piclist 2009\02\02\093937a >
Thread: Single Layer Autorouter?
www.piclist.com/techref/pcbs.htm?key=autoroute
face picon face BY : email (remove spam text)(Olin Lathrop)

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Forrest W. Christian wrote:
> Like I said, I have yet to see an autorouter even come close to what a
> human can produce.   Not to say there aren't nice autorouters out
> there, but for many applications it just doesn't seem to be worth it
> - at least to me.

That is because you are looking at the auto router the wrong way.  You
shouldn't expect it to produce something just as neat and tidy and clean
looking as as what a human can do given sufficient time.  The real purpose
is to take care of the 90% of the routing where there is no advantage for a
human to do it.  It will do this much faster and guarantee to pay attention
the whole time and always adhere to the design contraints like track width,
space width, keepout areas, etc.

You also seem to put some misplaced priority on asthetics.  The electrons
don't care how pretty you think the route is.  There are usually a few key
paths where physical layout does matter.  Spend your expert human time
making sure those are logically routed.  Most paths are not critical and
don't matter if they get routed a few inches longer or have a few extra vias
in them.  Making them look pretty isn't going to make the circuit more
reliable or cheaper or better.  Remember that "good enough" is by definition
good enough.

This weekend I did a layout and route on a respin of a board that is over 90
square inches.  Of course I manually routed a few critical sections.  These
included high current paths of the switching power supply, ground and
sub-ground connections, sensitive areas like the crystal connections to the
PIC, and a bus just to keep it from getting all tangled and making it hard
or impossible to route other parts.  But going further would have been a
waste of time and not been good value to the customer.  It took the auto
router nearly 5 hours to do all the stuff I didn't.  It would have taken me
much longer, and then I probably would have made a few mistakes.

As a example, I attached a small area of the board drawing.  IC22 and IC23
are connected to the same 8 bit data bus at top.  This bus goes to a lot of
places and I manually routed it.  The bottom pins of these two chips go to
connector P215.  The pin order happened to be flipped backwards from the
order on the connector.  Sure, I could have spent time manually routing this
and possibly even changing the placement so that they wouldn't be flipped,
but why bother?  I knew the auto router could do this and get it right.
What it came up with may not look pretty by human asthetics, but it's
perfectly fine electrically.  Maybe this could have been done with a few
less vias, but again, what's the point?


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