USR2 is always on but dim on my BBB, it occasionally blinks brighter and then returns to dim (when idle). Is this normal? I don’t recall seeing this behavior on the previous images, I thought it was completely turning off.
This is a fresh install of 2013-06-06, I just installed my 8192cu.
I would try a previous image. Not sure which revision you have, but on Rev A5B we made them a lot dimmer. So it could be some bleed over now that it is not quite so bright. I guess if depends on how dim it is. It could be that there is some leakage in the transistors that turn them on.
Gerald
USR2 is by default configured as an indicator of activity on the
processor. Even when the board is doing nothing, the CPU is not
completely idle. There are periodic tasks that run several times a
second, there are always many processes in the background that need to
be serviced from time to time. Each of these actions will cause USR2
to light up. When there is not much activity you will see only a faint
glow. During periods of higher load you will see the LED flashing or
even constantly on. It's a convenient way to see how loaded your
system is.
You can change the default behavior by writing to
/sys/class/leds/beaglebone:green:usr2/trigger. There are several
triggers that can be configured on the use LEDS. You can see the list
by looking at the contents of the trigger file:
root@beaglebone:/sys/class/leds/beaglebone:green:usr2# cat trigger
none nand-disk mmc0 mmc1 timer oneshot heartbeat backlight gpio [cpu0]
default-on transient
The active trigger is marked by square brackets. If the default
behavior bothers you, you can switch the LED off:
root@beaglebone:/sys/class/leds/beaglebone:green:usr2# echo none >trigger
root@beaglebone:/sys/class/leds/beaglebone:green:usr2# cat trigger
[none] nand-disk mmc0 mmc1 timer oneshot heartbeat backlight gpio cpu0
default-on transient
Hope it helps,
j.
Thank you both for the replies. I understand that a CPU is constantly
processing tasks, but is it really that constant? The LED never goes
off, just dim.
I have rev A5A. The LED is about half or third as bright as the other ones.
Putting the trigger to none will completely turn it off as expected,
when changing back to cpu0, it goes back to dim light. Setting it as
default-on makes it bright as expected as well.
I just find it odd that there are no idle time at all where the CPU is
actually at 0% and USR2 is off...
CPU is idle many times per second. The idle times are interrupted with
short periods of activity, so short that the average load is very
close to 0. The effect is similar to have PWM powered LED. Even with a
very small duty ratio, very close to 0, you will see a faint glow. If
you have an access to an oscilloscope you can connect it to the LED
and see how long it is actually turned on.
The fact that your load meter shows 0 does not mean that the CPU does
completely nothing. It only means that the load meter shows 0. If you
look at the load without rounding to integer you will see that value
is close to, but not exactly 0.
j.
Do you guys have the same thing on your BBB? Is your USR2 just dim (not off)?
Mine stays dim.
Gerald
Ok i shouldn't worry then, thanks for checking. 
No, I would not worry. There are plenty of other things to worry about.
Gerald