Datasheet

Enhanced Serial Communications Interface (ESCI) Module
MC68HC908QB8 Data Sheet, Rev. 3
116 Freescale Semiconductor
13.3.3.3 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 these times
(see Figure 13-6):
• After every start bit
• After the receiver detects a data bit change from 1 to 0 (after the majority of data bit samples at
RT8, RT9, and RT10 returns a valid 1 and the majority of the next RT8, RT9, and RT10 samples
returns a valid 0)
To locate the start bit, data recovery logic does an asynchronous search for a 0 preceded by three 1s.
When the falling edge of a possible start bit occurs, the RT clock begins to count to 16.
Figure 13-6. Receiver Data Sampling
To verify the start bit and to detect noise, data recovery logic takes samples at RT3, RT5, and RT7.
Table 13-1 summarizes the results of the start bit verification samples.
If start bit verification is not successful, the RT clock is reset and a new search for a start bit begins.
To determine the value of a data bit and to detect noise, recovery logic takes samples at RT8, RT9, and
RT10. Table 13-2 summarizes the results of the data bit samples.
Table 13-1. 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 RT1 RT1 RT1 RT1 RT1 RT1 RT1 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