The MIG controller is set up by default to automatically send a refresh command every 7.8 microseconds. This auto-generated refresh adheres to the JEDEC specification. An internal counter is used for the auto-refresh period and is set by the TREFI parameter. When the counter expires, the refresh is automatically sent.
Changing the Auto-Refresh Period
MIG does not support turning auto-refresh off. However, it is possible to change the period between refresh commands. The parameter for the refresh period is TREFI.
By default, TREFI is 7.8 microseconds;depending on the temperature you are running the memory, you might want to change this setting. The allowable range for the refresh rate is 3.9 to 7.8 microseconds.
NOTE: Using the Create Custom Part feature, available in the MIG tool, allows the refresh period (trefi - average periodic refresh interval) to be defined within the tool and the corresponding MIG parameters to be set properly.
Manually Sending Refresh
The MIG design includes the ability to manually send a refresh command. The signal app_ref_req, not pulled out to the top level of the user interface, can be used to manually request a refresh. This signal is located in the mem_intfc.v/.vhd module.Asserting this signal tells the memory controller to send out a refresh command. When the refresh is sent, the counter (set through the TREFI parameter) is reset.
It is recommended that users let the controller send these commands at regular intervals to ensure the JEDEC refresh requirements are met, but this signal can be used when needed.
Postponing and Pulling-in Refresh
(Xilinx Answer 42309) Virtex-6 and 7 Series DDR3 - Postponing and Pulling-in REFRESH commands