Abstract: A true random number generator comprises a ring oscillator which is triggered to start oscillating in a first mode of oscillation at an oscillation start time. The first mode of oscillation will eventually collapse to a second mode of oscillation dependent on thermal noise. A collapse time from the oscillation start time to the time at which the oscillator collapses to the second mode is measured, and this can be used to determine a random number. The TRNG can be synthesized entirely using standard digital techniques and is able to provide high randomness, good throughput and energy efficiency.