ST72321R7-Auto datasheet(141/243 Pages) STMICROELECTRONICS

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ST72321xx-Auto

Serial communications interface (SCI)

Doc ID 13829 Rev 1

141/243

When an overrun error occurs:

●

The OR bit is set.

●

The RDR content is not lost.

●

The shift register is 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 register followed by a SCIDR register read

operation.

Noise error

Oversampling techniques are used for data recovery 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 detection 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 operation followed by a SCIDR register read

operation.

During reception, if a false start bit is detected (for example, 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

application software when the first valid byte is received.

See also Noise error causes on page 145.

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