Answers Database

M1.4: TRCE reports large delay on net driven by TBUFs with PULLUP


Record #2896

Product Family:  Software

Product Line:  FPGA Implementation

Problem Title:
M1.4: TRCE reports large delay on net driven by TBUFs with PULLUP


Problem Description:
Keywords: 100865, trace, timing wizard, pullup, 3-state, tristate

Urgency: Standard


On a net that is always driven by 3-state buffers, a user
might find there is a large delay on the net if there is a
pullup attached to it. Even if the net is always driven to
a value by the 3-states, Trace will report the worst case
delay on the net as if the PULLUP was driving it, since is
has no way of knowing if the user's design will depend on the
PULLUP to drive it.

Reference #: 100865


Solution 1:

If the design requires a PULLUP (such as a WAND), then
TRACE is reporting the desired value, since the PULLUP
will sometimes drive the net.




Solution 2:

If the 3-state drivers always drive the net to a "1" or "0",
then there is no need for the PULLUP. There is a "weak-keeper"
circuit on the XC4000E/EX/XL longlines that will not allow
the longline to float ('Z'). By removing the PULLUP, Trace
will be able to report the delay on the net as driven by
the 3-state buffers, which is more desirable.



End of Record #2896