82P33714_17 Datasheet, PDF (25/64 Pages) Integrated Device Technology – Synchronous Equipment Timing Source for Synchronous Ethernet

82P33714 Datasheet
3.2.3.2 Input Clock Quality Monitoring
The qualities of all the input clocks are always monitored in the fol-
lowing aspects:
• Activity
• Frequency
Activity and frequency monitoring are conducted on all the input
clocks.
The qualified clocks are available for selection for the 2 DPLLs.
3.2.3.2.1 Activity Monitoring
Activity is monitored by using an internal leaky bucket accumulator,
as shown in Figure 10.
Each input clock is assigned an internal leaky bucket accumulator.
The input clock is monitored for each period of 128 ms, the internal leaky
bucket accumulator is increased by 1 when an event is detected; and it
is decreased by 1 when no event is detected within the period set by the
decay rate. The event is that an input clock drifts outside (>) ±500 ppm
with respect to the system clock within a 128 ms period.
There are four configurations (0 - 3) for a leaky bucket accumulator.
The leaky bucket configuration for an input clock is selected by the cor-
clock signal with no event
responding BUCKET_SEL[1:0] bits. Each leaky bucket configuration
consists of four elements: upper threshold, lower threshold, bucket size
and decay rate.
The bucket size is the capability of the accumulator. If the number of
the accumulated events reach the bucket size, the accumulator will stop
increasing even if further events are detected. The upper threshold is a
point above which a no-activity alarm is raised. The lower threshold is a
point below which the no-activity alarm is cleared. The decay rate is a
certain period during which the accumulator decreases by 1 if no event
is detected.
The leaky bucket configuration is programmed by one of four groups
of register bits: the BUCKET_SIZE_n_DATA[7:0] bits, the UPPER_
THRESHOLD_n_DATA[7:0] bits, the LOWER_THRESHOLD_n_
DATA[7:0] bits and the DECAY_RATE_n_DATA[1:0] bits respectively; ‘n’
is 0 ~ 3.
The no-activity alarm status of the input clock is indicated by the
INn_NO_ACTIVITY_ALARM bit (6  n  1).
The input clock with a no-activity alarm is disqualified for clock selec-
tion for the DPLLs.
clock signal with events
Input Clock
Leaky Bucket Accumulator
No-activity Alarm Indication
Decay
Rate
Figure 10. Input Clock Activity Monitoring
Bucket Size
Upper Threshold
Lower Threshold
0
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Revision 6, January 23, 2017