Universal synchronous/asynchronous receiver transmitter (USART/UART) RM0440
1614/2126 RM0440 Rev 4
Note: The data specified in Table 345 and Table 346 may slightly differ in the special case when
the received frames contain some Idle frames of exactly 10-bit times when M bits = 00 (11-
bit times when M= 01 or 9- bit times when M = 10).
37.5.9 USART Auto baud rate detection
The USART can detect and automatically set the USART_BRR register value based on the
reception of one character. Automatic baud rate detection is useful under two
circumstances:
• The communication speed of the system is not known in advance.
• The system is using a relatively low accuracy clock source and this mechanism
enables the correct baud rate to be obtained without measuring the clock deviation.
The clock source frequency must be compatible with the expected communication speed.
• When oversampling by 16, the baud rate ranges from usart_ker_ck_pres/65535 and
usart_ker_ck_pres/16.
• When oversampling by 8, the baud rate ranges from usart_ker_ck_pres/65535 and
usart_ker_ck_pres/8.
Before activating the auto baud rate detection, the auto baud rate detection mode must be
selected through the ABRMOD[1:0] field in the USART_CR2 register. There are four modes
based on different character patterns. In these auto baud rate modes, the baud rate is
measured several times during the synchronization data reception and each measurement
is compared to the previous one.
Table 346. Tolerance of the USART receiver when BRR[3:0] is different from 0000
M bits
OVER8 bit = 0 OVER8 bit = 1
ONEBIT=0 ONEBIT=1 ONEBIT=0 ONEBIT=1
00 3.33% 3.88% 2% 3%
01 3.03% 3.53% 1.82% 2.73%
10 3.7% 4.31% 2.22% 3.33%
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