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AR# 103

XC4000: The Five Configurations of TBUFs

Description

Description of the XC4000 TBUF Configurtions

Solution


The simple three-state buffer is a thing of the past: with
the advent of the XC4000, you can do an EditBlk on a TBUF.
An XC4000 TBUF can be used to implement two configurations of
three-state buffers, two varieties of wired-AND (WAND)
functions, and a wired-OR-AND (WORAND) function. These
possibilities and the programs which support them are
described below.


In an XC4000 LCA file, the configuration of a TBUF is
identified by two new tags: TBUF and I. For each function
shown below, the appropriate settings of these tags are also
listed.

Standard TBUF

The standard three-state buffer is identical to an XC3000
TBUF, and is controlled by the T pin (active-Low output
enable). This configuration is identified as TBUF: I: and is
supported by both PPR and XDE.

TBUF with grounded input

A TBUF with its input (I) pin tied to ground is
essentially an open-drain buffer: a pullup must be attached
to the output to create a logical High. In an XC3000 device
this function is implemented by routing a ground signal to
the I pin of the TBUF. Using an XC4000, the I pin of the
TBUF can be grounded internally, saving routing resources.

This configuration is identified as TBUF: I:GND and is
currently supported only by XDE. If PPR finds a TBUF with
the I pin grounded, it does not take advantage of this
internal ground connection. However, since PPR does
recognize a WAND function -- which is equivalent to a
TBUF with a grounded input -- using a WAND is preferrable.

NOTE: In an XC4000 IOB, a similar configuration allows the
OBUFT input to be grounded. The appropriate IOB
configuration tag is O:GND. This configuration is supported
only by XDE.

WAND using I pin

One possible configuration of a wired-AND (WAND) function
uses the I pin of the TBUF as an input, with an open-drain
output: a pullup must be attached to the output to create a
logical High. No connection is made to the T pin, which is
"tied off" internally. This configuration is identified as
TBUF:WAND I: and is supported by both PPR and XDE.

WAND using T pin

The other possible configuration of a wired-AND (WAND)
function uses the T pin of the TBUF as an input, with an
open-drain output: a pullup must be attached to the output
to create a logical High. No connection is made to the I
pin, which is tied off internally. This configuration
is identified as TBUF:WANDT I: and is supported only by XDE.
PPR Version 1.00 configures all WANDs to use the I pin as
input (TBUF:WAND).

WORAND

A wired-OR-AND (WORAND) function uses both the I and T
pins of the TBUF as an inputs, with an open-drain output: a
pullup must be attached to the output to create a logical
High. This configuration is identified as TBUF:WORAND I: and
is supported by both PPR and XDE.


AR# 103
Date Created 09/20/1995
Last Updated 03/03/2004
Status Archive
Type General Article