Answers Database

XC4000: The Five Configurations of TBUFs


Record #103

Product Family:  Hardware

Product Line:  4000

Problem Title:
XC4000: The Five Configurations of TBUFs


Problem Description:
Description of the XC4000 TBUF Configurtions


Solution 1:


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.





End of Record #103