A Magnetic Tunnel Junction Based True Random Number Generator
with Conditional Perturb and Real-Time Output Probability Tracking
Won Ho Choi*, Yang Lv*, Jongyeon Kim,
Abhishek Deshpande, Gyuseong Kang,
Jian-Ping Wang, and Chris H. Kim*equal contribution
University of Minnesota, Minneapolis
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Outline of Presentation
• True Random Number Generator (TRNG)
• Magnetic Tunnel Junction (MTJ)
• MTJ-based TRNG
• Conditional perturb scheme
• Real-time output probability tracking
• Conclusions
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An Application of True Random Number Generator (TRNG)
• Generates independent, unpredictable, nondeterministic, and aperiodic random numbers
• Use random numbers to generate secret keys
Q. Tang, et. al., CICC, 2014
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Prior Art of Physical TRNG
• Direct noise amplification from devices
– Random Telegraph Noise (R. Brederlow, ISSCC, 2006)
– Resistor thermal noise (V. Kaenel, CICC 2007)
– Requires post-processing to achieve sufficient randomness
• ROSC based TRNG (M. Bucci, Tran. on Comp., 2003; Q. Tang, CICC, 2014)
– Harvesting noise from oscillator jitter
– Generally requires noise amplification otherwise yield with low efficiency, thus increases design complexity
• Metastability TRNG (C. Tokunaga, JSSC, 2008; S. Mathew, JSSC, 2012)
– Inverter pair driven to metastable state
– Requires continuous calibrating loop
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Magnetic Tunnel Junction (MTJ)
• Spin polarized electrons rotate the magnetization direction of free layer with spin torque
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Switching Probability of an MTJ
H. Zhao, et. al., JAP, 2011
• Random thermal fluctuation in an MTJ can be utilized for generating random bits
• Trade-off relationship between speed, switching energy, and reliability
• Switching probability is sensitive to operating conditions6
MTJ-Based TRNG- Unconditional Reset Scheme -
• Applies large reset voltage in every cycles
thereby, adversely effecting on TRNG
performance
S. Yuasa, et. al., IEDM, 2013, concept only
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Proposed Conditional Perturb Scheme
• Perturbs the MTJ according to the previously
sampled MTJ state, thereby eliminating the
reset phase
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MTJ Time-to-Breakdown Analysis
• Absence of a reset phase enhances the
lifetime of the MTJ
Fail
ure
(%
)
9
C. Yoshida, et al., IRPS, 2009
Measurement Setup
• Random number generator measurement
setup with sub-50 picosecond pulse width
resolution.11
Measured Probability
• A small number of segments fail to meet
50±±±±1% probability
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Unconditional reset scheme Conditional perturb scheme
Measured Randomness
• Both schemes show a similar level of
randomness
• The output data fail to pass the frequency and
cumulative sums tests 13
Test Pass/Fail
1 Frequency Fail
2 Block frequency Pass
3 Cumulative Sums Fail
4 Runs Pass
5 Longest-Run-of-Ones Pass
6 Rank Pass
7 FFT Pass
8Non-overlapping
Template Matching
9 Serial Pass
10 Approximate Entropy Pass
# of segments: 55
Pass
Unconditional reset scheme Conditional perturb scheme
• Simple single-parameter feedback control
• The proposed techniques were implemented
in LabVIEWTM and experimentally verified
using a fabricated MTJ device
Real-Time Output Probability Tracking
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Measured Probability and Randomness- Real-Time Output Probability Tracking-
• Proposed conditional perturb and real-time
probability tracking achieves a good
randomness while improving the reliability,
speed, and power
Test Pass/Fail
1 Frequency
2 Block frequency Pass
3 Cumulative Sums
4 Runs Pass
5 Longest-Run-of-Ones Pass
6 Rank Pass
7 FFT Pass
8Non-overlapping
Template Matching
9 Serial Pass
10 Approximate Entropy Pass
Conditional perturb scheme, # of segments: 55
Pass
Raw data after probability tracking
Pass
Pass
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TRNG Performance Comparison
• Conditional perturb scheme improves the
speed, switching energy and reliability
*S. Yuasa, et. al., IEDM, 2013
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• It could potentially allow massive generation of random numbers with negligible circuit overhead
A Possible Application with STT-MRAM
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• We demonstrate for the first time a True
Random Number Generator (TRNG) based on
the random switching probability of Magnetic
Tunnel Junctions (MTJs)
• Proposed conditional perturb and real-time
output probability tracking achieves a good
randomness while improving the reliability,
speed, and power
Conclusions
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