Ring Oscillator TRNG Jitter Characterization Using Allan Variance
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Solution Overview
Problem
Existing methods for characterizing the jitter of ring oscillators in true random number generators (TRNGs) are complex, require significant additional surface area, and suffer from inaccuracies due to uncontrollable frequency ratios and noise contributions, making them inefficient and resource-intensive.
Innovation Solution
A device utilizing two identical ring oscillators with a synchronous flip-flop and counter to control the period difference, coupled with a circuit to calculate Allan variance on accumulated values, sets a target entropy and autocorrelation threshold to simplify the characterization and improve accuracy, allowing for efficient random number generation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If existing methods for characterizing jitter are used, then measurement precision can be achieved, but device complexity and surface area requirements increase significantly
Solution Approach 1:
The patent merges the jitter characterization function with the random number generation function by using the same two ring oscillators and counter circuitry for both purposes. The Allan variance calculation is performed on the counter output during normal TRNG operation, eliminating the need for separate characterization circuits and reducing overall device complexity while maintaining measurement precision.
Solution Approach 2:
The counter circuit serves dual functions: it counts oscillation periods for random number generation and simultaneously provides the data needed for jitter characterization through Allan variance calculation. This multi-functionality approach allows the same hardware resources to perform both TRNG operation and entropy source characterization without requiring additional dedicated circuits.
2Measurement precision
If frequency divider circuits are used for characterization, then measurement precision improves, but noise contributions and inaccuracies increase
Solution Approach 1:
The patent extracts the frequency division function from a separate divider circuit and integrates it directly into the counter operation. The counter naturally performs frequency-related counting based on the oscillator periods, eliminating the need for external frequency divider circuits that would introduce additional noise and inaccuracies. This extraction of the problematic component while preserving the needed functionality resolves the contradiction.
3Measurement precision
If additional characterization circuits are added, then measurement precision improves, but productivity and throughput are reduced
Solution Approach 1:
The patent enables continuous useful action by performing jitter characterization calculations during the ongoing random number generation process. The Allan variance calculation uses the counter output values that are continuously being generated during TRNG operation, so characterization is not a separate batch process but occurs continuously in parallel with random bit generation, maintaining full throughput while achieving precise characterization.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution simplifies the characterization process, reduces resource requirements, and enhances the accuracy and throughput of random bit generation by controlling noise components, ensuring compliance with minimum entropy and negligible autocorrelation.
Implementation Method 1
a synchronous flip-flop configured to deliver an output signal corresponding to a sampling of an output of the first oscillator at a frequency of an output of the second oscillator
Implementation Method 2
a counter configured to deliver, for each period of the output signal of the flip-flop, a value equal to a number of periods of the second oscillator counted during said period
Implementation Method 3
a first circuit configured to modify a period of at least one of the two oscillators so that a mean difference between the periods of the two oscillators is equal to a target difference
Implementation Method 4
a second circuit configured to: initialize a value of an integer K, calculate sums of K successive values of the counter, calculate an Allan variance on the calculated sums
Implementation Method 5
These phenomena include thermal noise resulting from thermal agitation and flicker noise induced by the charging/discharging of traps of the gate oxide
Implementation Method 6
The uncertainty (or jitter) on the actual period of an output signal of a ring oscillator as compared with the theoretical period of this signal is used as a source of randomness
Data Source
AI summary
The present description concerns an electronic device (1) comprising: a first ring oscillator (RO1) and a second ring oscillator (RO2); a synchronous flip-flop (FF); a counter (COUNTER); a first circuit (Nm CTRL) configured to modify a period of at least one of the two oscillators (RO1, RO2) so that a mean difference between the periods of the two oscillators is equal to a target difference; and a second circuit (PROCESS) configured to: initialize a value of an integer K, calculate sums (VAL) of K successive values of the counter (COUNTER), calculate an Allan variance (VAR) on the calculated sums (VAL), and set K to its current value if the calculated variance is greater than a first threshold and increment K otherwise.