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AR# 9022

EXEMPLAR - How do I use low skew lines in a Virtex design?


Keywords: Virtex-E, secondary, backbone, Leonardo, Spectrum, global

Urgency: Standard

General Description:
How do I use low skew lines in a Virtex design?


Leonardo Spectrum can enforce the usage of secondary global lines (low skew lines) in Virtex. This is used in the following cases:

1. If there are not enough global buffers for the clock nets (primary and internal), Spectrum advises that the low skew lines be used.

2. For high fanout clock-enable nets.

Internally, Spectrum sets a MAXSKEW property for the nets.

To turn this feature on, set the variable below to the appropriate SKEW value. (This value is device-dependant.) Furthermore, maximum skew values are not characterized, so maximum skew numbers are not found in the Data Book. A good range is 3 - 7 ns. (The default value is 0, which means that this is not enabled.)

set virtex_apply_maxskew 7

Please Note:
If you set a very low skew value, PAR will try to achieve that; if it cannot meet the constraint, an error will be caused.
AR# 9022
Date Created 04/11/2000
Last Updated 04/24/2007
Status Archive
Type General Article