How to mix external entropy to a random number generator?
Based on the current details, this will work where I = internal generator and E is the external one:-

random bytes = I ⊕ SHA1(E)

The equation assumes that I is a good quality RNG.  The key here is to get enough entropy from E.  You should measure it's (min.)entropy generation rate in bit/s sample and then take enough samples to ensure you have F times 16 bytes of it.  F is a safety factor which ideally should be >2 but I'd just go with F = 100.  If you have an analogue entropy source you can easily get 10 kSa/s with an on chip ADC, and a digital entropy source can be sampled at something in the order of 10 MSa/s.  Depends really on the features of your micro controller.

The hash allows a dissimilar amount of entropy to be easily combined.  It also acts as a randomness extractor for E, so you don't have to be debias the entropy and it'll deal with huge auto correlation.

Note.  I would make the argument that there's no point in having the internal generator (I).  If you have the luxury of a true entropy generator, use it.  The hash extractor function means that additional whitening is not necessary.  As Shane tells Joey, if you can use it, one's all you need.  And how would I be seeded anyway?