ST72344K2 datasheet(136/247 Pages) STMICROELECTRONICS

136 / 247 page

On-chip peripherals

ST72344xx, ST2345xx

136/247

Doc ID 12321 Rev 6

Overrun error

An overrun error occurs when a character is received and RDRF has not been reset.

Data cannot be transferred from the shift register to the RDR register until the RDRF bit

is cleared.

When a 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

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 (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

application software when the first valid byte is received.

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