Thursday, September 22, 2011

Eagle autorouter tips

It's pretty easy to comb the net for trash-talk and criticism of the CadSoft Eagle autorouter. Honestly it's not very good. It's less good than I am at routing, and I'm not that good.

But, if you lack patience like I do, the autorouter is the way to go. You can set it loose on a problem, go get a soda, read some blogs, come back, and have an acceptable if not beautiful board.

I have gathered a couple of useful hints which has helped the success of my autorouting attempts enormously:

  • Do a good job placing components. Consider the hardware subroutine advice below.
  • Set the routing grid to a very small number. If you are using inches, set it to 1 mil. I think the routing time goes up with the inverse square of the grid size, so half the grid size takes four times longer, but often the router can find solutions with a smaller grid that elude it with a larger grid.
  • Consider the shape of your board, and the general "grain" of the signals on it. On long, skinny boards like the Stick IMUinator, it may not be best to prefer horizontal and vertical. Most of your signals will want to go horizontal, forcing most of them to one side of the board. Consider doing the preferred directions diagonally. I learned this by watching the autorouter work on my design.
  • Delete (not just rip up) your ground plane polygons. The autorouter doesn't seem to handle polygons well at all, but if the polygon is not there and it treats ground just like any other signal, it can usually handle it well.
  • Be prepared to do a manual cleanup after the autorouter is done. Put back the ground planes, and rip up all the lines, but not the vias, in the ground signal. Sometimes, you will find it necessary to leave a wire which connects parts of the ground plane. This especially happens if your ground planes use thermals. Also just fix any general ugliness on the board. For instance, the autorouter loves to bring a signal out the side of a QFP pad (like on the controller in the IMUinator) and bring it as close as possible to another pin before turning. Fix these manually.
  • Check if the router has done anything monumentally stupid. For instance on the Stick IMUinator, the main crystal has four pads, two of which need to be grounded and two which are the crystal terminals. These are diagonally opposed, so that the orientation of the crystal doesn't matter. But, the autorouter connected the two ground terminals across the middle of the part, leaving no space for the actual crystal terminals. So, it routed one crystal terminal over the river, through the woods, past Grandmother's house, and ended up with a signal about 10x longer than the other one on both sides of the board, braided with other signals for maximum crosstalk problems. To fix this, I ripped it all up, manually routed just the crystal signals, then let the autorouter have at it.

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