AR# 9022: EXEMPLAR - How do I use low skew lines in a Virtex design?
AR# 9022
EXEMPLAR - How do I use low skew lines in a Virtex design?
Description
Keywords: Virtex-E, secondary, backbone, Leonardo, Spectrum, global
Urgency: Standard
General Description: How do I use low skew lines in a Virtex design?
Solution
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.