An 8-bit, 3-stage asynchronous gated ring oscillator (GRO) time-to-digital converter (TDC) is presented. It employs asynchronous techniques to achieve minimum GRO stages. This lead to about 40% to 70% gate count reduction compared to synchronous GRO-TDC. Count-missing, glitch, and unnecessary addition are eliminated. The uncorrupted noise shaping characteristic is obtained. The chip is implemented in a 0.18 μm CMOS technology. It occupies small area (140μm×310μm) and consumes low power (4mW to 13mW).