During the calibration stage of the memory, the ppc440mc_ddr2 does not provide a refresh. In simulation, a memory model might occur, such as:
# 1127510100.0 ps : system_tb.DDR2.cmd_task : ERROR: tRFC maximum violation during No Op
The lack of a refresh during calibration is not expected to cause issues in hardware.
This issue is planned to be fixed in the newest ppc440mc_ddr2 core to be released in EDK 11.1.