High speed links employ sophisticated analog circuits and logic in order to achieve performance targets and in many cases utilize a plurality of calibrated local clocks in order to transfer data while maximizing data transmit and capture margins. Phase rotator circuits are commonly used to produce the plurality of local clocks. Phase rotators act as clock phase mixers and provide a mechanism of creating, manipulating, and calibrating tightly timed clock edges from a much smaller set of high accuracy root phases. For example, a phase rotator may be designed to provide 128 output phases using just 16 selectable input phases. Further, the compact nature of the phase rotator structure allows for a plurality of phase rotators to be placed within the high speed link. For example, a DDR data link may be implemented with separate Rx (read) and Tx (write) clock phase rotators for each data bit/lane, and provided with additional phase rotators for digital synchronization and calibration.
Manufacturing defects have the potential to totally disable a phase rotator and, as such, it is common to test for such defects. However, it can be very difficult to ensure no manufacturing defects exist in a phase rotator design. A manufacturing tester normally does not have fine enough resolution to discern whether a phase rotator is properly operating at its functional speed since the manufacturing tester typically operates at a slower speed than the phase rotator operates. Also, some manufacturing testers do not have fine enough granularity to discern the individual step increments of a phase rotator. As a result, the quality of the manufacturing test is reduced to match the quality of the manufacturing tester or test-only logic is inserted into the design. This is counterproductive since reducing the quality of the manufacturing test may lead to defective parts being released from the manufacturer, and inserting test-only logic onto the design increases area and power demands.