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Author Topic: Eagle Routing Tips Needed  (Read 1528 times)
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ottawa, canada
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I'm doing my first PCB using Eagle and I'm struggling with the routing.  I'm not good at layout type things so I'm relying on the auto-router.  To get 100% routed I have to go with 10 mil traces and separation  for signals and 16/10 for VCC and ground and set the routing grid at 4 or less.  Even then I end up with dozens of vias and a jagged looking board.  There's lots of room and I've moved a few things around but it's hard to see any improvement.

Are there any general approaches I could take to improving it?  I did try hand-routing VCC and ground first but it just took me a long time to do and the result didn't seem any better.

The board holds an smd atmega328 with a power supply and ttl serial connections.  There's a 74hc166 shift register and a 74hc02 glue chip connecting to a 30 pin connector which interfaces with an existing board.



In the board layout the power supply is top right, the atmega is below it, iscp beside it.  the top chip near the 30 pin connector is the 74166 shift register and the 7402 is below that.   I would like to leave the bottom part of the board clear although that's not critical except for the icsp and other connectors.


any pointers or tips gratefully accepted.  Maybe for my next stab I'll try printing off the board and hand routing a chip at a time.
« Last Edit: May 05, 2012, 06:32:33 pm by bill2009 » Logged

Grand Blanc, MI, USA
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I put ground planes on both sides which helps a fair amount.
Then I try to place the components as logically as possible.
Then I try the autorouter, 10 mil grid.
If it's a real mess, I reconsider my "logical" placement of components and move things around.
Lather, rinse, repeat.
Once it looks decent, I'll more often than not route it all by hand.
I typically use 10 mil minimum trace size, 16mil clearance, and 20mil thermals.
All the boards I've done to date use through-hole components, I'm just starting to play with some SMDs.
Absolute minimum board size is rarely a criteria for me. I use iTead quite a bit, and given the breaks in their board sizing, I usually seem to have some extra room, sometimes quite a bit.

HTH
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ottawa, canada
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@jack: thanks.
"I put ground planes on both sides which helps a fair amount."
I know how to do the polygon/rats-nest thing after the routing - is that the same thing?
"Then I try to place the components as logically as possible." ok
"Then I try the autorouter, 10 mil grid." ok
"If it's a real mess, I reconsider my "logical" placement of components and move things around."
I guess that's where I am.  How would you pick what  to move where?
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Grand Blanc, MI, USA
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I just make two polygons, assign one to the top layer, one to the bottom, name them both GND. I like to make them slightly larger than the board, it makes them easier to click on and not get confused with the outline or whatever.

As for what to move where, that's probably more art or trial-and-error than science. I try to physically arrange components to minimize the length of signal traces. But it's not always intuitive. I did one recently where putting a component on the "wrong" side of the MCU (i.e. where the traces were longer to that particular component) and routing the traces between MCU pins made the rest of the layout a lot better. I wasn't kidding about lather-rinse-repeat, I go through LOTS of iterations, ripping everything up, repositioning components, then autorouting.

I also do try to make supply traces wider, but I don't get too carried away. Have to think about the current involved. A microcontroller that draws 12mA doesn't need a 100 mil supply trace on a board that's two or three inches long. Even if it drives loads that bring it up to several times that, the IR drop will still be pretty minimal.
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Thanks again.  Once I started hand-routing it turns out not to be that bad and I can see what I'm doing.  It makes me think twice about pin assignment and such.  i bet when i'm done i could turn it back to the autorouter and it would do a good job.
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Cool.  Yeah, I've been known to change pin assignments once I get to routing the board!  Once you do a couple boards you may discover that you're better than the autorouter smiley-lol
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