
RM0008
Universal synchronous asynchronous receiver transmitter (USART)
Doc ID 13902 Rev 12
779/1096
Figure 286. Break detection in LIN mode vs. Framing error detection
27.3.9
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.
The USART allows the user to control a 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 will or will not be generated during the last valid data bit
(address mark). The CPOL bit in the USART_CR2 register allows the user to select the
clock polarity, and the CPHA bit in the USART_CR2 register allows the user to select the
phase of the external clock (see
).
During idle, 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 time).
Case 1: break occurring after an Idle
IDLE
data2 (0x55)
data 1
data 3 (header)
In these examples, we suppose that LBDL=1 (11-bit break length), M=0 (8-bit data)
RX line
RXNE / FE
LBD
1 data time
1 data time
Case 1: break occurring while a data is being received
data 2
data2 (0x55)
data 1
data 3 (header)
RX line
RXNE / FE
LBD
1 data time
1 data time
BREAK
BREAK