# Limitations of clock configuration with board keywords

You can configure clocking in a system by specifying each board's role in the board's record in the NMS OAM database using keywords. Configuring clocking in this manner is best if you do not want to implement clock fallback in your system, or in test configurations where clock reliability is not a factor. Otherwise, for maximum system reliability, control clocking at the system level using an application such as clockdemo. For more information, refer to Running clockdemo.

For a detailed overview of H.100 and H.110 bus clocking, refer to CT bus clocking overview. For more information about retrieving and setting NMS OAM keyword values, refer to the NMS OAM Service Developer's Reference Manual.

Configuring clocking with board keywords has the following limitations:

- Unlike the clockdemo application, which allows you to specify several boards to take over mastery of the clock from one another in a fallback situation, the board keyword method allows you to specify only a fixed primary and secondary master.

- The board keyword method does not create an autonomous clock timing environment. If you implement clock fallback using this method, an application must still intervene when clock fallback occurs to reset system clocking before other clocking changes occur. If both the primary and secondary clock masters stop driving the clocks (and an application does not intervene), the boards default to standalone mode.

Note: Not all boards can act as primary or secondary master. For information about board models, refer to the board documentation.

Refer to the System configuration file example for an example of using board keywords to set up clock fallback in a multiple-board system.