PPS undersampling

[Hal Murray]

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 
performance.


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.