Nortel Networks Circuit Card - Manual (Page 731)

NTAK10 2.0 Mb DTI card Page 731 of 906
Circuit Card Description and Installation
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