True random number generator (RNG) RM0453
634/1450 RM0453 Rev 5
conditioning algorithm, a health monitoring block and two interfaces that are used to interact
with the entropy source: GetEntropy and HealthTest.
Figure 99. NIST SP800-90B entropy source model
The components pictured above are detailed hereafter.
Noise source
The noise source is the component that contains the non-deterministic, entropy-providing
activity that is ultimately responsible for the uncertainty associated with the bitstring output
by the entropy source. This noise source provides 1-bit samples. It is composed of:
• Multiple analog noise sources (x6), each based on three XORed free-running ring
oscillator outputs. It is possible to disable those analog oscillators to save power, as
described in Section 22.3.8: RNG low-power use.
• The XORing of all the noise sources into a single analog output.
• A sampling stage of this output clocked by a dedicated clock input (rng_clk with
integrated divider), delivering a 1-bit raw data output.
This noise source sampling is independent to the AHB interface clock frequency (rng_hclk),
with a possibility for the software to decrease the sampling frequency by using the
integrated divider.
Note: In Section 22.6: RNG entropy source validation the recommended RNG clock frequencies
and associated divider value are given.
Post processing
In the NIST configuration no post-processing is applied to the sampled noise source. In non-
NIST configuration B (as defined in Section 22.6.2) a normalization debiasing is applied,
that is half of the bits are taken from the sampled noise source, half of the bits are taken
from the inverted sampled noise source.
MSv44200V3
Entropy source
Noise Source
Digitization
Post-processing
(optional)
Raw data
Conditioning
(optional)
Heath
tests
Output
(GetEntropy)
Error
message
(HealthTest)
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