DDR2 Memory Controller on Cyclone IV: Pin placement error with VREF

Hello Everybody,

I'm working on a CycloneIV design with an DDR2 SDRAM controller. To find the correct pin placement for the PCB design, I'm verifying it with a stripped down project that so far only instantiates the top level of the megafunctions output.

I was running the tcl script "*_pin_assignments.tcl" and placed the pins after reading a lot in the Cyclone IV documentation (DQS- and DQ groups, diff-pair for the clk).

So I got now to a point where Analysis&Synthesis runs fine, but in the Fitter I'm stuck with some error messages that I don't understand:

Error: Cannot place I/O pin mem1_clk[0] in pin location AA17 -- I/O standard assigned to pin requires VREF value and VREF pin corresponding to pin location not available

Error: Cannot place I/O pin mem1_clk_n[0] in pin location AB17 -- I/O standard assigned to pin requires VREF value and VREF pin corresponding to pin location not available

Error: Cannot place I/O pin mem1_dqs[0] in pin location AB9 -- I/O standard assigned to pin requires VREF value and VREF pin corresponding to pin location not available

Error: Cannot place I/O pin mem1_dqs[1] in pin location V10 -- I/O standard assigned to pin requires VREF value and VREF pin corresponding to pin location not available

Error: Cannot place I/O pin mem1_dq[0] in pin location AA9 -- I/O standard assigned to pin requires VREF value and VREF pin corresponding to pin location not available

(...and so on for all DQ[0..15])

I thought that the "SSTL-18 Class I" does not require a VREF voltage. Or is it necessary to define something more than the VCCIO at 1.8V for the bank?

Or could there be some problem with Quartus II if not all the pins in a bank are used? For sure they are all at a standard with 1.8V VCCIO.

I hope someone can give me a hint on how to interpret this error. I read several ressources on the memory-IF design with Quartus II / Cyclone IV but cannot see how to continue.

Thanks,

Cheers,

Simon