Measuring Power Consumption of Processor

Discussion in 'Electronic Basics' started by Nilesh Khambekar, Nov 18, 2004.

1. Nilesh KhambekarGuest

Hi,

question about basics of power measurement...

In the power equation, we have
1) static power by = I X Vdd
[static]
2) switching power = 0.5 X C X (Vdd^2) X f
[dynamic]
3) short circuit power = Q X vdd X f X N
[dynamic]
4) leakage power = I(leak) X Vdd
[static]

How do we measure these components?

-For finding static power, we find total current flowing (from voltage
drop across register). and Multiply it by Supply voltage (core
voltage). right?

-How do we measure switching power of processor assuming maximum
switching activity? we know f and Vdd. What about C ?

-How do we measure short circuit power ?

-Can we disable clock to find leakage power component of static power?

Thanks,
Nilesh

2. John LarkinGuest

1.0, not 0.5. Every clock has two edges.

John

3. john jardineGuest

Does not, the bucket fill on the positive edge, using the top FET?. Bucket
contents dumped on neg edge.
regards
john

4. Fred BartoliGuest

Sure, but... 1/2 C V^2 is just the energy stored into the capacitor and
doesn't account for the energy wasted into the switches.

For a cap charged from a supply U :

Wu = U int(i*dt)

and also U = int(i/C*dt)

which gives Wu = C U^2.

So, 1/2 C U^2 dissipated into the CPU at the rising edge, and the same at
the falling edge.

5. john jardineGuest

"Fred Bartoli"
Thanks Fred.
I'd come in from a simple SMPS 'transfer of energy' POV, assuming zero
switch losses and neglecting that it needs the averaged work to charge the
cap (*0.5) and then averaged work to discharge it (*0.5).
regards
john

6. Nilesh KhambekarGuest

Hi,

Thanks.

Let me pinpoint my question...
What is practical procedure to measure these power components ?
1) How do I get C for finding switching power ?
2) How do I get I(leak) to meaure short circuit power
3) How can one measure leakage power

thanks,
Nilesh

7. John LarkinGuest

Measure the slope of the Icc-versus-Fclock curve. Or look for it on a
datasheet, where it's sometimes specified. Note that "dynamic power
capacitance" isn't always real capacitance, as it may include
significant shoot-through current spikes.
Short circuit power is zero.
Measure Icc at zero clock frequency. Multiply by Vcc.

John

8. Nilesh KhambekarGuest

Hi,

Thanks!

Can you suggest some useful book related to this topic?
Or some good online references?

This will help me doing the actual measurements.

Basically I am CS student having some work experience in embedded systems.

again thanks,
Nilesh