M68ICS08GR datasheet(245/408 Pages) FREESCALE

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Serial Communications Interface (SCI)

Functional Description

MC68HC908GR8 — Rev 4.0

Technical Data

MOTOROLA

Serial Communications Interface (SCI)

245

NOTE:

The RT8, RT9, and RT10 samples do not affect start bit verification. If

any or all of the RT8, RT9, and RT10 start bit samples are logic 1s

following a successful start bit verification, the noise flag (NF) is set and

the receiver assumes that the bit is a start bit.

To verify a stop bit and to detect noise, recovery logic takes samples at

RT8, RT9, and RT10. Table 18-4 summarizes the results of the stop bit

samples.

18.5.3.4 Framing Errors

If the data recovery logic does not detect a logic 1 where the stop bit

should be in an incoming character, it sets the framing error bit, FE, in

SCS1. A break character also sets the FE bit because a break character

has no stop bit. The FE bit is set at the same time that the SCRF bit is

set.

18.5.3.5 Baud Rate Tolerance

A transmitting device may be operating at a baud rate below or above

the receiver baud rate. Accumulated bit time misalignment can cause

one of the three stop bit data samples to fall outside the actual stop bit.

Then a noise error occurs. If more than one of the samples is outside the

stop bit, a framing error occurs. In most applications, the baud rate

Table 18-4. Stop Bit Recovery

RT8, RT9, and RT10

Samples

Framing

Error Flag

Noise Flag

000

001

010

011

100

101

110

111

Freescale Semiconductor, Inc.

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