RM0312 datasheet(237/275 Pages) STMICROELECTRONICS

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RM0312

Universal synchronous/asynchronous receiver transmitter (USART)

249

Note:

The figures specified in Table 47 and Table 48 may slightly differ in the special case when

the received frames contain some Idle frames of exactly 10-bit times when M=0 (11-bit times

when M=1).

21.3.6

Parity control

Parity control (generation of parity bit in transmission and parity checking in reception) can

be enabled by setting the PCEN bit in the USART_CR1 register. Depending on the frame

length defined by the M bit, the possible USART frame formats are as listed in Table 49.

Legends: SB: Start Bit, STB: STOP bit, PB: Parity bit

Note:

In case of wakeup by an address mark, the MSB bit of the data is taken into account and not

the parity bit

Even parity: the parity bit is calculated to obtain an even number of “1s” inside the frame

made of the 7 or 8 LSB bits (depending on whether M is equal to 0 or 1) and the parity bit.

Ex: data=00110101; 4 bits set => parity bit will be 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 7 or 8 LSB bits (depending on whether M is equal to 0 or 1) and the parity bit.

Example: data=00110101; 4 bits set => parity bit will be 1 if odd parity is selected (PS bit in

USART_CR1 = 1).

Transmission: If the PCEN bit is set in USART_CR1 then the MSB bit of the data written in

the data register is not transmitted but is changed by the parity bit to give an even number of

‘1’s if even parity is selected (PS=0) or an odd number of ‘1’s if odd parity is selected

(PS=1).

Reception: If the parity check fails, the PE flag is set in the USART_SR register and an

interrupt is generated if the PIEN bit is set in the USART_CR1 register.

Table 48. USART receiver’s tolerance when USART_DIV[3:0] is different from 0

M bit

NF is an error

NF is don’t care

3.33%

3.88%

3.03%

3.53%

Table 49. Frame formats

M bit

PCEN bit

USART frame

| SB | 8 bit data | STB |

| SB | 7-bit data | PB | STB |

| SB | 9-bit data | STB |

| SB | 8-bit data PB | STB |

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