Fig. 19 illustrates the on-chip true random number generator.
It consists of three entropy cells. Each cell by itself provides sufficient
entropy. Having three cells provides redundancy and also
enables reduction of entropy accumulation time to get the desired
amount of entropy. The source of entropy is thermal noise
from n-well resistors. This noise, after amplification by a differential
amplifier, modulates the VCO frequency. On-chip clock
samples the VCO output and sends it to a linear feedback shift
register (LFSR) which accumulates entropy over a pre-set accumulation
time. Privileged software can program a timer with
the desired entropy accumulation time. This timer blocks loads
from the LFSR before the pre-set entropy accumulation time
has elapsed to make sure the random number in the LFSR has
enough entropy before it is used