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AR# 584

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

解决方案




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.

AR# 584
创建日期 08/31/2007
Last Updated 03/22/2000
状态 Archive
Type ??????