Re: [SI-LIST] : coming up with average power estimates for buffers

Adrian Shiner (adrian.shiner@virgin.net)
Tue, 03 Aug 1999 20:37:33 +0100

D.C, You have made the classic mistake of oversimplification of the load presented to the supply. In your scheme of things, the system would dissipate very little power within the computing hardware. Current flows in circuits
which are not in seies with the capacitances which you use in your equations. Statistical averaging...that is part of the power of RSS in the real engineering world where practical fast solutions matter.

Best wishes

Adrian

"D. C. Sessions" wrote:

> Doug Yanagawa wrote:
> > Pat Zabinski wrote:
>
> > > One other point to note: as we increase the transmission line
> > > length, the RMS power goes up as well (as expected). However,
> > > this trend continues to a certain point, then the power actually
> > > reduces with increased line length. Can someone explain why
> > > the RMS power would be reduced with increased length? We're only
> > > seeing a small percentage change (~10-20%), but it's got
> > > me curious.
> >
> > When the transmission line gets long relative to the switching
> > frequency, the driver is not completely charging and discharging the
> > net. Hmmm, maybe we could solve the worlds energy problems. But the
> > theoretical power mins might have nasty harmonics associated with them.
>
> All of the RMS-vs-peak etc. stuff has been fun, but mostly irrelevant.
> Assuming a reasonably stiff power supply (safe bet) and no shunt termination
> (iffy) each rising transition will induce a delta-V equal to the supply
> voltage on some amount of capacitance (the falling transitions return the
> charge to ground.) The line capacitance is very nearly the trace capacitance
> plus load devices up to prop delays half of the line high time; for most
> systems this is a safe assumption. If the system actually has ballistic
> signals -- the UI is less than the round-trip delay -- then running
> without shunt terminations is REALLY bold.
>
> SO! In the absense of highly structured activity, there will be a rising
> edge approximately once in four UIs, each of which will charge the line to
> Vdd. Power is thus (Vdd)(Vdd)(Cline+Cload)/UI -- good old CFV**2
> Clocks, of course, are the quintessential example of "highly structured
> activity."
>
> If the line *is* shunt terminated, the power consumption becomes largely
> independent of frequency and is the statistically weighted sum of the
> power in the high state and the low state.
>
> --
> D. C. Sessions
> dc.sessions@vlsi.com
>
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