Homebrew PCBs

Leon Heller wrote:

> ----- Original Message ----- 
> From: "David McNab" <david@...>
> To: <Homebrew_PCBs@yahoogroups.com>
>>
>>I'm trying to autoroute the bottom side of the board to minimise the
>>number of unrouted connections. This is challenging, because due to
>>physical requirements, several components have to stay in fixed
>>positions on the board. At best, I'm getting down to 4 or 5 unrouted
>>connections, which I could solve with manual vias and top-side wires,
>>but I'd really like to get it down to 0 unrouted.


   Hard to tell 100% from your wording, but if you mean a single sided board and 
get it to 0 unrouted, realize that this is quite likely impossible.  Very few 
topologies with more than one IC or connector that has fixed pin locations will 
result in a board that can successfully be fully routed on one side with any 
amount of effort.

>>
>>I tried 2-side autorouting, and setting high costs for vias, as well as
>>maximum numbers of vias. But the autorouter seems to ignore these, and I
>>end up with 30 vias or more.

   The cost for vias is relative to the cost for other steps etc.  Vias not only 
have to go up, other costs have to go down.  You also have to have it work the 
right way through the several passes of the autoroute, so that it is constrained 
to head your board towards the correct final result.

>>My experiences with the autorouter have been weird to say the least. It
>>feels like I could get more scientific certainty from inspecting chicken
>>entrails to predict movements in the NASDAQ.

> Most autorouters are a waste of time, especially on single-sided boards. The 
> Eagle autorouter is crap, anyway.
> 


   Eagle autorouter is one of the more brilliant things I've ever used.  The 
defaults are simply poorly set up for how most people want it to work, they need 
to make a couple of easy load classes for single sided etc.  What I've made it 
do on more than a few times is far better than the settings it shipped with.  It 
takes quite a bit of playing around to start to understand it, but once 
understood you can make it work any way you like and it's very good at it.  Very 
high costs for top with adjusted lowered costs for bottom, and only letting it 
even try top on the last pass or two can easily get it to route a minimum number 
and length for top side jumpers.

   You also seriously have to play around adjusting single settings to extremes 
so you know what they do.  Bonus step is an extra step in the same direction 
going toward the goal.  Malus step is an extra step in the continuing direction 
but going further from the goal.  Sometimes it gives lower cost to take a few 
bad steps and then get around something etc.  You have to really learn what 
everything is doing, and learn to adjust it vs the other settings, and learn to 
do that vs the multidimensional effect of many passes.  Takes quite a bit of 
playing but it's not at all impossible.

   I couldn't even tell you the exact settings right now though, the last board 
or two I did were very straight forward single sided with special locations so I 
did them manually.  Haven't set the autorouter up fully since the last time I 
upgraded, I end up adjusting it to match what I need for complex boards anyway 
so don't usually bother taking the settings along with the upgrade.  I just 
figure them back out when I need them.

   On the other hand, 15 minutes of playing around with one of my old boards 
would get it where I could give you some more specific ideas of how to use it. 
Have too many other things going on right now, but maybe I can mess with it 
sometime shortly.  It also pops up in the SRS group now and then, everyone says 
it sucks then I say it's the best thing since sliced bread.  Something to write 
up a how to guide for at some point, it's just a very back burner thing.

Alan