hmurray at megapathdsl.net
Wed Aug 31 08:19:49 UTC 2016
gem at rellim.com said:
> Saving power is good, but I suspect the extra power is minimal. I hace USB
> power meters, so we can measure this.
It depends on the details. You can save a lot of power by turning stuff off.
The more you turn off, the more power you save but the longer it takes to
get running again. I think most CPUs these days have an instruction that is
roughly "halt and wait for an interrupt". That lets the CPU turn off the
clock for the instruction unit so it doesn't have to burn power running a
tight loop. As an experiment, you could write some code that goes into a
tight loop and see if that takes more power than not running anything.
The next step is things like putting the memory into self refresh mode or
turning off the disk controller.
USB is ugly to power down since there aren't any interrupt wires. It polls
for "interrupt" requests, so you can't power that off if you want a wakeup
from an interrupt on a USB device.
In the extreme, you can power off the clock generation circuit. You need
something like a push button to wake up. (or timer based on a low power
clock) If the clock is off, the only power is leakage which can be very low
if you use a silicon process targeted at low power rather than high
I plugged a PC into an Ethernet Hub the other day. While powered off, the
hub said 100 megabits. I assume that was the wake-via-LAN stuff and it was
running at 100 vs 1000 to save power.
Your USB power meter should work for a Raspberry Pi or BeagleBone. I assume
you have a wall-power meter for a laptop or desktop.
These are my opinions. I hate spam.
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