A method for operating a data interface circuit whereby calibration adjustments for data bit capture are made without disturbing normal system operation includes initially establishing, using a first calibration method where a data bit pattern received by the data interface circuit is predictable, an optimal sampling point for sampling data bits received by the data interface circuit, and during a normal system operation and without disturbing the normal system operation, performing a second calibration method where the data bit pattern received by the data interface circuit is unpredictable. The second calibration method determines an amount of a timing drift for received data bit edge transitions and adjusts the optimal timing point determined by the first calibration method to create a revised optimal timing point. The second calibration method samples fringe timing points associated with the transition edges of a data bit.