Universal synchronous asynchronous receiver transmitter (USART) RM0033
634/1381 RM0033 Rev 9
24.3.1 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
Figure 224).
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
start bit.
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 224. Word length programming
MS19822V2
Bit0 Bit1 Bit2 Bit3 Bit4 Bit5 Bit6 Bit7 Bit8
Start
bit
Stop
bit
Next
Start
bit
Idle frame
9-bit word length (M bit is set), 1 Stop bit
Possible
Parity
bit
Break frame
Data frame
Next data frame
Clock
** LBCL bit controls last data clock pulse
**
Start
bit
Start
bit
Stop
bit
Bit0 Bit1 Bit2 Bit3 Bit4 Bit5 Bit6 Bit7
Start
bit
Stop
bit
Next
Start
bit
Idle frame
8-bit word length (M bit is reset), 1 Stop bit
Possible
Parity
bit
Break frame
Data frame Next data frame
Clock
** LBCL bit controls last data clock pulse
**
Start
bit
Start
bit
Stop
bit
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