[mythtv-users] Capture Card Power Requirements

Eric Sharkey eric at lisaneric.org
Thu Aug 20 20:24:02 UTC 2009


On Thu, Aug 20, 2009 at 2:06 PM, David Brodbeck<gull at gull.us> wrote:
> James Crow wrote:
>>
>> One thing that bothers me is the BIOS hardware monitor shows the voltages
>> as low:
>> 12V rail -> 11.71V
>> 5V rail -> 4.89V
>> 3.3V rail -> 3.26V
>> Vcore -> 1.36V.
>>
>> I don't know if the sensors on the MB are not very sensitive (or poorly
>> calibrated) or if the voltages are indeed low. My multimeter shows the
>> voltages to be 11.97V and 5.00V.
>
> Those voltages are within spec.  The ATX specification allows +/- 5% for
>  positive voltages and +/- 10% for negative voltages.

Apparently things can drift from the true ideal voltage quite
noticeably without causing much harm.

I had the pleasure of having lunch with Ed Felten one time.  He
discussed some some security work he was working on then.  The idea
was that different CPU instructions require different amounts of
power, and that by very carefully crafting a piece of software, you
could vary the power draw by alternately doing sets of expensive/cheap
instructions, switching between them at 60Hz.  The goal of this was to
set up a resonance with the power supply's input line that would cause
the voltages on the mains to drift further from true.  The thought was
that if the voltages were off by just the right amount, the machine
would keep going but start to make mistakes in some of its
calculations, which might then allow privilege escalation.  He said,
with the hardware he was using, he was able to push the voltages
around quite a bit, but everything stayed just barely within spec and
he hadn't been able to actually introduce any mistakes.  Everything
just kept working.

So, anyway, that's a long story just to agree that 11.7V on the 12V
rail is nothing to worry about.

Eric


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