Feedbacks in other circuits issue with Duo SRAM

[David Jung]

Hello.

I'm just starting to learn ISE with the Papilio Duo.  My first step was to create a circuit to read & write 1 bit from the onboard SRAM.  However, I've hit a wall that I can't understand how to work around and was hoping someome could helpme figure it out.

Here is the circuit I have thus far (attached) - basically connected an 8bit IBUF and OBUF to the sram data lines and just all 0 to the address lines.  However, it won't synthesize: It gives this error:

ERROR:Xst:1850 - Unable to insert IOBUF on port <sram_data<0>> in unit <SRAMOutput> having KEEP_HIERARCHY constraint. There are feedbacks in other units.

 

Searching the Xst forums suggests turning off the KEEP_HEIRARCHY constraint, but it is already off in the settings and I don't understand why it is an issue since I have no other components / levels.

Any ideas greatly appreciated.

Thanks.