Finding a source of entropy in an embedded system?

For a small embedded device (TI MSP430F2274), I am trying to create a PRNG, but am having difficulty identifying a potential source of entropy to use as a seed. Unfortunately, the device does not have enough available memory space to include time.h and incorporate the function srand(time(0)). Has anyone had experience with this family of devices, or has incorporated a PRNG in an embedded device that is constrained by its memory space? Thank you.