ST7LITE39 datasheet(93/167 Pages) STMICROELECTRONICS

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ST7LITE3

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LINSCI

™ SERIAL COMMUNICATION INTERFACE (SCI Mode) (Cont’d)

11.5.5.3 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 SCICR1

Character reception

During a SCI reception, data shifts in least signifi-

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

SCIDR register consists or a buffer (RDR) be-

tween the internal bus and the received shift regis-

ter (see Figure 54).

Procedure

– Select the M bit to define the word length.

– Select the desired baud rate using the SCIBRR

and the SCIERPR 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[1:0] bits are 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 SCISR register

2. A read to the SCIDR register.

The RDRF bit must be cleared before the end of the

reception of the next character to avoid an overrun

error.

Idle Line

When an idle line 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[|1:0] bits are

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 an 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[|1:0] bits are cleared in the CCR register.

The OR bit is reset by an access to the SCISR reg-

ister followed by a SCIDR 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 character:

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

bit.

– Data is transferred from the Shift register to the

SCIDR 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 SCISR register read oper-

ation followed by a SCIDR 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

SCIDR 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 SCISR register read oper-

ation followed by a SCIDR register read operation.

Break Character

– When a break character is received, the SCI

handles it as a framing error. To differentiate a

break character from a framing error, it is neces-

sary to read the SCIDR. If the received value is

00h, it is a break character. Otherwise it is a

framing error.

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