Electronic – Why the MCU gets too hot when I plug it in to the USB

A debug LED (You can convert it into a watchdog blinky later to verify that your main loop/1ms interrupt or whatever you're using is still running) is something that I would consider pretty mandatory for an exploratory board. Hello World on your new PCB does not need to be as complex as an LCD. You could repurpose a backlight controlling MOSFET for this purpose if you don't want to add the real components.

I'm assuming you're giving yourself some form of breakout for your extra pins - An LCD screen is great, and I understand the desire to keep it simple, but there's little that can go wrong simply by adding a trace to nowhere, and nowhere can become somewhere someday. Even if you don't want to add real headers, some test points (in the form of staggered rows of .05x.1" copper pads) will let you solder and hot glue some wires on later. This doesn't have to be a big deal. I'd put some jumpers/resistors on those lines, so you can add some 1k resistors to protect your pins from being shorted or hit with ESD if you decide to do so. This also gives you the ability to pull any of your other pins high or low if later you find this is necessary!

One thing that I do on a first board is add a lot of vias. Vias are your friends when making modifications (assuming you're getting this done at a PCB house and don't have to drill them yourself). If you've got two vias on every trace, even if you don't change sides with your trace, you can cut the trace later with an Xacto and run 30-ga wire wrap wire between the traces that need to be swapped (Make sure your vias are big enough for this, though). You can also add 0805 0-ohm jumpers (solder bridges are cheap; you don't need to buy components) and solder wires to the pads later if you don't like the via method. Probably won't be necessary, but it's cheap/free insurance.

Oh, and connect the LCD/USB setup first, then tack wires on temporarily from your working breadboard to make sure that the externals are working.

One popular method to get data from one device running off one power supply to some other device running off some other power supply -- perhaps at much higher voltage -- is to solidly connect the grounds together and use a "discrete inverter" (level shifter) built from one discrete FET and one resistor for each signal (in your case, 5 signals).

Your MSP430 drives the gate of the FET. When the MSP430 sends a "hi" output (+3.3 V), turning the FET "on", the FET pulls the output close to GND. When the MSP430 sends a "lo" output, turning the FET "off", the resistor -- with one end connected to the "other" power supply, the power supply connected to the 4056 -- pulls the output "hi". In your case, the output is pulled close to +12 V when your +12 V power supply is turned on and connected, or 0 V if it isn't. No constant current can leak through this inverter / level shifter.

This arrangement "inverts" the signal. In principle one could add another inverter (a single 74HC14 or 5 more transistors and resistors) in series so that "hi" input gives "hi" output, and "close to GND" input gives "close to GND" output, but many people make the hardware a little simpler at the cost of adding a few more lines of code to compensate.

As an alternative to the one FET one resistor discrete inverter, some people use a "discrete inverter" built from one discrete BJT and two or three resistors for each signal; it works very similarly. a b c