MC9S12HZ256 Datasheet, PDF (411/692 Pages) Freescale Semiconductor, Inc – HCS12 Microcontrollers

Chapter 13 Serial Communication Interface (SCIV4)
In Figure 13-18, a large burst of noise is perceived as the beginning of a start bit, although the test sample
at RT5 is high. The RT5 sample sets the noise flag. Although this is a worst-case misalignment of
perceived bit time, the data samples RT8, RT9, and RT10 are within the bit time and data recovery is
successful.
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 13-18. Start Bit Search Example 3
Figure 13-19 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 13-19. Start Bit Search Example 4
MC9S12HZ256 Data Sheet, Rev. 2.04
Freescale Semiconductor
411