I have two pin one is IN and the other is INOUT and they each one goes to the D input of two FFs clocked by two different clocks Clk1, Clk2. When I start the PAR process, Xilinx tool complains with the following error:
ERROR:Place:17 - The placement constraints of the IOBs sck and sdi makes this design unroutable due to a physical routing limitation. This device has a shared routing resource connecting the ICLK and OTCLK pins on pairs of IOBs. This restriction means that these pairs of pins must be driven by the same signal or one of the signals will be unroutable. Before continuing please remove the placement constraints or move one of these IOBs to a new location.
The strange thing is that after looking at the design in the FPGA editor, IOBs OTCLK are not used at all in the mentioned IOBs and for one of the pins, the tool is using the internal FFs in the IOB while in the other it's using only an input buffer and that feeds a FF in another slice.
Ofcourse removing the pin placement constraint fixes the problem but that is something that cannot be done for the time being.
I wonder if somebody can help me figure out what's going on.
I'm using ISE 9.1.03i with Spartan 3
Thank you.