mc68hc908qb4 Freescale Semiconductor, Inc, mc68hc908qb4 Datasheet - Page 117

• Manufacturer Part Number: mc68hc908qb4
• Description: M68hc08 Microcontrollers
• Manufacturer: Freescale Semiconductor, Inc

To verify a stop bit and to detect noise, recovery logic takes samples at RT8, RT9, and RT10.
summarizes the results of the stop bit samples.
13.3.3.4 Framing Errors
If the data recovery logic does not detect a 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.
13.3.3.5 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 tolerance is much more than the degree of misalignment
that is likely to occur.
As the receiver samples an incoming character, it resynchronizes the RT clock on any valid falling edge
within the character. Resynchronization within characters corrects misalignments between transmitter bit
times and receiver bit times.
Freescale Semiconductor
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 1s following a
successful start bit verification, the noise flag (NF) is set and the receiver
assumes that the bit is a start bit.
RT8, RT9, and RT10 Samples
RT8, RT9, and RT10 Samples
000
001
010
011
100
101
110
111
000
001
010
011
100
101
110
111
Table 13-3. Stop Bit Recovery
Table 13-2. Data Bit Recovery
MC68HC908QB8 Data Sheet, Rev. 2
NOTE
Data Bit Determination
Framing Error Flag
0
0
0
1
0
1
1
1
1
1
1
0
1
0
0
0
Noise Flag
Noise Flag
0
1
1
1
1
1
1
0
0
1
1
1
1
1
1
0
Functional Description
Table 13-3
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