MC68HC908GT16_07 Datasheet, PDF (157/292 Pages) Freescale Semiconductor, Inc – Microcontrollers

Functional Description
14.4.3.3 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 these times
(see Figure 14-7):
• 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.
START BIT
LSB
RxD
SAMPLES
START BIT
START BIT
DATA
QUALIFICATION VERIFICATION SAMPLING
RT
CLOCK
RT CLOCK
STATE
RT CLOCK
RESET
Figure 14-7. Receiver Data Sampling
To verify the start bit and to detect noise, data recovery logic takes samples at RT3, RT5, and RT7.
Table 14-2 summarizes the results of the start bit verification samples.
Table 14-2. Start Bit Verification
RT3, RT5, and RT7 Samples
000
001
010
011
100
101
110
111
Start Bit Verification
Yes
Yes
Yes
No
Yes
No
No
No
Noise Flag
0
1
1
0
1
0
0
0
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 14-3 summarizes the results of the data bit samples.
MC68HC908GT16 • MC68HC908GT8 • MC68HC08GT16 Data Sheet, Rev. 5.0
Freescale Semiconductor
157