22.15.3 Clock Exceptions

Timer enables you to specify global clock constraints. If you have paths that are not required to meet the global constraint (for example, multi-cycle paths), then list them as exceptions. The Clocks Exceptions area on the Clocks tab provides a mechanism for doing this. A terminal specified as a clock exception will cause all paths beginning or ending at this terminal to be unconstrained by the global timing constraint.

To add or remove terminals from the Clock Exception List:
  1. Click the Clocks tab.
  2. Select the Clock name from the drop-down menu.
  3. Enter a constraint in the Constraints area and click Set.
  4. Select Source or Sink in the Clock Exceptions area. The Clock Exceptions area displays the Pins. The terminals of the sequential device are displayed using an <instance_name>:<pin_name> format.

    For example, a DFM with an instance name of U1\FF1 will have a single source terminal, U1\FF1:CLK, and three sink terminals: U1\FF1:A, U1\FF1:B, and U1\FF1:S.

  5. Use the Filter field to further sort the list of clock pins. Naming filters are provided to limit the list of terminals for consideration. The naming filters use the * character as a wildcard and the / character to delimit levels of hierarchy.

    For example, use * to filter for all terminals; *:E to filter for all terminals with pin E;U1/* to filter for all terminals in block U1; and u1/*:E to filter for all terminals in block U1 with a pin E. After entering your naming filter, click the Set or Select All button. Multiple * and / characters may be used.

  6. Add or Remove the clock exception. To add a clock exception, highlight the desired entry from the Clock Pins list and click Add. To remove, highlight it in the Exceptions list and click Remove.
  7. From the File menu, choose Commit to commit changes.