Switch for turning off MSP430
The problem is that the PL2303 (and most UART configurations) idle with a high output on TX out. This ends up powering your MSP430. Yes, the logic high output of PL2303 TX is powering your MSP430 through one its input pins.

That's because the inputs of most CMOS ICs have protection diodes that are connected to VDD, like in this diagram (from this useful website)



So when the PL2303 drives the MSP input high and MSP430 power is disconnected, that voltage ends up going through the protection diode and into the MSP430 power rail. So you are powering your MSP with the PL2303!

So to solve this, you can do a couple of things:

1) Insert a current limiting resistor to limit the current going into the MSP input. This generally a lousy solution because a small amount of current still flows, and your UART speed will be affected.

2) Gate the TX of PL2303 so that the MSP input is low when you want it to power off. Here's a simple example:







simulate this circuit &ndash; Schematic created using CircuitLab

So when MSP430 power goes low, the AND gate input goes low and its output goes low. There are many ways to do the same; just an example.