
Functional Description
MC68HC908GR8 MC68HC908GR4 Data Sheet, Rev. 7
Freescale Semiconductor
169
18.4.3.2 Character Reception
During an SCI reception, the receive shift register shifts characters in from the PE1/RxD pin. The SCI data
register (SCDR) is the read-only buffer between the internal data bus and the receive shift register.
After a complete character shifts into the receive shift register, the data portion of the character transfers
to the SCDR. The SCI receiver full bit, SCRF, in SCI status register 1 (SCS1) becomes set, indicating that
the received byte can be read. If the SCI receive interrupt enable bit, SCRIE, in SCC2 is also set, the
SCRF bit generates a receiver CPU interrupt request.
18.4.3.3 Data Sampling
The receiver samples the PE1/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
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 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 18-6. Receiver Data Sampling
To verify the start bit and to detect noise, data recovery logic takes samples at RT3, RT5, and RT7.
Table 18-2 summarizes the results of the start bit verification samples.
Table 18-2. 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
RT CLOCK
RESET
RT
1
RT
1
RT
1
RT
1
RT
1
RT
1
RT
1
RT
1
RT
1
RT
2
RT
3
RT
4
RT
5
RT
8
RT
7
RT
6
R
T11
R
T10
RT
9
R
T15
R
T14
R
T13
R
T12
R
T16
RT
1
RT
2
RT
3
RT
4
START BIT
QUALIFICATION
START BIT
VERIFICATION
DATA
SAMPLING
SAMPLES
RT
CLOCK
RT CLOCK
STATE
START BIT
LSB
PE1/RxD