ST72E331 datasheet(62/107 Pages) STMICROELECTRONICS

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ST72E331 ST72T331

SERIAL COMMUNICATIONS INTERFACE (Cont’d)

5.5.4.3 Receiver

The SCI can receive data words of either 8 or 9

bits. When the M bit is set, word length is 9 bits

and the MSB is stored in the R8 bit in the CR1 reg-

ister.

Character reception

During a SCI reception, data shifts in least signifi-

cant bit first through the RDI pin. In this mode, DR

ternal bus and the received shift register (see Fig-

ure 1.).

Procedure

– Select the M bit to define the word length.

– Select the desired baud rate using the BRR and

the ERPR registers.

– Set the RE bit, this enables the receiver which

begins searching for a start bit.

When a character is received:

– The RDRF bit is set. It indicates that the content

of the shift register is transferred to the RDR.

– An interrupt is generated if the RIE bit is set and

the I bit is cleared in the CCR register.

– The error flags can be set if a frame error, noise

or an overrun error has been detected during re-

ception.

Clearing the RDRF bit is performed by the following

software sequence done by:

1. An access to the SR register

2. A read to the DR register.

The RDRF bit must be cleared before the end of the

reception of the next character to avoid an overrun

error.

Break Character

When a break character is received, the SCI han-

dles it as a framing error.

Idle Character

When a idle frame is detected, there is the same

procedure as a data received character plus an in-

terrupt if the ILIE bit is set and the I bit is cleared in

the CCR register.

Overrun Error

An overrun error occurs when a character is re-

ceived when RDRF has not been reset. Data can

not be transferred from the shift register to the

TDR register as long as the RDRF bit is not

cleared.

When a overrun error occurs:

– The OR bit is set.

– The RDR content will not be lost.

– The shift register will be overwritten.

– An interrupt is generated if the RIE bit is set and

the I bit is cleared in the CCR register.

The OR bit is reset by an access to the SR register

followed by a DR register read operation.

Noise Error

Oversampling techniques are used for data recov-

ery by discriminating between valid incoming data

and noise.

When noise is detected in a frame:

– The NF is set at the rising edge of the RDRF bit.

– Data is transferred from the Shift register to the

DR register.

– No interrupt is generated. However this bit rises

at the same time as the RDRF bit which itself

generates an interrupt.

The NF bit is reset by a SR register read operation

followed by a DR register read operation.

Framing Error

A framing error is detected when:

– The stop bit is not recognized on reception at the

expected time, following either a de-synchroni-

zation or excessive noise.

– A break is received.

When the framing error is detected:

– the FE bit is set by hardware

– Data is transferred from the Shift register to the

DR register.

– No interrupt is generated. However this bit rises

at the same time as the RDRF bit which itself

generates an interrupt.

The FE bit is reset by a SR register read operation

followed by a DR register read operation.

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