Answers Database

XC4000E/EX/XL/XLT/XV: M1- TBUF net Delay 4000EX - Effect of the Pullup.


Record #1659

Product Family:  Hardware

Product Line:  4000EX

Problem Title:
XC4000E/EX/XL/XLT/XV: M1- TBUF net Delay 4000EX - Effect of the Pullup.


Problem Description:
Urgency: Standard

Keywords: pullup, long line, TBUF, 4000EX

Description

How does the internal 4000ex pullup modify the TBUF net delay?


Solution 1:

On a more general note, the nature of the longline nets in
the EX devices is somewhat different than in the E family.
The quadlines, not the longlines, are generally the fastest
routing resource.  The advantages of using tbufs to
efficiently implement things like wide muxes, bidirectional
buses, and wired functions remains.  This has been a strong
advantage for Xilinx



Solution 2:

Adding pullups will only help timing if the only thing
"driving" the longline high is the pullups themselves (e.g.
wired functions like WANDs).  In these cases, adding a second
pullup where there was only one before will improve the L->H
transition by a quite a bit.  Adding a third will only give a
smaller improvement, and other effects like power dissipation
come into play.  Adding pullups to configurations where
the tbufs are always actively driving high and low has only a marginal impact on
 the L->H transition, and can actually hurt
the H->L transition.



End of Record #1659