RM0367 Rev 7 769/1043
RM0367 Universal synchronous/asynchronous receiver transmitter (USART/UART)
872
29.5.1 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 Figure 243).
• 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 Section 29.4: USART implementation for
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.
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