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authorRob Austein <sra@hactrn.net>2020-09-13 23:15:43 +0000
committerRob Austein <sra@hactrn.net>2020-09-13 23:21:33 +0000
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-```
-#!htmlcomment
-
-This page is maintained automatically by a script. Don't modify this page by hand,
-your changes will just be overwritten the next time the script runs. Talk to your
-Friendly Neighborhood Repository Maintainer if you need to change something here.
-
-```
-
-```
-#!html
-<h1>rosc_entropy</h1>
-
-<p>Digital entropy source based on on jitter between multiple, digital ring
-oscillators. The entropy source is used in the TRNG as one of several
-entropy sources feeding the mixer.</p>
-
-<h2>Functionality</h2>
-
-<p>The digital oscillators are created using adders. The carry out from the
-adder are inverted and fed back into the adder as carry in. In
-combination with operand values we basically get an inverted signal (the
-carry out) that toggles reapeatedly. By having the operands externally
-defined, synthesis tools in general will not optimize them away.</p>
-
-<p>The carry out signal is sampled with a clock that toggles at a much
-slower rate than the intrinsic toggle rate of the carry out signal. In a
-modern FPGA, the toggle rate may be 400+ MHz while the sample rate might
-be 10 kHz. This sample time allows the differences in intrinsic toggle
-frequency between separate oscillators to drift apart after sampling.</p>
-
-<p>The entropy source contains 32 separate oscillators. The outputs from
-the oscillators are XOR-combined to create a single entropy bit. Entropy
-bits are collected into 32-bit words which are provided to entropy
-consumers.</p>
-
-<h2>Implementation Results</h2>
-
-<p>The entropy source has been implemented, tested and shown to produce
-good quality entropy (using the ent estimation tool etc) in Altera
-Cyclone-IV and Cyclone-V devices as well as in Xilinx Spartan-6.</p>
-
-<p>The Xilinx synthesis tool will try to optimize the combinational loop
-away. (More specifically, it claims that the oscillator sample registers
-will have a fixed value.). There is therefore an attribute in the source
-code to force the tool to preserve the register.</p>
-```
-
-[[RepositoryIndex(format=table,glob=core/rng/rosc_entropy)]]
-
-| Clone `https://git.cryptech.is/core/rng/rosc_entropy.git` |
-|---|