Datasheet

Functional Description
MC68HC812A4 Data Sheet, Rev. 7
Freescale Semiconductor 163
Figure 14-10 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.
Figure 14-10. Start Bit Search Example 4
Figure 14-11 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.
Figure 14-11. Start Bit Search Example 5
In Figure 14-12 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.
Figure 14-12. Start Bit Search Example 6
RESET RT CLOCK
RT1 RT1 RT1 RT1 RT1 RT1 RT1 RT1 RT1 RT1 RT2
RT3
RT4 RT7 RT6 RT5
RT10
RT9 RT8
RT14 RT13 RT12 RT11 RT15 RT16
RT1 RT2 RT3
SAMPLES
RT CLOCK
RT CLOCK COUNT
PERCEIVED AND ACTUAL START BIT
RXD
11111001
LSB
11 1 1
RESET RT CLOCK
RT1 RT1 RT1 RT1 RT1 RT1 RT1 RT1 RT1 RT1 RT2
RT3
RT4 RT7 RT6 RT5 RT1 RT1 RT1 RT1 RT1 RT1 RT1 RT1 RT1 RT1 RT1 RT1
SAMPLES
RT CLOCK
RT CLOCK COUNT
START BIT
RXD
11111010
LSB
11 1 1 1 0000000 0
NO START BIT FOUND
RESET RT CLOCK
RT1 RT1 RT1 RT1 RT1 RT1 RT1 RT1 RT1 RT1 RT2 RT3 RT4 RT7 RT6 RT5
RT10
RT9 RT8
RT14 RT13 RT12 RT11 RT15 RT16
RT1 RT2 RT3
SAMPLES
RT CLOCK
RT CLOCK COUNT
START BIT
RXD
11111000
LSB
11 1 1 0 110