Re: [SI-LIST] : Decoupling capacitor selection & placemen

Paul Galloway ([email protected])
Fri, 7 Nov 1997 12:49:27 -0500


Haven't we concluded that the only reason some existing chips are
functioning is due to the inherent on-die capacitance?

As a reference you may want to check the following:

"Circuit Implementation of a 300 MHz 64-bit Second-Generation CMOS Alpha
CPU", Digital Technical Journal, vol. 7 No.1, 1995.

which refers to use of 160nf on chip decoupling capacitance.

There were several generations of chips prior that also incorporated
on-die capacitance. As D.C. relates, using gate capacitance to build
these isn't as much of an issue as is changing the design process and
methodology to accomodate.


On Nov 7, 9:36am, D. C. Sessions wrote:
> Subject: Re: [SI-LIST] : Decoupling capacitor selection & placemen
> Mellitz, Richard wrote:
> >
> > D.C.
> >
> > Are you your own straight man? :-)
> No -- SI people are all loopy.
> > Sorry, ...couldn't resist.
> Lines like that should be deposited -- they leave people sputtering.
> > Anyhow,
> > that sounds great! How much C are you going to add and where on the
> > chips do you plan put it? The I/O ring, core, ...?
> Currently there's a capital appropriation going through for hammers.
> After that, it's whatever fits wherever we can get it. It's really
> a methodology issue, but the fact remains that onchip capacitance
> has far higher Q and less distribution parasitics than anything
> off-chip can approach.
> Here's a general idea:
> For a synchronous design, CMOS power dissipation is pretty much
> a capacitive-discharge effect. A device that burns 2400 mW at
> 3.6v and 100 MHz has an effective load capacitance of about 2000 pF
> mostly right after clock events. If we go with a 10% supply dip,
> that means bypass of 20 nF; a gate capacitance of 6-7 mF/sq.M gives
> an area of about 3 square millimeters. NOT small, but not huge
> either; it's about 10% of a typical IC.
> Keep in mind that quiescent CMOS gates provide distributed
> capacitance via their input capacitance, both by coupling to
> the supply rails of the devices driving them and by the series
> capacitance of their own input transistors. (This isn't anything
> like 10% of the die area, but it is enough to make a difference
> and has the nice characteristic of random interconnect.)
> The challenge to people like me is to change our methodology
> to at least take advantage of the available floorspace to add
> caps without pushing out development time. It's frustrating
> partly because in deep submicron designs the routing takes up
> so much of the die that there is quite a bit of silicon doing
> nothing but supporting oxide.
> > >The other possibility is that people like me(!)
> > >will incorporate termination onchip (as the Creator intended it) to
> > >eliminate stubs and add capacitors onchip (on the RIHGT side of
> > >bondwire inductance.)
> --
> D. C. Sessions
> [email protected]
>-- End of excerpt from D. C. Sessions