50 kW? Get rid of those vacuum tubes and use solid state! (couldn't resist)
Larry Miller
At 10:57 AM 8/2/99 -0500, you wrote:
>I'm working on a rather large system (over 50,000 Watts!), and we
>are trying to come up with some detailed power estimates for
>each component.
>
>The system is essentially comprised of LOTS(!) of identical
>parallel processing ASICs, each ASIC having roughly 600 I/O.
>The basic I/O is full-swing CMOS with a 2.5 VDD supply running
>at roughly 100 Mbps. A small change in one ASIC will have
>a three-orders of magnitude higher impact on the system, so we need
>to pay very close attention to details. The I/O have been
>designed and tweaked to account for impedance, edge rates,
>packaging effects, etc., so we have a high level of confidence
>they are going to work; at this point, we simply need to
>obtain a power estimate for them.
>
>In order to come up with good system-level power estimates (which
>will determine cooling requirements and power supply requirements),
>we need to have an accurate ASIC power estimate. We've got pretty
>good numbers for the core circuitry, but we're trying to develop
>an estimate for the custom I/O buffers.
>
>To get the power for one buffer, we simulate the buffer with
>a 1010101... pattern, toggling every possible bit period.
>The buffer is loaded with an average-length transmission
>line, and we use spice to plot the power vs time for at
>least two bit-transitions. Overall, we get a power
>vs time plot that is relatively flat except during the logic
>transitions (no surprise here), and the peaks vary in amplitude
>depending upon a rising or falling edge.
>
>In the past, we have used the "simple average" power, meaning
>taking the integral of the power over two bit periods (to ensure
>we've captured one falling and one rising edge)and dividing
>it by the time. We have used this figure as
>our average power for the worst-case-bit-pattern.
>
>However, a colleague recently suggested using the "RMS average"
>of the power, which is computed slightly differently. For our
>case, the RMS average resulted in a power estimate that was
>50% higher than the average value.
>
>>From my experience, taking the integral of the power curve will
>result in the effective energy consumed by the buffer, and dividing
>this by the time will provide the average power. However,
>RMS is used so frequently in power estimates, I could not provide
>a good answer why it shouldn't be used.
>
>Can anyone tell me how to best determine the average power
>for a buffer? Am I anywhere on the right track? Which is better,
>simple average or RMS average?
>
>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.
>
>
>Thanks,
>Pat Zabinski
>
>
>
>
>--
> Pat Zabinski ph: 507-284-5936
> Mayo Foundation fx: 507-284-9171
> 200 First Street SW [email protected]
> Rochester, MN 55905 www.mayo.edu/sppdg/sppdg_home_page.html
>
>
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