Universal synchronous/asynchronous receiver transmitter (USART/UART) RM0367
790/1043 RM0367 Rev 7
Figure 252. Break detection in LIN mode vs. Framing error detection
29.5.11 USART synchronous mode
The synchronous mode is selected by writing the CLKEN bit in the USART_CR2 register to
1. In synchronous mode, the following bits must be kept cleared:
• LINEN bit in the USART_CR2 register,
• SCEN, HDSEL and IREN bits in the USART_CR3 register.
In this mode, the USART can be used to control bidirectional synchronous serial
communications in master mode. The CK pin is the output of the USART transmitter clock.
No clock pulses are sent to the CK pin during start bit and stop bit. Depending on the state
of the LBCL bit in the USART_CR2 register, clock pulses are, or are not, generated during
the last valid data bit (address mark). The CPOL bit in the USART_CR2 register is used to
select the clock polarity, and the CPHA bit in the USART_CR2 register is used to select the
phase of the external clock (see Figure 253, Figure 254 and Figure 255).
During the Idle state, preamble and send break, the external CK clock is not activated.
In synchronous mode the USART transmitter works exactly like in asynchronous mode. But
as CK is synchronized with TX (according to CPOL and CPHA), the data on TX is
synchronous.
In this mode the USART receiver works in a different manner compared to the
asynchronous mode. If RE=1, the data is sampled on CK (rising or falling edge, depending
on CPOL and CPHA), without any oversampling. A setup and a hold time must be
respected (which depends on the baud rate: 1/16 bit duration).
MSv31157V1
data 1 IDLE BREAK data 2 (0x55) data 3 (header)
1 data time 1 data time
RX line
RXNE /FE
LBDF
Case 1: break occurring after an Idle
data 1 data2 BREAK data 2 (0x55) data 3 (header)
1 data time 1 data time
RX line
RXNE /FE
LBDF
Case 2: break occurring while data is being received
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