ST STM32F301 6 Series Reference Manual page 735

RM0366
Modbus/ASCII
In this mode, the end of a block is recognized by a specific (CR/LF) character sequence.
The USART manages this mechanism using the character match function.
By programming the LF ASCII code in the ADD[7:0] field and by activating the character
match interrupt (CMIE=1), the software is informed when a LF has been received and can
check the CR/LF in the DMA buffer.
26.5.9 USART parity control
Parity control (generation of parity bit in transmission and parity checking in reception) can
be enabled by setting the PCE bit in the USART_CR1 register. Depending on the frame
length defined by the M bits, the possible USART frame formats are as listed in
M bits
00
00
01
01
10
10
1. Legends: SB: start bit, STB: stop bit, PB: parity bit. In the data register, the PB is always taking the MSB
position (9th, 8th or 7th, depending on the M bits value).
Even parity
The parity bit is calculated to obtain an even number of "1s" inside the frame of the 6, 7 or 8
LSB bits (depending on M bits values) and the parity bit.
As an example, if data=00110101, and 4 bits are set, then the parity bit is 0 if even parity is
selected (PS bit in USART_CR1 = 0).
Odd parity
The parity bit is calculated to obtain an odd number of "1s" inside the frame made of the 6, 7
or 8 LSB bits (depending on M bits values) and the parity bit.
As an example, if data=00110101 and 4 bits set, then the parity bit is 1 if odd parity is
selected (PS bit in USART_CR1 = 1).
Parity checking in reception
If the parity check fails, the PE flag is set in the USART_ISR register and an interrupt is
generated if PEIE is set in the USART_CR1 register. The PE flag is cleared by software
writing 1 to the PECF in the USART_ICR register.
Universal synchronous/asynchronous receiver transmitter (USART/UART)
Table 104. Frame formats
PCE bit
0
1
0
1
0
1
RM0366 Rev 5
(1)
USART frame
| SB | 8-bit data | STB |
| SB | 7-bit data | PB | STB |
| SB | 9-bit data | STB |
| SB | 8-bit data | PB | STB |
| SB | 7-bit data | STB |
| SB | 6-bit data | PB | STB |
Table 104.
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