# how to measure power dissipated in a digital circuit

Discussion in 'CAD' started by Christopher Denis, Jan 24, 2005.

1. ### Christopher DenisGuest

I was reading a paper on "LOW-POWER DIGIT-SERIAL MULTIPLIER"
And I came accross a problem of how they measured the power dissipated
in the Multiplier which they have designed.
It said on a paper that,they are using HEAT:Hierarchical Energy
Analysis
Tool,which is based on SPICE.
I discussed with my Prof about this,but he adviced me not to use a
HSPICE
(which is available in our Uni) because a Multiplier is too big a
circuit
to use HSPICE.
My question is how to Am I going to measure power dissipated in a
digital circuit(in this case a Multiplier)

Chris

2. ### Ken SmithGuest

I assume you mean calculate and not measure. If you really want to
measure it you need to build one.

If you want to calculate it, you need to get a completed design ready as
though you were going to put it into real silicon. A lot of CPLD and FPGA
tools contain a power estimator.

How many parts are in the design? Are you doing it transistor by
transistor?

www.linear.com has an unlimited spice on its web site. It is called
SwitcherCad-III for historical reasons. You could feet it your schematic
or netlist and let it run until you get an answer. Computer time is cheap
these days.

3. ### Helmut SennewaldGuest

Hello Chris,
a multiplier may have many very similar circuit blocks.
You simply calculate the power dissipation of one block and
assume that N-blocks require N-times that power.

Another approach is trying to calculate how many transition
you have per multiplication. If you know also how many gates
are conencted to those nets, then you can make a good estimation
about the power dissipation. The power dissipation for one
net is U^2*C*f plus some additional power in the transition.
The power dissipation of a single gate could be simulated with
SPICE for this approach.

A serial multiplier may have only a few thousend transistors.
You should give HSPICE or LTSPICE a try.
LTSPICE is a free and powerful SPICE simulation program.

Best Regards,
Helmut

4. ### Jim ThompsonGuest

I'm not that familiar with HSpice anymore. Quite a few years ago,
when I last used it, I found it to have the worst GUI ever conceived
by man ;-)

However, try this:

Display "I(VDD)"... you will see lots of current spikes.

Then (this works with PSpice, don't know about HSpice):

Display "avg(I(VDD))", since VDD is a constant, this display
multiplied by VDD will be your average power, or HEAT.

...Jim Thompson

5. ### Guest

Mr Smith,
linear.com is going to be a useful link for me.

In fact,In my research,I want to measure and compare the power
dissipated by more than two multipliers.
You have mentioned that CPLD and FPGA tools contains a power
estimator.I think,they might be useful.

I will post again,when in need for further assistance,
Thanks a lot.
Chris

6. ### Guest

I guess your advisor was trying to say: "Do not analyze this
digital circuit by transforming it into analog circuit" because it
is a big task.

But it would be necessary to do some analog simulation "locally"
instead "globally" to have a good estimate. You might as well
try

http://www.ScienceOxygen.com/electrical359.html
to see if you can get any useful information over there...