MC68HC705C9A_1 Datasheet, PDF (88/157 Pages) Freescale Semiconductor, Inc – General Release Specification

Freescale Semiconductor, Inc.
General Release Specification _{Start Bit Detection}
9.12 Start Bit Detection
When the input (idle) line is detected low, it is tested for three more
sample times (referred to as the start edge verification samples in
Figure 9-4). If at least two of these three verification samples detect a
logic zero, a valid start bit has been detected; otherwise, the line is
assumed to be idle. A noise flag is set if all three verification samples do
not detect a logic zero. Thus, a valid start bit could be assumed with a
set noise flag present.
If a framing error has occurred without detection of a break (10 zeros for
8-bit format or 11 zeros for 9-bit format), the circuit continues to operate
as if there actually was a stop bit, and the start edge will be placed
artificially. The last bit received in the data shift register is inverted to a
logic one, and the three logic one start qualifiers (shown in Figure 9-4)
are forced into the sample shift register during the interval when
detection of a start bit is anticipated (see Figure 9-6); therefore, the start
bit will be accepted no sooner than it is anticipated.
If the receiver detects that a break (RDRF = 1, FE = 1, receiver data
register = $003B) produced the framing error, the start bit will not be
artificially induced and the receiver must actually detect a logic one
before the start bit can be recognized (see Figure 9-7).
MC68HC705C9A — Rev. 2.0
Serial Communications Interface (SCI)
For More Information On This Product,
Go to: www.freescale.com