Chapter 15 Serial Communication Interface (S12MC9S12XDP512V5)
MC9S12XDP512 Data Sheet, Rev. 2.11
700 Freescale Semiconductor
In Figure 15-22 the verification samples RT3 and RT5 determine that the first low detected was noise andnot the beginning of a start bit. The RT clock is reset and the start bit search begins again. The noise flagis not set because the noise occurred before the start bit was found.

Figure 15-22. Start Bit Search Example 1

In Figure 15-23, verification sample at RT3 is high. The RT3 sample sets the noise flag. Although theperceived bit time is misaligned, the data samples RT8, RT9, and RT10 are within the bit time and datarecovery is successful.

Figure 15-23. Start Bit Search Example 2

Reset RT Clock
RT1
RT1
RT1
RT1
RT2
RT3
RT4
RT5
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
110111100000
LSB
0 0
Reset RT Clock
RT1
RT1
RT1
RT1
RT1
RT1
RT2
RT3
RT4
RT5
RT6
RT7
RT8
RT11
RT10
RT9
RT14
RT13
RT12
RT2
RT1
RT16
RT15
RT3
RT4
RT5
RT6
RT7
Samples
RT Clock
RT Clock Count
Actual Start Bit
RXD
1111110000
LSB
00
Perceived Start Bit