ZL30116 Data Sheet
16
Zarlink Semiconductor Inc.
1.4 Ref and Sync Monitoring
All input references (ref0 to ref7) are monitored for frequency accuracy and phase regularity. New references are
qualified before they can be selected as a synchronization source and qualified references are continuously
monitored to ensure that they are suitable for synchronization. The process of qualifying a reference depends on
four levels of monitoring.
Single Cycle Monitor (SCM)
The SCM block measures the period of each reference clock cycle to detect phase irregularities or a missing clock
edge. In general, if the measured period deviates by more than 50% from the nominal period, then an SCM failure
(scm_fail) is declared.
Coarse Frequency Monitor (CFM)
The CFM block monitors the reference frequency over a measurement period of 30 μs so that it can quickly detect
large changes in frequency. A CFM failure (cfm_fail) is triggered when the frequency has changed by more than 3%
or approximately 30000 ppm.
Precise Frequency Monitor (PFM)
The PFM block measures the frequency accuracy of the reference over a 10 second interval. To ensure an
accurate frequency measurement, the PFM measurement interval is re-initiated if phase or frequency irregularities
are detected by the SCM or CFM. The PFM provides a level of hysteresis between the acceptance range and the
rejection range to prevent a failure indication from toggling between valid and invalid for references that are on the
edge of the acceptance range.
When determining the frequency accuracy of the reference input, the PFM uses the external oscillator’s output
frequency (f
ocsi
) as its point of reference.
Guard Soak Timer (GST)
The GST block mimics the operation of an analog integrator by accumulating failure events from the CFM and the
SCM blocks and applying a selectable rate of decay when no failures are detected.
As shown in Figure 5, a GST failure (gst_fail) is triggered when the accumulated failures have reached the upper
threshold during the disqualification observation window. When there are no CFM or SCM failures, the accumulator
decrements until it reaches its lower threshold during the qualification window.
Figure 5 - Behaviour of the Guard Soak Timer during CFM or SCM Failures
ref
CFM or SCM failures
upper threshold
lower threshold
t
d
- disqualification time
t
q
- qualification time = n * t
d
t
d
t
q
gst_fail
