The following message is reported in NGDBuild:
"WARNING:Ngd:231 - PERIOD TIMESPEC '%s' has TIMEGRP '%s' which contains a mixture of PADs and synchronous elements. The PADs in this group will be exempt from analysis by the TRCE application. The ability to mix PADs and synchronous elements in a PERIOD group will soon be made obsolete."
The message warns that this TIMEGRP contains pads and synchronous elements, but that the group should only contain synchronous elements.
To avoid future warnings and errors, remove the pads from this group:
TIMEGRP new_group = timegrp_with_pads_name EXCEPT PADS;
TIMEGRP clock2 = clock1 EXCEPT PADS;
TIMESPEC TS_clock1= PERIOD clock2 12 ns;