjianchun
tomjianchun
Associate II
March 17, 2015
Question

two STM32 micro USART communication

  • March 17, 2015
  • 2 replies
  • 819 views
Posted on March 17, 2015 at 06:05

currently in my design, I have two STM32 micro communicate with USART at Baud rate of 115.2k, the trace length is around 1.5inch.

STM32F205 use HSE, external crystal resonator of 50PPM tolerance,

STM32F030 use HSE, external Murata ceramic resonator(0.5% tolerance, 0.2% temperature coefficient, 0.1% aging).

these two micro in same board, and share same VDD/VSS.

Does tolerance of STM32F030 ceramic resonator can meet the 115.2k bps USART communication ?

or what is the tolerance requirement for external clock for USART communication?

Thanks.

#usart-brg-jitter
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2 replies

jpeacock
jpeacock
Associate
March 18, 2015
Posted on March 18, 2015 at 13:26

The 0.5% tolerance isn't good, you will see some jitter in the signal.  You can try it but I'd suggest making sure there's a gap of two or more bit times between individual bytes (use USART TC status to make sure a byte is completely shifted out before sending the next byte top the transmitter).  That gives the USART receiver time to sync to the next start bit and not detect an overrun or framing error.  You might also try using two stop bits to add an inter-character gap.

You may still have to drop the baud rate if there's too much variation from the resonator.

  Jack Peacock

tomjianchun
tomjianchunAuthor
Associate II
March 20, 2015
Posted on March 20, 2015 at 10:37

then I can change to 0.1% tolerance ceramic resonator,

it is 0.1% tolerance ceramic resonator, and 0.13% temperature coefficient, and 0.07% aging , so totally is less than 0.3% tolerance,

Does these enough ?

Thanks.

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