Information

4. If a single noise event occurs, and it is possible to uniquely identify the noise event,
then resynchronization takes place.
Starting at sample 15 of the previous time bit period, five data samples are collected. The
number and location of the samples are key to decide if an adjustment in time base is
required. Table below lists the possible values and the actions associated with each
possibility. In the table, S means the data is the same as the logical value that was
received in the second half of the previous bit period. D means that the sample is
different from the logical value that was received in the second half of the previous bit
period.
Sample Values (15,16,1,2,3) Action / Event
SSSSS No start of bit tarnation is detected. Therefore, no adjustment
to time base is made.
SSSSD Two or more error events occurred or the time base was off.
In this case, the time base is slowed down by two. Sample 3
becomes sample 1. The next sample is treated as sample 2.
SSSDS Two or more error events occurred, time base was off along
with noise occurrence, or sample 2 is noise and there is no
start of bit transition. Therefore, no adjustment to time base is
made.
SSSDD It is possible that either noise was received during sample 1
or the time base needs shifting. In this case, the time base is
slowed down by one. Sample 2 becomes sample 1, and
sample 3 becomes sample 2. The next sample is treated as
sample 3.
SSDSS It is most likely that sample 1 is noise and there is no start of
bit transition. Therefore, no adjustment to time base is made.
SSDSD It is possible that sample 1 is noise, and time base needs
shifting by two, or that sample 2 is noise. It is more likely that
sample 2 is noise and therefore no adjustment to time base is
made.
SSDDS It is most likely that sample 3 is noise. Therefore, no
adjustment to time base is made.
SSDDD This is the expected case. Therefore, no adjustment to time
base is made.
SDSSS It is most likely that sample 16 is noise and there is no start of
bit transition. Therefore, no adjustment to time base is made.
SDSSD Either multiple errors occurred or sample 16 is noise and time
base is off by two. In this case, the time base is slowed down
by two. Sample 3 becomes sample 1. The next sample is
treated as sample 2.
SDSDS In this case, multiple errors have occurred. Therefore, no
adjustment to time base is made.
SDSDD In this case, there must either be multiple noise or one noise
at sample 16 or sample 1 with a time shift. Assuming that one
noise occurred, it is unclear what direction the time shift is.
Therefore, no adjustment to time base is made.
Table continues on the next page...
Chapter 45 Universal Asynchronous Receiver/Transmitter (UART)
K20 Sub-Family Reference Manual, Rev. 2, Feb 2012
Freescale Semiconductor, Inc. 1101