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how to measure power dissipated in a digital circuit

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

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  1. 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
    Tool,which is based on SPICE.
    I discussed with my Prof about this,but he adviced me not to use a
    (which is available in our Uni) because a Multiplier is too big a
    to use HSPICE.
    My question is how to Am I going to measure power dissipated in a
    digital circuit(in this case a Multiplier)

    Thanks in advance,
  2. Ken Smith

    Ken Smith Guest

    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? 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. 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. SwitcherCADIII/LTspice

    Best Regards,
  4. Jim Thompson

    Jim Thompson Guest

    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,
    Thanks for your kind reply. 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.
  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
    to see if you can get any useful information over there...
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