MC68HC812A4 Datasheet, PDF (243/342 Pages) Motorola, Inc – 16-bit device composed of standard on-chip peripheral modules connected by an intermodule bus. Modules include

Serial Communications Interface Module (SCI)
Functional Description
14.6.4.4 Framing Errors
If the data recovery logic does not detect a logic 1 where the stop bit
should be in an incoming frame, it sets the framing error flag, FE, in SCI
status register 1 (SCSR1). A break character also sets the FE flag
because a break character has no stop bit. The FE flag is set at the same
time that the RDRF flag is set.
14.6.4.5 Baud Rate Tolerance
A transmitting device may be operating at a baud rate below or above
the receiver baud rate. Accumulated bit time misalignment can cause
one of the three stop bit data samples to fall outside the actual stop bit.
Then a noise error occurs. If more than one of the samples is outside the
stop bit, a framing error occurs. In most applications, the baud rate
tolerance is much more than the degree of misalignment that is likely to
occur.
As the receiver samples an incoming frame, it resynchronizes the RT
clock on any valid falling edge within the frame. Resynchronization
within frames corrects misalignments between transmitter bit times and
receiver bit times.
Slow Data Tolerance
Figure 14-13 shows how much a slow received frame can be misaligned
without causing a noise error or a framing error. The slow stop bit begins
at RT8 instead of RT1 but arrives in time for the stop bit data samples at
RT8, RT9, and RT10.
RECEIVER
RT CLOCK
MSB
STOP
DATA
SAMPLES
Figure 14-13. Slow Data
Advance Information
MOTOROLA
Serial Communications Interface Module (SCI)
MC68HC812A4 — Rev. 3.0
243