Feedbacks in other circuits issue with Duo SRAM

David Jung

Recommended Posts


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.



Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.