Content originally posted in LPCWare by arw on Tue Dec 30 11:06:53 MST 2014
Hello,
If the device is not powered, then the state of the chip is completely undefined. It is dangerous for another device to drive any voltage in a circuit that connects to an IC that is powered off. Either IC may be damaged since the IOs are not in a controlled state.
If you want to ensure that the device is in a known state, then try using Deep Power-Down mode, for example, which holds all pins (except WAKEUP) in tri-state mode. This will require that you keep an active power supply to the device.
If you must power-off the part of the system where this MCU is included, then all connecting circuits in the powered-on area should be high impedance, and must not drive a voltage on these connecting paths.
-arw