Figure 297. Word Length Programming - ST STM32F405 Reference Manual

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USART character description

Word length may be selected as being either 8 or 9 bits by programming the M bit in the

USART_CR1 register (see

The TX pin is in low state during the start bit. It is in high state during the stop bit.

An Idle character is interpreted as an entire frame of "1"s followed by the start bit of the

next frame which contains data (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 either 1 or 2 stop bits (logic "1" bit) to acknowledge the

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 297. Word length programming

9-bit word length (M bit is set), 1 Stop bit

8-bit word length (M bit is reset), 1 Stop bit

RM0090 Rev 18

** LBCL bit controls last data clock pulse

** LBCL bit controls last data clock pulse

Next data frame

Next data frame

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