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AR# 15885

5.1isp1 Timing Simulation, NGDAnno, Virtex-II/Pro - Output delays for differential signals are not annotated correctly


Keywords: Timing, Simulation, NGDAnno, Speedfiles, differential, LVDS, LVPECL, negative, delay, DIFF, OBUFTDS

Urgency: Standard

General Description:
A change in the speed files that were delivered in 5.1i Service Pack 1 causes negative delays to be annotated to the X_OBUFTDS:

(INSTANCE xlxi_3)
(PORT I (240:240:240)(240:240:240))
(PORT T (240:240:240)(240:240:240))
(IOPATH I O (-1512:-1512:-1512)(-1512:-1512:-1512))
(IOPATH T O (-1512:-1512:-1512)(-1512:-1512:-1512))
(IOPATH I OB (-1512:-1512:-1512)(-1512:-1512:-1512))
(IOPATH T OB (-1512:-1512:-1512)(-1512:-1512:-1512))

The IOPATH delay should be a positive value. In this case, the IOPATH delay should have been "805" instead of "-1512."

A number of simulators will issue an error when a negative IOPATH delay is encountered. Most simulators will simply set the value to "0". This will cause the delay for the differential output to be much less than it should be. In the above case, the total delay from the FF to the pad should have been 1.920 ns, but was 1.115 ns.


This problem is fixed in the latest 5.1i Service Pack, available at:
The first service pack containing the fix is 5.1i Service Pack 2.
AR# 15885
Date Created 09/04/2007
Last Updated 08/11/2005
Status Archive
Type General Article