I am designing a DDR controller which runs at 133 MHz. I am trying to synthesize this on Spartan 3. I am using FDDR IO and OBUFT for generation of IO signals. Everything works fine till FFDR block. It gives proper double data rate output. But when this goes to OBUFT, output of OBUFT is distorted. I miss the first hald of data completely and second half is also not stable enough. Can anybody help me on this. How can I make this tristate buuffers work.