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AR# 37328

13.4 PlanAhead - Cannot place the clock pins with IBUFGDS_DIFF_OUT in Spartan-6


When IBUFGDS_DIFF_OUT is used for the clock in a Spartan-6 device, PlanAhead 12.1 cannot place the clock pins in the package view window.

When I enter the I/O pin location in the "site" field of I/O Port Properties, the following message appears:

Illegal to place instance IBUFG_DS_nst/B2 on site "xxx"

In cases where IBUFGDS is used instead of IBUFGDS_DIFF_OUT, PlanAhead can place the pins as usual.


There are two work-arounds for this issue:

  • Use IBUFGDS instead of IBUFGDS_DIFF_OUT.
    PlanAhead can then place the pins as usual.
  • Use the UCF file for the I/O assignment if IBUFGDS_DIFF_OUT is used in Spartan-6.

This issue is fixed in PlanAhead 14.1.

AR# 37328
Date 01/30/2015
Status Archive
Type Known Issues
  • Spartan-6 LX
  • Spartan-6 LXT
  • ISE Design Suite - 12
  • ISE Design Suite - 13
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