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8T49N281 Datasheet(PDF) 7 Page - Integrated Device Technology

Part # 8T49N281
Description  Operating modes: locked to input signal
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Manufacturer  IDT [Integrated Device Technology]
Direct Link  http://www.idt.com
Logo IDT - Integrated Device Technology

8T49N281 Datasheet(HTML) 7 Page - Integrated Device Technology

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8T49N281 DATA SHEET
REVISION 4 07/08/15
7
FEMTOCLOCK® NG OCTAL UNIVERSAL FREQUENCY TRANSLATOR
Input Clock Monitor
Each clock input is monitored for Loss of Signal (LOS). If no activity
has been detected on the clock input within a user-selectable time
period then the clock input is considered to be failed and an internal
Loss-of-Signal status flag is set, which may cause an input
switchover depending on other settings. The user-selectable time
period has sufficient range to allow a gapped clock missing many
consecutive edges to be considered a valid input.
User-selection of the clock monitor time-period is based on a counter
driven by a monitor clock. The monitor clock is fixed at the frequency
of the PLL’s VCO divided by 8. With a VCO range of 3GHz - 4GHz,
the monitor clock has a frequency range of 375MHz to 500MHz.
The monitor logic for each input reference will count the number of
monitor clock edges indicated in the appropriate Monitor Control
register. If an edge is received on the input reference being
monitored, then the count resets and begins again. If the target edge
count is reached before an input reference edge is received, then an
internal soft alarm is raised and the count re-starts. During the soft
alarm period, the PLL tracking will not be adjusted. If an input
reference edge is received before the count expires for the second
time, then the soft alarm status is cleared and the PLL will resume
adjustments. If the count expires again without any input reference
edge being received, then a Loss-of-Signal alarm is declared.
It is expected that for normal (non-gapped) clock operation, users will
set the monitor clock count for each input reference to be slightly
longer than the nominal period of that input reference. A margin of 2-3
monitor clock periods should give a reasonably quick reaction time
and yet prevent false alarms.
For gapped clock operation, the user will set the monitor clock count
to a few monitor clock periods longer than the longest expected clock
gap period. The monitor count registers support 17-bit count values,
which will support at least a gap length of two clock periods for any
supported input reference frequency, with longer gaps being
supported for faster input reference frequencies. Since gapped
clocks usually occur on input reference frequencies above 100MHz,
gap lengths of thousands of periods can be supported.
Using this configuration for a gapped clock, the PLL will continue to
adjust while the normally expected gap is present, but will freeze
once the expected gap length has been exceeded and alarm after
twice the normal gap length has passed.
Once a LOS on any of the input clocks is detected, the appropriate
internal LOS alarm will be asserted and it will remain asserted until
that input clock returns and is validated once 8 rising edges have
been received on that input reference. If another error condition on
the same input clock is detected during the validation time then the
alarm remains asserted and the validation time starts over.
Each LOS flag may also be reflected on one of the GPIO[3:0] outputs.
Changes in status of any reference can also generate an interrupt if
not masked.
Holdover
8T49N281 supports a small initial holdover frequency offset in
non-gapped clock mode. When the input clock monitor is set to
support gapped clock operation, this initial holdover frequency offset
is indeterminate since the desired behavior with gapped clocks is for
the PLL to continue to adjust itself even if clock edges are missing. In
gapped clock mode, the PLL will not enter holdover until the input is
missing for two LOS monitor periods.
The holdover performance characteristics of a clock are referred as
its accuracy and stability, and are characterized in terms of the
fractional frequency offset. The 8T49N281 can only control the initial
frequency accuracy. Longer-term accuracy and stability are
determined by the accuracy and stability of the external oscillator.
When the PLL loses all valid input references, it will enter the
holdover state. In fast average mode, the PLL will initially maintain its
most recent frequency offset setting and then transition at a rate
dictated by its selected phase-slope limit setting to a frequency offset
setting that is based on historical settings. This behavior is intended
to compensate for any frequency drift that may have occurred on the
input reference before it was detected to be lost.
The historical holdover value will have three options:
• Return to center of tuning range within the VCO band
• Instantaneous mode - the holdover frequency will use the DPLL
current frequency 100msec before it entered holdover. The
accuracy is shown in the AC Table.
• Fast average mode - an internal IIR (Infinite Impulse Response)
filter is employed to get the frequency offset. The IIR filter gives a
3 dB attenuation point corresponding to nominal a period of 20
minutes. The accuracy is shown in the AC Table.
When entering holdover, the PLL will set a separate internal HOLD
alarm internally. This alarm may be read from internal status register,
appear on the appropriate GPIO pin and/or assert the nINT output.
While the PLL is in holdover, its frequency offset is now relative to the
crystal input and so the output clocks will be tracing their accuracy to
the local oscillator or crystal. At some point in time, depending on the
stability & accuracy of that source, the clock(s) will have drifted
outside of the limits of the holdover state and the system will be
considered to be in a free-run state. Since this borderline is defined
outside the PLL and dictated by the accuracy and stability of the
external local crystal or oscillator, the 8T49N281 cannot know or
influence when that transition occurs. As a result, the 8T49N281 will
remain in the Holdover state internally.


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