ST72321AR datasheet(103/189 Pages) STMICROELECTRONICS

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ST72321

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SERIAL COMMUNICATIONS INTERFACE (Cont’d)

10.6.4.3 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 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 61).

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 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 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.

Break Character

When a break character is received, the SPI 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

RDR 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 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. Normal data bits are considered valid if

three consecutive samples (8th, 9th, 10th) have

the same bit value, otherwise the NF flag is set. In

the case of start bit detection, the NF flag is set on

the basis of an algorithm combining both valid

edge detection and three samples (8th, 9th, 10th).

Therefore, to prevent the NF flag getting set during

start bit reception, there should be a valid edge de-

tection as well as three valid samples.

When noise is detected in a frame:

– The NF flag 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 flag is reset by a SCISR register read op-

eration followed by a SCIDR register read opera-

tion.

During reception, if a false start bit is detected (e.g.

8th, 9th, 10th samples are 011,101,110), the

frame is discarded and the receiving sequence is

not started for this frame. There is no RDRF bit set

for this frame and the NF flag is set internally (not

accessible to the user). This NF flag is accessible

along with the RDRF bit when a next valid frame is

received.

Note: If the application Start Bit is not long enough

to match the above requirements, then the NF

Flag may get set due to the short Start Bit. In this

case, the NF flag may be ignored by the applica-

tion software when the first valid byte is received.

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