Nios crashes when reading from the DDR2

I assume your design is for a custom board?

Well the reason the NIOS is hanging is most likely because it's waiting for the DDR2 controller to return data but it's never returning it. I have seen this before but your circumstances are different. The most common reason I saw was when there was insufficient pipeline buffering between the processor and the DDR2 controller.

Here are some things to try:

1 - Try making the read buffer larger inside the clock-crossing bridge.

2 - Check that the init_done signal from the DDR2 controller is getting asserted.

3 - Put signaltap in to look at the avalon interface to the DDR2 controller and see what is going on there when a read occurs.

Jake