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Subject: Re: [geda-user] Board DRC
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From: John Doty <jpd AT noqsi DOT com>
In-Reply-To: <20151023225207.de1fcd7a8542af77b2c51127@gmail.com>
Date: Fri, 23 Oct 2015 15:47:48 -0600
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On Oct 23, 2015, at 2:52 PM, Nicklas Karlsson =
(nicklas DOT karlsson17 AT gmail DOT com) [via geda-user AT delorie DOT com] =
<geda-user AT delorie DOT com> wrote:

>>>> There should be exactly one short at the star ground point. If
>>>> algorithm calculate their lines meet at the edge end point have to
>>>> checked in some way.
>>>=20
>>> Or if a particular bit of copper could be tagged as "star point" or
>>> something, the algorithm could know that it should be the edge of
>>> multiple nets, and verify it.
>>>=20
>>> I'm just thinking, our DRC isn't designed to handle these types of
>>> problems, but a new DRC might, and give us other benefits too.
>>>=20
>>=20
>> I spent the morning in a video meeting reviewing a complicated layout =
from a geda-gaf->Allegro flow. The issues were far beyond the things =
discussed in this (thoroughly mutated) thread. We have segmented =
grounds, but not star connections. Star connections only work well if =
you can neglect inductive coupling. And then there ware other things: I =
was able to say at one point =93Don=92t worry about crosstalk between =
signals X and Y: they crosstalk so badly in the sensor that any =
conceivable crosstalk on the board will be negligible.=94
>>=20
>> So, it=92s complicated. I found myself wishing for a tool that would =
show concentrations of AC magnetic field. As far as I know, Allegro =
can=92t handle these issues: humans have to pay attention.
>>=20
>> I don=92t think a tool that works from the top down can ever do this =
stuff well. You need a bottom-up model of a circuit board to even start =
to think about how to analyze how your layout really works. Pcb has no =
such foundation, which is the root of the frustration it causes for a =
reductionist like me.
>>=20
>> John Doty              Noqsi Aerospace, Ltd.
>> http://www.noqsi.com/
>> jpd AT noqsi DOT com
>=20
> To do this kind of electromagnetic calculation the file format or =
internal storage in pcb is probably the smallest problem.

Except that pcb doesn=92t really do bottom-up description well. If it =
did, folks wouldn=92t have been asking for buried vias for years: you =
could simply draw one.


> To show concentrations of AC magnetic field current need be known in =
each segment. Once current and shapes are known it I guess it will be =
possible to make calculations. Capacitance and inductance between nets =
would probably be more realistic and maybe not practical impossible?

For IC design, we extract resistance and capacitance of the =
interconnections. =46rom that, we can do SPICE simulations. But part of =
what makes this possible is that the tools build up the design from the =
bottom. Make a mask transparent in a certain spot, make a diode. Make =
two diodes, you have source and drain. Make a gate with a different =
mask, you get a transistor. Group together several transistors, make a =
gate. Group together several gates, make a flip-flop, =85

The tools work directly on structure rather than indirectly from =
intentions the way pcb does. You realize intentions by constructing =
structures (=93cells=94) rather than by utilizing pre-defined =
=93features=94. In the general case, this is easier to understand both =
for users and for processing tools.

>=20
> Trace impedance calculation however are probably more realistic, thes =
kind of calculations are available on the net =
http://www.eeweb.com/toolbox/microstrip-impedance together with =
equations.
>=20

John Doty              Noqsi Aerospace, Ltd.
http://www.noqsi.com/
jpd AT noqsi DOT com



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