A fully-digital True Random Number Generator (TRNG) measures the frequency difference between two free-running ring oscillators, or in other words the beat frequency, to extract random frequency jitter. For generating a continuous stream of random bits with a high entropy level, the lower significant bits meeting the NIST randomness criteria are concatenated. The generation efficiency is further improved by utilizing a multi-phase structure. The proposed circuit fabricated in 65nm achieves an energy efficiency of 15.1Mb/mW at 0.8V. Experimental data collected from eight TRNG test chips passed all 15 NIST tests without the use of any feedback or tracking scheme.