NET "dbg_tck_o" TNM_NET = dbg_tck_pin;
TIMESPEC dbg_tck_pin = PERIOD dbg_tck_pin 100 KHz HIGH 50%;

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ERROR:ConstraintSystem:300 - In file:
   /home/limb/Documents/fusesoc-builds/build/nexys4/src/nexys4/data/nexys4.ucf(2
   66): Constraint 'PERIOD' has a value 'dbg_tck_pin' which is invalid. Use the
   following:
   Text that matches the regular expression: *i:ts[a-z_0-9\\]+
   Please see the Constraints Guide for more information on this constraint.