MC908GR8CDWE Datasheet, PDF (243/408 Pages) Freescale Semiconductor, Inc – M68HC08 Microcontrollers

Freescale Semiconductor, Inc.
Serial Communications Interface (SCI)
Functional Description
18.5.3.3 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 the
following times (see Figure 18-6):
• 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.
PE1/RxD
START BIT
LSB
SAMPLES
RT
CLOCK
RT CLOCK
STATE
RT CLOCK
RESET
START BIT
QUALIFICATION
START BIT
DATA
VERIFICATION SAMPLING
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.
MC68HC908GR8 — Rev 4.0
MOTOROLA
Serial Communications Interface (SCI)
For More Information On This Product,
Go to: www.freescale.com
Technical Data
243