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AR# 1523

XACT-CPLD XC9500: Using Local Feedback Paths


Keywords: XACT CPLD local feedback

Urgency: Standard

General Description:

The XC9500 Function Blocks feature a Local Feedback Path
which allows for faster feedback delays within a single
function block. This allows function block outputs to drive
into the same function block's AND array, without going outside
of the function block.

*Note: XC9536 devices do not contain local feedback paths.

The current version of the XACT-CPLD 9500 fitter software supports
this LFBK path, but certain steps must be followed to ensure
its use.



First you must get a software patch from our BBS or FTP site.

The name of the file is FITTER<platform>.zip

(Xilinx File http://www.xilinx.com/txpatches/pub/swhelp/cpld/fitterpc.zip)
(Xilinx File http://www.xilinx.com/txpatches/pub/swhelp/cpld/fittersn.tar.Z)
(Xilinx File http://www.xilinx.com/txpatches/pub/swhelp/cpld/fitterhp.tar.Z)

BBS: 408-559-9327

After installing the Patch, the following guidelines must
be used in order to take advantage of the Local Feedback Paths:

1.) The source and destination logic must be placement
constrained into the same Function Block.

To do this...
Schematic Entry: Use the LOC attribute.
ABEL Entry: Use the PARTITION Xepld Property Statement.

*NOTE: LOC attributes may only be placed on Primitives.
See Solution #788 for more information on which
components are valid Primitives.

2.) The path must be time-constrained with a Timespec. The
Timespec may either be placed on the schematic, or in a
Constraint file (.CST). Making an unrealistically small
timespec (like 1ns) on the path will force the software to
use the Local Feedback Path.

For more information on specifying Timespecs:
See Soln #1331

Say you have two flip flops and you want the path from the
output of the first to the input of the second to use the
Local Feedback Path in Function Block 6.

____ ____
| |---------(^^^^^^^^^^)---------| |
|____| (__________) |____|

FlipFlop1 Combinatorial FlipFlop2


Put a TIMESPEC on this path which is
unreasonable, say 1ns. The software will
know it can't meet this timespec but will
use the local feedback trying it's best to
meet the TIMESPEC.


The following is a situation in which the software is not yet
capable of using the Local Feedback Path, although it is
architecturally possible.

If one of the endpoints of the desired Local Feedback Path also
feeds the Enable pin of a bidirectional I/O pin, the current software (XACT-CPLD v6.0.1 + Patch) is not able to use the
Local Feedback Path. This limitation is being addressed for
the next major software release.

EXAMPLE: LFBK desired between FF1 and FF2.

___ ___
| |--------| |-----------|
|___| |___| |\| ____
| >-------<____| BIDIRECTIONAL I/O
FF1 FF2 |/


In order for the fitter to take TIMESPECs into account during fitting process, user must enable XACTperformace under IMPLEMENTATION Template.
AR# 1523
Date Created 08/31/2007
Last Updated 09/28/2008
Status Archive
Type General Article