UM0306 Universal synchronous asynchronous receiver transmitter (USART)
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new data is received during the reading sequence (between the USART_SR register read
access and the USART_DR read access).
Noise error
Over-sampling techniques are used (except in synchronous mode) for data recovery by
discriminating between valid incoming data and noise.
Figure 147. Data sampling for noise detection
Table 50. Noise detection from sampled data
When noise is detected in a frame:
● The NE is set at the rising edge of the RXNE bit.
● The invalid data is transferred from the Shift register to the USART_DR register.
● No interrupt is generated in case of single byte communication. However this bit rises
at the same time as the RXNE bit which itself generates an interrupt. In case of multi-
buffer communication an interrupt will be issued if the EIE bit is set in the USART_CR3
register.
The NE bit is reset by a USART_SR register read operation followed by a USART_DR
register read operation.
Framing error
A framing error is detected when:
The stop bit is not recognized on reception at the expected time, following either a de-
synchronization or excessive noise.
Sampled Value NE Status Received bit value Data Validity
000 0 0 Valid
001 1 0 Not Valid
010 1 0 Not Valid
011 1 1 Not Valid
100 1 0 Not Valid
101 1 1 Not Valid
110 1 1 Not Valid
111 0 1 Valid
RX LINE
Sample
clock
1234567891011
12
13 14 15 16
sampled values
One bit time
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7/16
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