AR# 1523: XACT-CPLD XC9500: Using Local Feedback Paths
XACT-CPLD XC9500: Using Local Feedback Paths
Keywords: XACT CPLD local feedback
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.
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
EXAMPLE: 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.
<-----------------------------------------> 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.
**NOTE** 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.