Answers Database

XC3000/XC4000/XC5200: PAR ERROR 4kpl:7 - Too many TBUFs (TRISTATEs) driving longline


Record #1312

Product Family:  Software

Product Line:  Merged Core

Problem Title:
XC3000/XC4000/XC5200: PAR ERROR 4kpl:7 - Too many TBUFs (TRISTATEs)
driving longline



Problem Description:
A design that uses several TBUFs driving a single node/line
is implemented in Xilinx's FPGA architectures as follows:


	|	|	|      ......	      |
      \~~~/   \~~~/   \~~~/		    \~~~/
     --\ /   --\ /   --\ /		   --\ /
	|	|	|		      |
========x=======x=======x=====================x======== horizontal
							longline

The TBUFs in all FPGA architectures may access only the horizontal
longlines, and each longline has a limited number of TBUFs that can
access it. The number of TBUFs that may access a horizontal longline
for a given part is listed in the Data Book.

If PAR error 4kpl:7 is referencing TBUFs, then the problem is
that you have exceeded the number of TBUFs that can drive a single
longline.



Solution 1:

The following design hint may or may not be practical
for your design.
  Divide up your TBUFs into two of more groups, each
group driving its own node/line. Then, mux the resulting
lines together, and develop control logic to drive the
select line of the mux.



	|	|	|
      \~~~/   \~~~/   \~~~/
     --\ /   --\ /   --\ /
	|	|	|
========x=======x=======x=======|\
				| \
				|  \
	|	|	|	|MUX|
      \~~~/   \~~~/   \~~~/	|   |---------out
     --\ /   --\ /   --\ /	|  /
	|	|	|	| /
========x=======x=======x=======|/



End of Record #1312