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• Trusting the Autorouter Hack Chat Transcript

  Lutetium • 02/23/2018 at 21:10 • 1 comment

  Sophi Kravitz says:

  for those who are here now, the reason I asked @Ben Jordan to join us was because of the last EDA chat where a bunch of you were talking about autorouting

  in a not so favorable way :)))

  I thought it would be excellent to discuss from the standpoint of understanding how it works

  @Ben Jordan welcome! please introduce yourself!

  Ben Jordan says:

  Hi all, thanks for coming. I've worked at Altium for 13.5+ years

  I *was* an FPGA designer, then R&D applications engineer, then FAE, then product manager

  I've seen a lot change in EDA, and worked with some cool people including @technolomaniac who's now with Autodesk, and Dave Jones from EEVBlog. Heard every opinion there is about auto routing tech

  This topic is contraversial

  Sophi Kravitz says:

  @Ben Jordan why do you think it's contraversial

  Ben Jordan says:

  Most professional PCB designers I have talked to would not use an autorouter. They wear the T-shirt that says "never trust the autorouter".... and usually for good reasons

  But realistic people, also know if you have computers, you should use them... right?

  First, let me 'splain some autorouter history from our side…

  Sophi Kravitz says:

  our first q comes from @Lutetium : Where can someone who has felt duped by the autorouter time and time start to begin seeing it the useful tool it was meant to be?

  Ben Jordan says:

  Excellent question

  hopefully knowing the how it works a bit will help with that

  First, let me 'splain some autorouter history from our side...

  First there are different types of autorouters: Grid, Rectilinear, and Topological are the main ones

  Grid routing is the oldest tech, and what you'd find in most older PCB tools. It works like this:

  12:13 PM

  So everything's on a grid

  The router is a dumb algorithm that leaves the net's starting pad horizontally or vertically (can be arbitrary or biased) then takes right-angles to get around where it has to go. If an object (like a component pad) is in the way, it will take a left or right turn based on the overall direction.

  Simple, fast, but low completion rates, and wasteful of space.

  Grid based routing worked really okay for 1970s and 1980s boards which were mainly through-hole, and introduced the notion of path costs, bias, decision making

  Dave Blundell says:

  this is my autorouter experience, in a nutshell.

  Ben Jordan says:

  you can see why most poeple said, yeah, I like the idea but this can't cut it

  Sophi Kravitz says:

  but that was 30 years or more ago

  Ben Jordan says:

  yeah. so Autorouters back then were $$$$$$ add-ons to CAD workstations. A few were sold and then a few years went by and they were abandoned in favor of human labor

  Grid based router was in old Protel, old P-CAD, and older versions of EAGLE as well. So then, Cooper and Chyan Technology  (now part of Cadence) came up with "Rectilinear" routing which did away with the grid.

  Rectilinear routers divide the board geometry into rectangles with no grid (hence the name)

  routes are still mainly done horizontal, vertical, etc. but more room was available.

  For each trace being routed, it would create a new rectangle array of the board map.

  Much better completion rates, but still suffered problems because the further the board was completed the more crap was in the way.

  And the results never "looked" like what a human would do. And in spite of all practical opinions, apparently it matters what the routing looks like, even though the board will not likely be seen by humans once it's in the final product ;-)

  So the next evolution came in the late 1990s (still, a long time ago!) with "topological" autorouters

  A topological router (like the ones currently in EAGLE, PADS, Altium Designer, CircuitMaker etc) maps the board space with polygons, usually triangles:

  darryln says:

  12:24 PM

  (CCT router in 1996 sucked mightily, trust me)

  Ben Jordan says:

  A lot of people abandoned automation for PCB at many times throughout history

  So that's the triangle-based topology...

  Read more »

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