68300

Tim,

My opinion is that 3x sampling is fluff.  First, the TouCAN is the only
controller I've ever seen with that feature.  Second, TouCAN documentation
doesn't mention how the samples are resolved.  Third, the CAN bit-timing
definitions allow the user to choose the sample point.  So the bus designer
can define the middle of the "settled" bit since he knows all the
electrical parameters of his bus. ("Settled" meaning both in waveshape and
arbitration).

Your sample point depends on so many other factors.  Length, bus speed, and
clock tolerances are the big ones.  With very accurate clocks, you can
increase your bus speed by pushing the sample point back toward 80-85%.
However, you can't use 80-85% if your clocks are inaccurate.  Maybe you can
get away with only 70%, which will then govern maximum speed and length.

Always remember the farthest transmitter competing with you.  He must sense
your "winning" bit in time for his sample point according to his
observation of time and his current synchronization (which may be 5 bit
times old, and his bit times to boot).

Jeff





"Tim Flynn" <timflynn@...> on 09/04/2002 10:13:07 AM

Please respond to 68300@yahoogroups.com

To:    <68300@yahoogroups.com>
cc:
Subject:    [68300] 68376: CAN bus sample mode and sample point


Does anyone have an opinion about chosing the three sample mode over single
sample?
What are the pros and cons of each?
Also where should I put the sample point? I have been running at 62% but I
see a benefit to running out at ~80%
What are the pros and cons of the positions of the sample point?

Tim Flynn
RMS Welding Systems
ph.(780) 955-3907
timflynn@...
www.rmsweldingsystems.com






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