Serial Communication Interface (S12SCIV5)
Start Bit
LSB
No Start Bit Found
RXD
Samples
1
1
1
1
1
1
1
1
1
0
0
1
1
0
0
0
0
0
0
0
0
RT Clock
RT Clock Count
Reset RT Clock
Figure 11-26. Start Bit Search Example 5
In Figure 11-27, a noise burst makes the majority of data samples RT8, RT9, and RT10 high. This sets the
noise flag but does not reset the RT clock. In start bits only, the RT8, RT9, and RT10 data samples are
ignored.
Start Bit
LSB
RXD
Samples
1
1
1
1
1
1
1
1
1
0
0
0
0
1
0
1
RT Clock
RT Clock Count
Reset RT Clock
Figure 11-27. Start Bit Search Example 6
11.4.6.4 Framing Errors
If the data recovery logic does not detect a logic 1 where the stop bit should be in an incoming frame, it
sets the framing error flag, FE, in SCI status register 1 (SCISR1). A break character also sets the FE flag
because a break character has no stop bit. The FE flag is set at the same time that the RDRF flag is set.
11.4.6.5 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 (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.
S12P-Family Reference Manual, Rev. 1.13
Freescale Semiconductor
391
FREESCALE [ Freescale ]
