Serial Communications Interface Module (SCI)
Functional Description
MC68HC908GP32
•
MC68HC08GP32
—
Rev. 6
Technical Data
MOTOROLA
Serial Communications Interface Module (SCI)
253
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 18-7
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.
Figure 18-7. Slow Data
For an 8-bit character, data sampling of the stop bit takes the receiver
9 bit times
×
16 RT 10 RT cycles = 154 RT cycles.
With the misaligned character shown in
Figure 18-7
, the receiver counts
154 RT cycles at the point when the count of the transmitting device is
9 bit times
×
16 RT 3 RT cycles = 147 RT cycles.
The maximum percent difference between the receiver count and the
transmitter count of a slow 8-bit character with no errors is
For a 9-bit character, data sampling of the stop bit takes the receiver
10 bit times
×
16 RT 10 RT cycles = 170 RT cycles.
MSB
STOP
RT
1
RT
2
RT
3
RT
4
RT
5
RT
6
RT
7
RT
8
RT
9
RT10
RT11
RT12
RT13
RT14
RT15
RT16
DATA
SAMPLES
RECEIVER
RT CLOCK
154
147
–
154
--------------------------
100
×
4.54%
=
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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