check out the following conditions before configuring TDM clock:
* BRI interfaces should be shut down before using the no tdm clock command for BRI.
* The configuration for a ds0/pri/channel/tdm group on a T1/E1 must be removed before deleting the TDM clock configuration for T1/E1.
* Any residual or default TDM clock configuration must be deleted before creating a new TDM clock configuration.
* If an export port is configured, the line protocol must be up before configuring any import ports.
* On a Cisco 1751 or Cisco 1760 router, if one port is configured as export, all other ports must be configured as imports.
* An export port clock can be turned off only after all the import ports (including BRI-auto ports) are turned off.
* The configured TDM clock and the status of the running TDM clock might be different, depending on the line conditions and connections.
