Looking at the ML461 board to do some DDR2 development, there is concern that we have to have external termination on the board rather than using DCI. Why do we use the external resistors rather than DCI?
The ML461 board is designed to demonstrate both DCI and non-DCI termination schemes. The default currently on the board is for non-DCI; that is, termination at the FPGA. We have also prepared another board at factory by removing these termination resistors at FPGA, and implemented DCI within FPGA. We have successfully characterized that design for up to 272 MHz.
The only caution with using the DCI termination is that FPGA consumes a large amount of power (11.5 watts for DCI vs. 7.5 Watts for non-DCI on LX25), so you must implement an appropriate cooling solution (heat sink + air flow) if you plan to use the DCI scheme.