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.
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AR# 103
Date
03/03/2004
Status
Archive
Type
General Article
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