Freescale Semiconductor MC9S08PT60, Справочное руководство

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Accumulated bit time misalignment can cause one of the three stop bit data samples
(RT8, RT9, and RT10) to fall outside the actual stop bit. A noise error will occur if the
RT8, RT9, and RT10 samples are not all the same logical values. A framing error will
occur if the receiver clock is misaligned in such a way that the majority of the RT8, RT9,
and RT10 stop bit samples are a logic zero.
As the receiver samples an incoming frame, it re-synchronizes the RT clock on any valid
falling edge within the frame. Resynchronization within frames will correct a
misalignment between transmitter bit times and receiver bit times.
15.4.5.1 Slow data tolerance
Figure 15-36 shows how much a slow received frame 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.
MSB
R
T1
R
T2
R
T3
R
T4
R
T5
R
T6
R
T7
R
T8
R
T9
R
T1
0
R
T1
1
R
T1
2
R
T1
3
R
T1
4
R
T1
5
R
T1
6
DATA
SAMPLES
RECEIVER
RT CLOCK
STOP
Figure 15-36. Slow data
For an 8-bit data and 1 stop bit character, data sampling of the stop bit takes the receiver
9 bit times x 16 RT 10 RT cycles =154 RT cycles.
With the misaligned character shown in Figure 15-36 , the receiver counts 154 RT cycles
at the point when the count of the transmitting device is 9 bit times x 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 data and 1 stop bit character with no errors is:
((154 - 147) / 154) x 100 = 4.54%
For a 9-bit data or 2 stop bits character, data sampling of the stop bit takes the receiver 10
bit times x 16 RT 10 RT cycles = 170 RT cycles.
With the misaligned character shown in Figure 15-36 , the receiver counts 170 RT cycles
at the point when the count of the transmitting device is 10 bit times x 16 RT 3
RT cycles = 163 RT cycles.
The maximum percent difference between the receiver count and the transmitter count of
a slow 9-bit or 2 stop bits character with no errors is:
Functional description
MC9S08PT60 Reference Manual, Rev. 4, 08/2014
436 Freescale Semiconductor, Inc.