The soft calibration module implements some aspects of Phases 1, 2, and 3 of calibration.Note:This Answer Record is a part of the Xilinx MIG Solution Center (Xilinx Answer 34243). The Xilinx MIG Solution Center is available to address all questions related to MIG. Whether you are starting a new design with MIG or troubleshooting a problem, use the MIG Solution Center to guide you to the right information.
The first major operation is Phase 1 of calibration or calibrated input termination. The soft calibration module measures the value of an external resistor connected between theRZQ pin and ground to determine the desired on-chip Input Termination value forthe bidirectional dq, ldqs_p/n, and udqs_p/n pins. This only occurs you select the Calibrated Input Termination option in the MIG GUI flow (see the "Setting FPGA Options" section in theSpartan-6 FPGA Memory Interface Solutions User Guide(UG416). Otherwise, an approximate uncalibrated on-chip termination or external termination is assumed, and this startup step is skipped.
(Xilinx Answer 43638)Calibrated Input Termination?
The second major step of the startup sequence is to load the memory device mode registers with the desired parameters.
After the memory device has been configured, Phase 2 of calibration occurs. This phase adds delay to the input path of the DQS strobes entering the FPGA. The goal is to shift the DQS strobes into the center of what becomes the Read Data capture window.
(Xilinx Answer 43667)DQS Centering?
Afterall of the operations in the startup sequence have completed, the MCB enters normal operation. Commands and data can be loaded into the User Interface FIFOs while the startup sequence is in progress, but no commands are executed until calibration completes and the block enters normal operation.
During normal operation, the soft calibration module continuously monitors the tap delay values of the IDELAY element used to delay the DQS input paths (for more information on IDELAY, see the Spartan-6 FPGA SelectIO Resources User Guide). The intent is to measure any change in the per tap delay value due to voltage or temperature variations during operation. If a shift in tap delay value is detected, the tap delay count on the DQS strobe input paths can be adjusted to keep them centered in the Read Data capture window. The update to the IDELAY values is done during memory REFRESH operations to avoid impacting normal data operations and controller efficiency. Phase 3 of calibration is known as continuous DQS tuning.