Nortel Networks 553-3001-211 manual Clock controller functions and features, Reference switchover

NTAK10 2.0 Mb DTI card Page 719 of 894

Clock controller functions and features

The NTAK10 2MB DTI clock controller functions and features include:

•phase-locking to a reference, generating the 10.24 Mhz system clock, and distributing it to the CPU through the backplane. Up to two references at a time can be accepted.

•providing primary to secondary switchover and auto-recovery

•preventing chatter

•providing error burst detection and correction, holdover, and free running capabilities

•complying with 2.0 Mb CCITT specifications

•communicating with software

•filtering jitter

•making use of an algorithm to aid in detecting crystal aging and to qualify clocking information

Reference switchover

Switchover may occur in the case of reference degradation or reference failure. When performance of the reference degrades to a point where the system clock is no longer allowed to follow the timing signal, then the reference will be said to be out of specification. If the reference being used is out of specification and the other reference is still within specification, an automatic switchover is initiated without software intervention. If both references are out of specification, the clock controller provides holdover.

Autorecovery and chatter

If the software command “track to primary” is given, the clock controller tracks to the primary reference and continuously monitors the quality of both primary and secondary references. If the primary becomes out of specification, the clock controller automatically tracks to secondary provided that it is within specifications. On failure (both out of specification), the clock controller enters the HOLDOVER mode and continuously monitors both references. An automatic switchover is initiated to the reference that recovers first. If the secondary recovers first, then the clock controller tracks to the

Circuit Card Description and Installation