MC9S12P128 Datasheet, PDF (388/564 Pages) Freescale Semiconductor, Inc – S12 Microcontrollers

Serial Communication Interface (S12SCIV5)
Perceived Start Bit
Actual Start Bit
LSB
RXD
Samples 1 1 1 0
0
1
0000
RT Clock
RT Clock Count
Reset RT Clock
Figure 11-24. Start Bit Search Example 3
Figure 11-25 shows the effect of noise early in the start bit time. Although this noise does not affect proper
synchronization with the start bit time, it does set the noise flag.
Perceived and Actual Start Bit
LSB
RXD
Samples 1 1 1 1 1 1 1 1 1 0
1
0
RT Clock
RT Clock Count
Reset RT Clock
Figure 11-25. Start Bit Search Example 4
Figure 11-26 shows a burst of noise near the beginning of the start bit that resets the RT clock. The sample
after the reset is low but is not preceded by three high samples that would qualify as a falling edge.
Depending on the timing of the start bit search and on the data, the frame may be missed entirely or it may
set the framing error flag.
S12P-Family Reference Manual, Rev. 1.12
388
Freescale Semiconductor