DocID024597 Rev 3 1183/1693
RM0351 Universal synchronous asynchronous receiver transmitter (USART)
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36.5.1 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 USARTx_CR1 register (see Figure 384).
• 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 Autobaudrate (0x7F
and 0x55 frames detection) are not supported. 7-bit mode is supported only on some
USARTs.
In default configuration, 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 will include
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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