Random Number Generation 

Random numbers might be generated by combining some pseudo-random sequence generation function with bitstream from hardware based random bitstream generator. The role of the pseudo-random sequence generation function is to compensate for the slowness of the hardware based bitstream generator. The hardware based bitstream generator might use light, sound, electrical noise (even regular electrical noise, like the 50Hz or some engine noise) for an input to some fast cryptographically relatively strong hash function (not unrollable, has collisions). The hash function output would be the random bitstream. 

Hash Function

Probably one needs to write that from scratch, because 8bit MCUs are usually not used for executing cryptographically strong hash functions. The MurMurHash is not designed to be cryptographically strong, but it might give some ideas.