Answers Database

PPR 5.x guide may leave clock enable on flip-flops where CE was removed (3000A)


Record #584

Problem Title:
PPR 5.x guide may leave clock enable on flip-flops where CE was removed
(3000A)



Problem Description:



Solution 1:




PROGRAM:  PPR

VERSION:  5.x

PLATFORM(s):  All

SHORT DESCRIPTION:

PPR 5.x guide may leave clock enable on flip-flops where CE was removed (3000A)

LONG DESCRIPTION:

PROBLEM:

A flip-flop has its clock enable removed in a 3000A design.  The design is
run through PPR using the previous iteration as a guide.  After the design is
routed, the flip-flop still has the clock enable attached!

CAUSE:

In the 3000 architecture, two flip-flops in the CLB share a common clock
enable.  This is an "all or nothing" arrangement; both flip-flops must use it
or neither flip-flop must use it.  In the situation in question, the
following happens: when the design is processed the first time, XNFMap places
two flip-flops, which share a common clock enable, into the same CLB.  The
LCA file eventually has a CLB with these two flip-flops paired together.

When the clock enable is removed on one of those flip-flops, XNFMap carefully
places them apart, noting that putting these two flip-flops in the same CLB
is illegal.  However, PPR follows the guide-file instructions and places the
two flip-flops in the same CLB, even though this is illegal.  This causes the
modified flip-flop to regain the clock enable previously removed from the
user's schematic!

SOLUTION:

Remove the offending CLB from the guide file and try again.




End of Record #584