What IOSTANDARD for using IOBUFDS on digital IO?

[Skillers]

I want to use IOBUFDS on the digital IO pins (on the extension connectors). These currently have the IOSTANDARD set to LVCMOS33, which is not compatible with differential signals. I am not sure which IOSTANDARD to switch to to have IO capable differential pins on this connector, does anyone know?

Thanks

[Nils Roos]

There's a problem there. All available differential IO-standards have voltage references of 2.5V or lower (LVDS_25, DIFF_HSTL_II_18, ...), but other ports on the IO bank where the GPIOs are located have 3.3V, and you can't mix voltages references within one bank. So there is no differential standard that you can use for the GPIOs. Among the other things on bank 35 is the DAC, so changing the other ports to fit a differential standard is out of the question.

[Skillers]

Hmm, thanks for the info.

I've seen it claimed (e.g. here) that the digital IO is capable as running as differential pairs, but this is apparently not the case? Or is it that differential digital IO is available from the Linux side, but not the FPGA side?

[Nils Roos]

I've seen it claimed (e.g. here) that the digital IO is capable as running as differential pairs

Indeed, the architecture diagram mentions differential capability for the GPIOs. I guess nobody checked if it is actually possible when the diagram was created, and nobody updated the diagram once it became clear that it's not. The listed device specs don't have that unfortunate mistake, though.
The daisy-chain signals are capable of differential operation, so you've got four pairs at least.