Output port "O" of PSEUDO_DIFF_OUT primitive... must drive only one OBUF primitive...

Ok, I figured this out. The altera generated HPCII instance requires the DDR2 clock pins to be bidirectional. After changing this, the design compiles.

Also, I found that the only way to properly use differential pins in my DIMM design, I had to:

1) Temporarily change all Differential SSTL-18 I/O types in the tcl file to single-ended SSTL-18.

2) Assign all differential pins one by one as single-ended in the pin planner. This has the effect that the pin planner does not try to automatically add an incorrectly named negative pin for the diff pair.

3) Change the single ended SSTL-18 pins back to differential SSTL-18.