ST STM32L4 5 Series Reference Manual page 1307

RM0351
40.5.1 USART character description
The word length can be selected as being either 7 or 8 or 9 bits by programming the M[1:0]
bits in the USART_CR1 register (see
•
7-bit character length: M[1:0] = 10
•
8-bit character length: M[1:0] = 00
•
9-bit character length: M[1:0] = 01
Note: In 7-bit data length mode, the Smartcard mode, LIN master mode and Auto baud rate (0x7F
and 0x55 frames detection) are not supported. 7-bit mode is supported only on some
USARTs.
By default, the signal (TX or RX) is in low state during the start bit. It is in high state during
the stop bit.
These values can be inverted, separately for each signal, through polarity configuration
control.
An Idle character is interpreted as an entire frame of "1"s (the number of "1"s includes the
number of stop bits).
A Break character is interpreted on receiving "0"s for a frame period. At the end of the
break frame, the transmitter inserts 2 stop bits.
Transmission and reception are driven by a common baud rate generator, the clock for each
is generated when the enable bit is set respectively for the transmitter and receiver.
The details of each block is given below.
Universal synchronous asynchronous receiver transmitter (USART)
Figure
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