When I fit a CPLD design with a PERIOD constraint on a TNM group, the following error is reported:
"ERROR: XdmHelpers:664 - Period specification "TS_CLK" references the TNM group "CLK", which contains only pad elements. Only synchronous elements are permitted in a period specification. If this group was intended to reference the synchronous elements driven by the pad(s), you may need to change the TNM property to a TNM_NET property, so that it will be traced forward from the pad(s)."
To work around this issue, change the group constraint from a TNM to a TNM_NET.
This problem is fixed in the latest 5.1i Service Pack, available at:
The first service pack containing the fix is 5.1i Service Pack 1.