An eye-opening monitor circuit in 0.13 μm CMOS operates from 1 to 12.5Gbit/s at 1.2V supply. It maps the input eye to a 2D error diagram with 68dB mask error dynamic range. Left and right halt of the eye are monitored separately to capture asymmetric eyes. Tested input amplitude is from 50 to 400mV. The chip consumes 330mW and works at 10Gb/s with a supply voltage as low as 1V.