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 - Last Modified: 10/14/96 17:26 |
