MSC8144E Reference Manual, Rev. 3
20-14
Freescale
Semiconductor
UART
Note:
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
(logic 0).
To verify a stop bit and to detect noise, sample logic takes samples at RT8, RT9, and RT10. The
noise flag, NF, is set if not all samples have the same logical value (the same as for data bit
sampling). If the majority of the samples are logic 0 framing error flag, FE, is set. Table 20-6
summarizes the results of the stop bit samples.
In Figure 20-10 the start bit verification samples RT3 and RT5 determine that the first logic 0
detected is noise and not the beginning of a start bit. The RT counter is reset and the start bit
search resumes. The noise flag is not set because the noise occurred before the start bit was
found.
Table 20-6. Stop Bit Recovery
RT8, RT9, and RT10 Samples
Framing Error Flag
Noise Flag
000
1
0
001
1
1
010
1
1
011
0
1
100
1
1
101
0
1
110
0
1
111
0
0
Figure 20-10. Start Bit Search Example 1
Reset RT Counter
RT
1
RT
1
RT
1
RT
1
RT
2
RT
3
RT
4
RT
5
RT
1
RT
1
RT
2
RT
3
RT
4
RT
7
RT
6
RT
5
RT
1
0
RT
9
RT
8
RT
1
4
RT
1
3
RT
1
2
RT
1
1
RT
1
5
RT
1
6
RT
1
RT
2
RT
3
Samples
RT Clock
RT Clock Count
Start Bit
URXD
1
1
0
1
1
1 1
0
0
0
0
0
LSB
0
0
Содержание MSC8144E
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