Specifications

Serial Communications (V40)

Buffer Ready) bit is set to a logical 1 when a character is transferred

into the SRB. Application software uses RBRDY to determine if a

character is available. RBRDY is automatically reset when the

character is read from the SRB.

The PE (Parity Error), OVE (Overrun Error) and FE (Framing Error)

bits flag data communication errors. The PE bit is set to a logical 1 if

a character is received into the SRB with a mismatch between the

parity bit and the character itself. The test for parity can be enabled

and disabled using the Serial Mode register.

An overrun error occurs when a character is written to the STB before

the character currently in the buffer is transmitted. The overrun error

is flagged with a logical 1 in the OVE bit. Avoid overrun errors by

having the application software test the TBRDY bit before writing the

character. A second type of overrun occurs if a character is received

into the SRB before the character currently in the buffer is read by the

application program. This type of overrun is avoided if the application

software reads the character before another is received. This can be

done on a polled basis by testing the RBRDY flag at a frequency

greater than the rate in which the characters are transferred or by

using the SCU interrupt capabilities to signal the presence of a

character to be read.

The FE bit is set to a logical 1 to flag a third type of error called a

framing error. A framing error signals a missing stop bit. This

happens when a break sequence is detected or when the programmed

baud rate is not the same as device on the other end of the serial link.

Once set to flag an error, the PE, OVE and FE bits remain set until a

logical 1 is written to the Error Clear bit of the Serial Command

The BKD bit is set to a logical 1 when a break sequence is detected.

A break sequence is detected when a complete character, from the

start bit through the stop bit, is received with RxD in a low state. The

BRK bit is reset when RxD returns to a high state or the SCU is reset.

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