Eye scan is a useful feature but it can be costly in terms of resources (see Resources). To save hardware resources, it is possible to shift the NIDRU sampling point inside the eye to measure the eye aperture. With this method, the eye scan can be performed only during the debug session, because bit errors are visible as soon as the sampling phase is pushed closer to the borders of the eye.
The port SHIFT_S_PH can be used to move the sampling point. The port is specified in twos complement and ranges from -128 to 127. 256 steps completely cover one UI. By setting SHIFT_S_PH to 0 (default), the NIDRU samples the incoming data stream in the middle of the eye.
The eye scan ports operate independently on the port SHIFT_S_PH. This means that each eye scan is always performed relative to the middle of the eye diagram and not to the setting of SHIFT_S_PH.