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A fully bidirectional mixed-voltage input/output (I/O) buffer using a novel floating N-well circuit is presented. To provide appropriate gate voltages for output stage transistors, a dynamic gate bias generator without gate-oxide overstress effect is implemented. The proposed I/O also takes advantage of a novel gate-tracking circuit and a PAD voltage detector by means of eliminating the leakage current such that the compatibility among all subcircuits is ensured. Our design is proved on silicon using 0.18 μm CMOS process that when VDDIO is 5.0/3.3/1.8/1.2/0.9 V, the maximum data rate is found to be 80/80/125/100/80 MHz, respectively, with a given capacitive load of 10 pF.