Framing Errors; Baud Rate Tolerance - Motorola MC68HC908GP32 Technical Data Manual

Serial Communications Interface Module (SCI)
NOTE:

18.5.3.4 Framing Errors

18.5.3.5 Baud Rate Tolerance

Technical Data
252
Freescale Semiconductor, Inc.
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.
samples.
Table 18-4. Stop Bit Recovery
RT8, RT9, and RT10
Samples
000
001
010
011
100
101
110
111
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.
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
Serial Communications Interface Module (SCI)
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Table 18-4 summarizes the results of the stop bit
Framing
Error Flag
1
1
1
0
1
0
0
0
MC68HC908GP32
Noise Flag
0
1
1
1
1
1
1
0
MC68HC08GP32 Rev. 6
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