
Serial Communications Interface (SCI)
MC68HC908JL8/JK8 MC68HC08JL8/JK8 MC68HC908KL8 Data Sheet, Rev. 3.1
128
Freescale Semiconductor
9.4.3.3 Data Sampling
The receiver samples the RxD pin at the RT clock rate. The RT clock is an internal signal with a frequency
16 times the baud rate. To adjust for baud rate mismatch, the RT clock is resynchronized at the following
After every start bit
After the receiver detects a data bit change from logic 1 to logic 0 (after the majority of data bit
samples at RT8, RT9, and RT10 returns a valid logic 1 and the majority of the next RT8, RT9, and
RT10 samples returns a valid logic 0)
To locate the start bit, data recovery logic does an asynchronous search for a logic 0 preceded by three
logic 1s. When the falling edge of a possible start bit occurs, the RT clock begins to count to 16.
Figure 9-6. Receiver Data Sampling
To verify the start bit and to detect noise, data recovery logic takes samples at RT3, RT5, and RT7.
Table 9-2 summarizes the results of the start bit verification samples.
Start bit verification is not successful if any two of the three verification samples are logic 1s. If start bit
verification is not successful, the RT clock is reset and a new search for a start bit begins.
Table 9-2. Start Bit Verification
RT3, RT5, and RT7
Samples
Start Bit
Verification
Noise Flag
000
Yes
0
001
Yes
1
010
Yes
1
011
No
0
100
Yes
1
101
No
0
110
No
0
111
No
0
RT CLOCK
RESET
RT1
RT2
RT3
RT4
RT5
RT8
RT7
RT6
RT11
RT10
RT9
RT15
RT14
RT13
RT12
RT16
RT1
RT2
RT3
RT4
START BIT
QUALIFICATION
START BIT
VERIFICATION
DATA
SAMPLING
SAMPLES
RT
CLOCK
RT CLOCK
STATE
START BIT
LSB
RxD