Clearance Matrix Mysteriously Changing

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Clearance Matrix Mysteriously Changing

Postby dickelbeck » Mon Aug 08, 2011 2:26 pm

Alfons,

The clearance matrix is changing during a routing session without explanation. I load a board, and route it manually over the course of 3 days, never unloading, only saving to

a) *.dsn, and
b) *.ses

During this time, my matrix changes from 4.1 to 4.2 and 6.1 to 6.2

This cannot be explained by anything I am doing. I had edited all values to 4.1 and 6.1 shortly after loading 3 days ago, and now they have changed to 4.2 and 6.2. And as I said, during this time, the only thing I have done is save, never reload.
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Re: Clearance Matrix Mysteriously Changing

Postby alfons » Sat Aug 13, 2011 8:22 am

It seems to me, that your clearance values do not fit the internal routing grid of the board exactly.
It looks, that the router has increased the critical clearance value a little bit, so that it matches the internal grid.
Because the router has made the clearance bigger that should not cause any clearance violations in your board design.
If that is a problem for you can try to increase the value in the resolution scope at the beginning of your critical .DSN-file (maybe by a factor 10).
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Re: Clearance Matrix Mysteriously Changing

Postby dickelbeck » Thu Aug 18, 2011 2:23 pm

(resolution mil 10)

The board is exported with 1/10 of a mil.

The clearance I pass you is 6.1
You change it to 6.2

What am I missing? Your comment is not making sense to me.

If I pass you 6.0, you do not change it.
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Re: Clearance Matrix Mysteriously Changing

Postby dickelbeck » Sat Aug 27, 2011 6:23 pm

Thank you for your help Alfons.


(resolution mil 10)

The board is exported with 1/10 of a mil.

The clearance I pass you is 6.1
You change it to 6.2

What am I missing? Your comment is not making sense to me.

If I pass you 6.0, you do not change it.


What I meant to say, and didn't, is that I would have expected your software to adopt the routing grid size according to the (resolution mil 10) statement. This only makes sense, because this seems to be the only way you could output coordinates at that resolution without introducing a rounding error during the output creation, and a subsequent clearance violation within Kicad. So I assume the (resolution ...) statement is being honored in this way, that it establishes the routing grid size. That said, then it becomes confusing that 6.1 would become 6.2, but that 6.0 does not become 6.1.

I hope this clarifies my confusion, if that makes sense.

Thanks again for the usage of this excellent software.

Dick
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Re: Clearance Matrix Mysteriously Changing

Postby alfons » Sun Aug 28, 2011 9:21 am

Yes, I remember that I reduced the resolution in my router to the resolution of the Host Cad grid to avoid these rounding errors, which would occur after importing the board to my router and back to the host CAD several times.
The resolution in Kicad is currently quite low. The problem is caused in my opinion by some rounding waste depending on this low resolution.
There may be also several other reasons to think about increasing the resolution of the Kicad grid in the future, or even making this resolution flexible definable for each individual board as in the Specctra standard (and in my router).
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Re: Clearance Matrix Mysteriously Changing

Postby dickelbeck » Wed Aug 31, 2011 4:27 pm

The exported resolution matches the internal unit of the PCBNew layout tool. We cannot let the exported resolution exceed the internal unit within PCBNew, otherwise we'd get rounding errors and clearance violations on the back import. Actually the constraint is that we can export at a resolution equal to or less than what we support.

Given that constraint, your comments on the resolution are well taken. We've had discussions about increasing the resolution within Kicad's PCBNew.

Question: does the resolution of your routing grid affect your speed in any way?
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Re: Clearance Matrix Mysteriously Changing

Postby alfons » Thu Sep 01, 2011 9:11 am

No, the autorouting speed depends only on the number and density of object shapes on the board. A coarse grid may an contrary slow down the autorouter because of the bigger grid waste some routed connections will be less optimal.
The restriction on the grid resolution is, that the absolute value of a board coordinate does not exceed 2^25, which is 33554432.
This value is chosen, so that the product of two such coordinates is contained in the mantissa of a double ( 2^53) with some space left for addition.
With bigger coordinate values rounding errors may occur, because the product of 2 such integers cannot be represented exactly by a double, as soon as the exponent of the double must be used.
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