M1020/21
Integrated
Circuit
Systems, Inc.
VCSO BASED
CLOCK PLL
P r o d u c t D a t a S h e e t
HS/PBO Operation
Optional Hitless Switching and Phase Build-out
Once triggered, the following HS/PBO sequence
occurs:
1.The HS function disables the PLL Phase Detector
and puts the device into NBW (narrow bandwidth)
mode. The internal resistor Rin is changed to
2100kΩ. See the External Loop Filter on pg. 6.
2.If included, the PBO function adds to (builds out) the
phase in the clock feedback path (in VCSO clock
cycle increments) to align the feedback clock with
the (new) reference clock input phase.
3.The PLL Phase Detector is enabled, allowing the
PLL to re-lock.
4.Once the PLL Phase Detector feedback and input
clocks are locked to within 2 ns for eight consecutive
cycles, a timer (WBW timer) for resuming wide
bandwidth (in 175 ns) is started.
5.When the WBW timer times out, the device reverts
to wide loop bandwidth mode (i.e., Rin is returned to
100kΩ) and the HS/PBO function is re-armed.
The M1020/21 is available with a Hitless Switching
feature that is enabled during device manufacturing.
In addition, a Phase Build-out feature is also offered.
These features are offered as device options and are
specified by device order code. Refer to “Ordering
Information” on pg. 10.
The Hitless Switching feature (with or without Phase
Build-out) is designed for applications where switching
occurs between two stable system reference clocks. It
should not be used in loop timing applications, or when
reference clock jitter is greater than 1 ns pk-pk. Hitless
Switching is triggered by the LOL circuit, which is
activated by a 4 ns phase transient. This magnitude of
phase transient can generated by the CDR (Clock &
Data Recovery unit) in loop timing mode, especially
during a system jitter tolerance test. It can also be
generated by some types of Stratum clock DPLLs
(digital PLL), especially those that do not include a post
de-jitter APLL (analog PLL).
Narrow Bandwidth (NBW) Control Pin
When the M1020/21 is operating in wide bandwidth
mode (NBW=0), the optional Hitless Switching function
puts the device into narrow bandwidth mode when
activated. This allows the PLL to lock the new input
clock phase gradually. With proper configuration of the
external loop filter, the output clock complies with MTIE
and TDEV specifications for GR-253 (SONET) and ITU
G.813 (SDH) during input reference clock changes.
A Narrow Loop Bandwidth control pin (NBW pin) is
included to adjust the PLL loop bandwidth. In wide
bandwidth mode (NBW=0), the internal resistor Rin is
100kΩ. With the NBW pin asserted, the internal resistor
Rin is changed to 2100kΩ. This lowers the loop
bandwidth by a factor of about 21 (approximately 2100 /
100) and lowers the damping factor by a factor of about
4.6 (the square root of 21), assuming the same loop
filter components.
The optional proprietary Phase Build-out (PBO)
function enables the PLL to absorb most of the phase
change of the input clock. The PBO function selects a
new VCSO clock edge for the PLL Phase Detector
feedback clock, selecting the edge closest in phase to
the new input clock phase. This reduces re-lock time,
the generation of wander, and extra output clock cycles.
External Loop Filter
To provide stable PLL operation, the M1020/21 requires
the use of an external loop filter. This is provided via the
provided filter pins (see Figure 5).
Due to the differential signal path design, the
implementation requires two identical complementary
RC filters as shown here.
The Hitless Switching and Phase Build-out functions
are triggered by the LOL circuit. For proper operation,
a low phase detector frequency must be avoided. See
“Guidelines for Using LOL” on pg. 5 for information
regarding the phase detector frequency.
RLOOP CLOOP
RPOST
CPOST
CPOST
HS/PBO Triggers
The HS function (or the combined HS/PBO function)
is armed after the device locks to the input clock refer-
ence. Once armed, HS is triggered by the occurance of
a Loss of Lock condition. This would typically occur as a
consequence of a clock reference failure, a clock failure
upstream to the M1020/21, or a M1020/21 clock refer-
ence mux reselection.
RLOOP CLOOP
nOP_IN OP_OUT
RPOST
OP_IN
nOP_OUT
nVC
VC
4
9
8
5
6
7
Figure 5: External Loop Filter
M1020/21 Datasheet Rev 1.0
6 of 10
Revised 28Jul2004
Integrated Circuit Systems, Inc. ● Networking & Communications ● www.icst.com ● tel (508) 852-5400
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