BeagleBoard-XM with a battery - USB undervolt problem (and solution!)

Hi All,

I just thought I'd write in and discuss an issue I was having, and the
solution I found - in case anyone else comes across it.
I'm building a beagleboard-xm powered hexacopter (like a quadrocopter
but with 6 motors) - and I have a series of USB devices in the beagle
for the various components (arduino for PWM for the ESCs, a USB->GPS
module, and an IMU (Gyros/Accels/Etc). I'm powering all this with a
5V power regulator plugged into the 5V barrel connector plug on the
beagle.

The problem was, when I powered the beagle off a LiPo battery, plugged
into a 14.8v->5v power regulator, the board would initially come on,
but when it switched on the USB bus, the various LEDs on my USB
devices would flicker quickly, then the board would reset and the
cycle would repeat. Powering off a mains 5v powerpack was fine,
however. What was odd is that if I let the board boot up on battery
with no USB devices plugged in, and then plugged them in one at a time
- it was fine.

Measuring current, the board was initially pulling 0.6A @ 5v, but just
before the reset - it jumped to 0.8A. On mains, it went from 0.6A-

1.2A in about 0.2 seconds. I wouldn't have thought that would be too

much for my 3A turnigy UBEC to handle, but there you have it.

I pulled out my trusty digital multimeter, suspecting an undervolt -
but for the life of me, I couldn't see one. If it was there, it was
too fast for my meter to pickup.
So I headed to my local electronics shop, and picked up a 10v 4700uF
electrolytic capacitor, and soldered it in parallel across the
positive/ground, just before my barrel connector going into the
beagle. I then fired up the board - perfect!

If any one else is having a similar problem - i.e. a bunch of USB
devices suddenly pulling current at poweron - this may well be your
solution.