ST STM32F301 6 Series Reference Manual page 717

RM0366
26.5.1 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: The 7-bit mode is supported only on some USARTs. In addition, not all modes are
supported in 7-bit data length mode. Refer to
additional information.
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/UART)
RM0366 Rev 5
Figure 277).
Section 26.4: USART implementation for
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