The Spartan-6 FPGA MIG design includes top-level RTL ports "selfrefresh_mode" and "selfrefresh_enter". The selfrefresh_mode signalis driven by the MIG design.However, selfrefresh_enter is a user-driven port which is left unconnected/floating inMIG 3.5 and oldergenerated designs.
Should the selfrefresh_enter ports (1 per MCB interface) be left unconnected?
The MIG design leaves these ports unconnected as it is up to the user whether or not the Self Refresh feature is used.When the design does not use Self Refresh, these signals must be grounded.Leaving the signals floating can result in issues in an implemented design.
In MIG 3.6 and newer designs, the selfrefresh_enter signal is tied to 1'b0. If the design is using Self Refresh, the ports should be controlled by user logic as stated in the Spartan-6 FPGA Memory Controller User Guide (UG388); see the MCB Operation -> Self Refresh chapter: