Is the power supply pins (5V, 3.3V, GND) unidirectional or can they be used to power external ICs when
a) the device is powered by USB
b) the device is powered by 3.3V battery
If the power supply pins cannot be used then can the GND ternminal be used to atleast get a common ground.
Any technical information on the circuit will be very helpful. The link on the website goes around in circles to the mbed website and back without any technical information other than the pinout.
http://developer.mbed.org/platforms/Outrageous-Circuits-mBuino/
About half way down there is a link to download the schematics and Eagle cad files.
To answer your question:
the 3.3V and GND IO are direct connections to the CPU power rails. They can be used to supply or source power.
+5V IO is an input only intended as a way to power the on board regulator without needing the USB connected. There is no 5V output available when running from USB unless you make hardware changes.
When running from USB or from the +5 IO pin the 3.3V IO will be 3.3V (minus a diode drop of between 0.1 and 0.2).
When running from the battery terminals the 3.3V IO will be the battery voltage (again minus a diode drop of between 0.1 and 0.2)
All grounds (USB,battery and IO) are tied together.
Thanks a lot for your help. 