The final stage of DQS read centering that is completed before normal operation is to repeat read DQS centering with a complex data pattern. The purpose of using a complex pattern is to stress the system for SI effects such as ISI and noise while calculating the read DQS center position. This ensures that the read center position can reliably capture data with margin in a true system.
The first step in this stage is to revert the PQTR and NQTR delays to 0. At tap 0 if invalid data is not detected on all data bits, increment DQ and DBI IDELAYs until invalid data is detected. Next, the PQTR and NQTR delays are incremented to detect invalid to valid and then valid to invalid edges of the read window. In the final step, read DQS is centered in the valid window.