MC68HC08AS32A Datasheet, PDF (182/296 Pages) Motorola, Inc – Microcontrollers

Freescale Semiconductor, Inc.
Serial Communications Interface (SCI)
To verify a stop bit and to detect noise, recovery logic takes samples at RT8, RT9,
and RT10. Table 13-4 summarizes the results of the stop bit samples.
Table 13-4. Stop Bit Recovery
RT8, RT9, and RT10
Samples
000
001
010
011
100
101
110
111
Framing
Error Flag
1
1
1
0
1
0
0
0
Noise Flag
0
1
1
1
1
1
1
0
13.4.3.4 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.
13.4.3.5 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.
Slow Data Tolerance
Figure 13-8 shows how much a slow received character can be misaligned without
causing a noise error or a framing error. The slow stop bit begins at RT8 instead of
RT1 but arrives in time for the stop bit data samples at RT8, RT9, and RT10.
For an 8-bit character, data sampling of the stop bit takes the receiver
9 bit times × 16 RT cycles + 10 RT cycles = 154 RT cycles.
With the misaligned character shown in Figure 13-8, the receiver counts 154 RT
cycles at the point when the count of the transmitting device is
9 bit times × 16 RT cycles + 3 RT cycles = 147 RT cycles.
Data Sheet
182
Serial Communications Interface (SCI)
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MC68HC08AS32A — Rev. 1
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