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Re: beaglebone: changing cpu frequency

On Aug 2, 2013, at 5:52, Manuel Bouyer <bouyer%antioche.eu.org@localhost> wrote:
> On Thu, Aug 01, 2013 at 06:36:45PM -0400, Dennis Ferguson wrote:
>> Note, however, that for the BB Black, the SRM has a power consumption test 
>> with the CPU
>> running at 1000 MHz with the HDMI active, the USB port driving a hub and 
>> with the other
>> USB port and the SD card slot in use, which found the maximum current draw 
>> to be
>> 485 mA.  The BB Black doesn't necessarily need the CPU frequency dropped 
>> when running
>> on USB peripheral power, only the BB does, but I'm not sure how one would 
>> tell them apart
>> in software.
> Note that USB only guarantees 100ma. For highter currents, the device
> has to request it explicitely (and I'm not sure the BB does request it),
> and the host is free to refuse it.

This is very true if the thing powering the USB port is in fact a host or hub, 
not a power supply, but I think the only way to run that processor on less than
0.5W at any frequency is to leave the SDRAM and flash turned off and it clearly
couldn't boot an operating system that way.  For the BB to do the right thing 
ROM bootstrap would need to ask for the current before turning anything else on,
and simply fail if the host refused it, but the ROM doesn't ask (nor does 
anything else)
and just assumes enough power is available anyway.

I think the assumption that the power from the barrel connector is 
"sufficient", while
the current available from the USB is 500 mA, is entirely arbitrary.  Neither 
is necessarily correct, but there's no way to boot the thing in its current 
form without
assuming something.  I think in the end booting conservatively until the user 
tells it
what it can do is the only thing that can be done.

Dennis Ferguson

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