Abstract: A phase detector for use in conjunction with a delay locked loop is provided. Programmable delay elements insert an adjustable delay in a received data stream. The programmable delays stress the setup and hold times of the incoming data. Phase detector sampling logic detects the phase difference between a nominal center of the data window, and the limits on the setup (early) edge of the data value window, and the hold time limit (late time) edge of the data valid window (“guardbands”). A data signal arriving earlier than an early guardband or later than a late guardband may not be correctly sampled, and a guardband failure may be said to have occurred. A state machine detects such guardband errors and provides corrective feedback signals.