When I place an OFFSET constraint on my design, the following message appears during implementation:
"WARNING: Cpld:819 - Clock CLK appearing in an OFFSET timespec currently must be explicitly declared as a global clock input (BUFG) in your design. The OFFSET constraint referring to this clock will be ignored."
The OFFSET constraint applies only to CPLDs when the clock is on a global clock net. This can be achieved by performing one of the following:
1. Placing the following constraint on the clock net
NET myclock BUFG=CLK;
(replace myclock with the name of the clock signal).
2. Inserting a BUFG component in the clock path immediately after the input pin.
Either of the above will force the CPLD Implementation tools to use a global clock pin of the CPLD for this input. You might also pin assign to a specific global clock pin if desired.
Currently OFFSET constraints may not be applied to clocks on non-global clock nets. To work around this issue, apply From:To constraints for the clock path and the data paths individually.