There are several entropy sources like this -Havege, jytter and Dakarand all tries to use jitter and unpredictable behavior in CPUs. The chief advantage is tha

There are several entropy sources like this -Havege, jytter and Dakarand all tries to use jitter and unpredictable behavior in CPUs. The chief advantage is that they don't need another HW source beside the CPU itself. I've been sceptic to how well they work on simpler architectures like ARM. (Havege for example tries to force cache misses in multi level memory system, which implies a need for caches). But at least Jytter actually seems to work fairly well on embedded CPUs too. What to remember is to use these as entropy sources, not source for random numbers. Seed a proper csprng and use that to generate numbers. Quite a few CPUs today can implement AES efficiently so AES-CTR is probably a good choice in many systems.