Autorouting ... and net prioritisation

Question

My apologies if this one has been asked before but if it has, I can't find it.

WARNING! Contains auto router question.... people with religious or fanatical zeal and/or opinions regarding auto routers ... look away now....

Ok, this is a it of a doozy. I have a sort-of workaround but I'm not particularly happy with it as it involves a fair bit more work than it really should need.

The question is pretty simple ... how can I enforce a net prioritisation for the Autorouter in Eagle?

About now ... a million people will probably be wondering "why" or of what use is that, but it is actually pretty important and let me explain. When I'm laying out a board, I typically start with the layout of tracks in the following order:

Clock Lines

Busses

Power/Ground Planes

... everything else ...

Put simply, for clock lines, I want them as *direct* as possible, with as few (preferably no) vias as possible.

After that, I think lay out the relevant busses and if there are differential paired lines, then they go next.

After that, power and ground (for 4 layer and more, I usually have a separate plane for those but I digress)

... and finally, everything else gets done.

The way I've been doing it up until now, is I typically start with a throw down of components, and then start by getting the auto router to start with individual clock nets. Typically I'll look at what it suggests and then either I'lll move stuff around or I'll adjust it manually but I try to have stuff laid out that is routed by the auto-router in the same or a similar fashion to the way that I would do it. When it doesn't do what I think it would do, I usually try to work out why.

After that, I then attack the various busses and so froth. Now ... the question is that if you do this, and you wind up with a board, if you just rip up the board and then let the auto-router have play at it, it will in 95% of the cases make a complete and utter mess of it. In most of my cases, simply applying a manual priority usually produces a much better job (even if you use nothing other than the auto-router).

The big hassle with this is that it is *horribly* time consuming. Is there a way of letting the auto-router work on a net class list (vs an individual net) and if there is, is there a way of then telling it to then assigning a priority so that it starts with say a net class of say "clock", then progresses to "bus" and so on ...

autodeskguest · Answer

After that, I then attack the various busses and so froth. Now ... the
question is that if you do this, and you wind up with a board, if you
just rip up the board and then let the auto-router have play at it, it
will in 95% of the cases make a complete and utter mess of it. In most
of my cases, simply applying a manual priority usually produces a much
better job (even if you use nothing other than the auto-router).

The big hassle with this is that it is horribly time consuming. Is
there a way of letting the auto-router work on a net class list (vs an
individual net) and if there is, is there a way of then telling it to
then assigning a priority so that it starts with say a net class of say
"clock", then progresses to "bus" and so on ...

Hi Brendon,

I hope you're doing well. I can think of a couple of ways to do this.

There's nothing built into EAGLE to perform this type of task.

What you can do is create a ULP that looks are your classes and

determines what nets belong to it. The showclass ULP is probably 90% of

the way there since it pretty much does this.

Once you know what nets belong to each netclass then the ULP could write

a script file that passes these nets to the autorouter and it would do

this for each netclass. Here's a simple example that includes loading an

Autorouter configuration file.

AUTO LOAD viadrops;

AUTO GND +5V +3.3V -12V +12V;

AUTO LOAD default;

AUTO N$1 N$2 N$3 N$4 ...;

That's an idea of what the resulting script might look like.

I know it's not built-in but I think this is pretty doable.

hth,

Jorge Garcia

autodeskguest · Answer

Jorge Garcia wrote on Mon, 23 November 2015 12:03

Once you know what nets belong to each netclass then the ULP could
write
a script file that passes these nets to the autorouter and it would do

this for each netclass. Here's a simple example that includes loading
an
Autorouter configuration file.

AUTO LOAD viadrops;
AUTO GND +5V +3.3V -12V +12V;
AUTO LOAD default;
AUTO N$1 N$2 N$3 N$4 ...;

While this answers the question as asked, it really needs to be pointed out

that doing this is a bad idea. Separate run of AUTO will consider anything

routed by previous runs as immutable. There is no flexibility to rip and

and re-route. A previous run can very easily block access to yet unrouted

pins and the like, which will then be unroutable.

This method also prevents using multiple autorouter passes to refine the

route. For example, I usually use 8 passes. The first few are optimized

to find a solution, even it is not very efficient and uses a lot of vias.

Subsequent passes then change the weights to refine the overall route.

Once a solution has been found, It can't be undone in subsequent passes.

You can therefore be a lot more aggressive in later passes in trying to

minimize vias, keep things off the ground plane or on the outer layers,

etc. Actually I optimize for minimum vias in the middle, then relax the

via cost towards the end to minimize tracks in polygons, like the ground

layer.

The multiple passes also work together as sortof a annealing process, with

parameters changed less between later passes.

Again, the method suggested here prevents all these things, to the point I

doubt you'll get a route you actually want to produce boards with.

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