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Famed hardware hacker Bunnie Huang has posted an overview of his notes on designing an open source entropy generator. His summary links to the full notes which include schematics, measurement results, as well as other key details.
The final optimized design takes <1cm2 area and draws 520uA at 3.3V when active and 12uA in standby (mostly 1.8V LDO leakage for the output stage, included in the measurement but normally provided by the system), and it passes preliminary functional tests from 2.8-4.4V and 0-80C. The output levels target a 0-1V swing, meant to be sampled using an on-chip ADC from a companion MCU, but one could add a comparator and turn it into a digital-compatible bitstream I suppose. I opted to use an actual diode instead of a NPN B-E junction, because the noise quality is empirically better and anecdotes on the Internet claim the NPN B-E junctions fail over time when operated as noise sources. I'll probably go through another iteration of tweaking before final integration, but afaik this is the smallest, lowest power open-source avalanche noise generator to date (slightly smaller than this one [PDF]).
(Score: 0) by Anonymous Coward on Sunday April 21 2019, @05:48PM (1 child)
He states other designs were too large and power hungry, but then he does not state which designs he rejected.
The info is not anywhere to be found in his notes.
(Score: 2) by janrinok on Monday April 22 2019, @07:29AM
The design of the circuit is irrelevant if building it results in a circuit that is too large to fit his mobile device or drains too much power, although it is true that it might be of interest to some. This was the smallest design that he could build that met his very limited power budget.