Usart Character Description; Figure 229. Word Length Programming - STMicroelectronics STM32F05 series Reference Manual

Universal synchronous asynchronous receiver transmitter (USART)
25.5.1

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
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 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 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.

Figure 229. Word length programming

Clock
576/742
Figure
9-bit word length (M bit is set), 1 stop bit
Data frame
Start
Bit0 Bit1 Bit2
bit
Idle frame
Break frame
8-bit word length (M bit is reset), 1 stop bit
Data frame
Start
Bit1
Bit0
Bit
Clock
Idle frame
Break frame
Doc ID 018940 Rev 1
229).
Bit3 Bit4 Bit5 Bit6
Bit2 Bit3 Bit5 Bit6
Bit4
Possible
Next data frame
parity
bit Next
Start
Stop
Bit8
Bit7
bit
bit
**
Start
bit
Start
Stop
bit
bit
** LBCL bit controls last data clock pulse
Possible Next data frame
Parity
Bit
Next
start
Stop
Bit7
bit
bit
****
**
Start
bit
Start
Stop
bit
bit
** LBCL bit controls last data clock pulse
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