AR# 2896


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


General Description:

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


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.

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.

AR# 2896
Date 01/18/2010
Status Archive
Type General Article
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