Some microcontroller projects require the generation of random numbers. The first thought is to simply use the random() call provided by the compiler libraries. That function provides psuedorandom numbers which will probably seem random enough at first pass.

Unfortunately, you may realize that after some usage, the random numbers generated by this function always gives the same sequence of results. This is caused by the microcontroller always using the same seed for the generator. If you want the numbers to no longer be completely deterministic, you will need to provide a seed to the random number library. This is typically done with srandom(uint32_t). By providing a seed, you can initialize the random library with a different state.

The question becomes, “what can you use as a seed?” If all your devices have unique serial numbers, those could be used. Then at least each of your devices should have a different sequence of random numbers. Unfortunately, those sequences would still be the same on each reboot for a single devices. Below I will present a method for getting enough entropyon an Atmel AVR to seed the generator for what I call “usefully random.”

The Atmel AVR Mega series has several different oscilators which are not phase locked. This means that you can use the interference between these oscilators to provide some variability. The easiest way to do this is to setup an interrupt off of one oscillator and count the ticks of another oscillator. For example you can setup a Timer/Counter to run from the main high speed oscillator, and have the Watchdog Timer running from the 128kHz oscillator trigger an interrupt. From the interrupt the counter can be queried. There is quite a bit of jitter between the oscillators which means that the lowest few bits in the counter can be used for entropy. Just do this a few times and rotate in the bits to a 32 bit value. Then that value can be used as the seed.

An additional source of entropy can be obtained by sampling a floating analog pin repeatedly. The bottom couple of bits can be used as noise measurement. By mixing both approaches, it is pretty easy to have enough entropy to seed the generator in less than 100ms after booting, allowing for use of the ADC and Timer/Counter for other application purposes.