[Magic-dev] Re: magic resistance extraction

[R. Timothy Edwards]

Dear Chitlesh,

Getting back as far as your June postings now. . .

 > I'm using the default scmos.tech of magic and i have created a
 > rectangle (2 by 7) with polysilicium.
 > A sim file was extracted and the output is as follows:
 > | units: 100 tech: scmos format: MIT
 > R a_0_n2# 84

Item number 1:  When you don't define a specific device, then
Magic will extract what's called a "lumped resistance".  This
does not mean that it thinks your resistor is 84 ohms.  It means
that if a signal was passed through this net, it would have a
time constant of t=R*C.  If you haven't placed any transistors or
other devices, then magic will presumably decide at random whether
your resistor is oriented vertically or horizontally.  You've
probably already figured this out, but I thought I would mention
it for the sake of completeness.

 > A second sim file was extracted from a similar rectangle but twice the
 > width that is (4 by 7) and the output is:
 > | units: 100 tech: scmos format: MIT
 > C a_0_n2# GND 2.9
 > R a_0_n2# 42

The first file found the capacitance to be less than its cutoff
point and didn't print the value (default is 2fF, I think?).  Now
you have a file with a lumped R*C value on the node a_0_n2#, R
in ohms and C in femtoFarads (the man page for "sim (5)"
describes the format in detail).

 > Ok if I follow the simple R = þ*L /A relation
 > where þ is the resistivity,
 > L  : the length in this case 7
 > A :the area (H*W),
 > so R = (þ/H) L/W
 > by this ratio I have þ/H = 24.

 > Where and how magic sets þ/H to be 24 when the scmos.tech allows
 > 2.0/1.6/1.2 µm ?
 > Is there a table somewhere which defines the resistance/µm for
 > * metal1,metal2,metal3
 > * ndiff,pdiff
 > * polysilicium
 > * ...
 > with respect to 2.0/1.6/1.2 µm ?

Item 2:  Magic's internal representation is in unitless values of
"lambda".  Magic's tech file defines various "styles" for extraction,
each of which defines a scalefactor relating internal units to microns.
Type:

   : ext style
   The current style is "lambda=1.0(scna20_orb)".

to get the current style.  The tech file itself has the scalefactor
in the "extract" section for this style:

  lambda 100

which means that 1 lambda (internal unit) = 100 centimicrons, correct
for the default (and defunct!) 1.0 micron process from (the also defunct,
at least in name) Orbit Semiconductor foundry.

For more information on resistance and capacitance extraction, try
the Magic tutorials online and the online version of the Magic Maintainer's
Manual #2:

	http://opencircuitdesign.com/magic/userguide.html
	http://opencircuitdesign.com/magic/tutorials/tut8.html
and
	http://opencircuitdesign.com/magic/techref/maint2.html
esp.	http://opencircuitdesign.com/magic/techref/maint2.html#extract

You should find these helpful.

					Regards,
					Tim

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