MSP430 microcontrollers doesn’t come with native USB peripheral, so if you want to use USB with these micros you need to find a way around the problem. One way as usually is using USB interface chips. This method puts the USB load to hardware level and leaves microcontroller more processing resources. But sometimes we don’t want additional cost or simply don’t want to occupy more PCB space. Then a software USB implementation might work.
Currently implementation works for MSP430F2274 and MSP430G2553. Code fits in 9kb of flash memory and runs fine with up to 512 bytes of RAM. So this is cheap solution like V-USB for AVR devices. If you would like to give a try – download Mecrisp from sourceforge.