Patent ID: 8081723

Claim:
Apparatus for determining at least part of a width of an eye of a serial data signal, the apparatus comprising: clock and data recovery circuitry for producing a first clock signal having a first phase, said first phase having a first phase relationship to a phase of the serial data signal; phase shift circuitry for receiving the first clock signal and producing a second clock signal based on the first clock signal and having a second phase with a controllably variable phase shift from the first phase; sampling circuitry for using the second clock signal to sample the serial data signal to produce an eye-monitor serial data signal; error checking circuitry for detecting data errors in the eye-monitor serial data signal; and control circuitry for varying the phase shift through a succession of different amounts of phase shift, wherein the control circuitry first gradually increases the amount of phase shift in a first direction from a starting amount until the error checking circuitry detects data errors exceeding a threshold, and wherein the control circuitry thereafter gradually increases the amount of phase shift in a second direction from the starting amount until the error checking circuitry again detects data errors exceeding the threshold.