RANG timing spec?

[bluebeam]

It looks like the random number generator is relative slow as implied by the documents. I wonder if it has been spec'ed some where? Exactly how much time does it take to generate a byte? Or at least some idea of worst case timing? It seems important to know that if we have to wait in a while loop waiting for random bytes to be available.

[MartinBL]

Hi,

What chip are you looking at? We have numbers for the nRF51 in the Product spec v3.3:

Values for the nRF52 can be found here.

Both chips also have an event register called VALRDY that you can use to get notified for every new random number written to the VALUE register.

A simple way to use it is like this:

while(NRF_RNG->EVENTS_VALRDY != 1)
{
    // Do nothing, but wait
}

[MartinBL]

As mentioned in the specs "The time [t_RNG,UNI] to generate a byte cannot be guaranteed". The RNG uses Von Neumann’s debiasing algorithm which compares pairs of bits generated by the RNG and if both values in the pair are the same ('0' or '1') they are discarded. Hence, there is a probability that you will get long sequences of 0's or 1's and that the process will take a (relatively) long time.