Application Report
SNAA294 – December 2019

Multi-Clock Synchronization
Timothy Toroni
ABSTRACT
Synchronization of clocks sourced from a single device is often supported directly by the integrated circuit(IC) producing the clocks. Synchronization of multiple clocking ICs or synchronization of a clock input tovarious clock outputs may require additional signal timing requirements or frequency planning.
In a system with multiple clock frequencies created by dividers and phase locked loops (PLLs), there is a
low frequency clock rate with the timing information necessary to synchronize all clocks. A low frequency
clock or DC signal carries with it information about the moment of a synchronization request. This lowest
frequency clock may be a reset signal to a divider or a clock frequency used for 0-delay feedback in a
PLL. JESD204B uses a SYSREF clock, which is a low frequency synchronization signal by which the local
multi-frame clock (LMFC) on JESD204B devices, such as data converters and logic devices, can be resetand synchronized to allow determinism within the period of the LMFC.
This application note discusses methods and requirements for synchronizing multiple devices by either adivider reset, 0-delay, or a combination of both of these methods.

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2
3
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6

Contents
Definitions and acronyms ................................................................................................... 2
Divider Phase Uncertainty .................................................................................................. 3
PLL Phase Uncertainty...................................................................................................... 3
Zero Delay Mode Synchronization ........................................................................................ 6
Synchronization using ZDM and Divider Reset ........................................................................ 11
Conclusion .................................................................................................................. 13
List of Figures

1

Clock Divider Block Diagram ............................................................................................... 3

2

Timing Plot for Phase Uncertainty of a Divider .......................................................................... 3

3

Basic PLL Block Diagram................................................................................................... 4

4

Timing Plot for Deterministic PLL Case................................................................................... 4

5

Timing Plot for Non-Deterministic PLL Case ............................................................................. 56

PLL Block Diagram with Clock Distribution............................................................................... 67

PLL Block Diagram with Clock Distribution and ZDM .................................................................. 6

8

Timing Plot for ZDM with Phase determinism ........................................................................... 8

9

Timing Plot for ZDM with Least Terms R' > 1............................................................................ 9

10

Timing Plot for ZDM with Phase Determinism for Multiple Frequencies ............................................ 10

11

Functional Block Diagram of LMK0482x Configured to Re-clock CLKin0 for JESD204B SYSREF............. 12
List of Tables

1

Synchronization Methods Pros and Cons ............................................................................... 13

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Definitions and acronyms

1

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Definitions and acronyms
SYNC — A synchronization signal to hold a divider in reset.
SYSREF — In JESD204B subclass 1 systems, SYSREF is the signal in which the rising edge marks thedevice clock rising edge, which resets the local multi-frame clock (LMFC), and possibly otherdividers in a JESD204B device, provided that the SYSREF is still high when the device clock risingedge occurs.
ZDM — Zero delay mode (or 0-Delay mode). A mode of operation of a PLL where an output divider is
included in the PLL feedback path to lock phase of divider output with reference.
GCD — The greatest common divisor of two or more numbers is defined as the largest number which candivide into all the other numbers without a remainder. Traditionally an operation on integers, thisalso applies to non-integers. For example the GCD of 122.88 MHz and 19.2 MHz is 3.84 MHz.
Checking this answer shows that 122.88 MHz / 3.84 MHz = 32 and 19.2 MHz / 3.84 MHz = 5.
PLL — Phase Lock Loop.
JESD204B — JEDEC standard which defines a method for synchronization of data between data
converters and logic devices. JESD204B introduces device subclass 1 for deterministic latency.
JESD204C increases data rates and adds 64B/66B and 64B/80B as options for encoding in thedata link layer. JESD204B supports only 8B/10B encoding. This application note references
JESD204B but applies equally to JESD204C.

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Divider Phase Uncertainty

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2

Divider Phase Uncertainty
An issue with dividers occurs when a clock is divided to a lower frequency. The division introduces phase
uncertainty, as illustrated in Figure 2. A divider of value D has an output clock with one of D possible
phases with respect to the input clock. This phase uncertainty is caused when dividers start dividing thereference input frequency at different edges, or when the initial state of the dividers may have a different
starting values at power up.
Input
Clock

/D

OutputClock

Figure 1. Clock Divider Block Diagram

Input ClockOutput Clock
Input Clock / 4+0 degrees

Input Clock / 4+90 degrees

Input Clock / 4+180 degrees

Input Clock / 4+270 degrees

Figure 2. Timing Plot for Phase Uncertainty of a Divider

2.1

Divider Reset Synchronization
A solution to non-deterministic phase is to reset the dividers to a common starting value. To do this, a
synchronization signal must reset the dividers while meeting a setup and hold time to the input clock. As
the input clock frequency increases, the valid window for the reset signal becomes small, and adjusting
the timing of the reset signal relative to the input clock to meet the setup and hold times may be difficult.
JESD204B subclass 1 achieves synchronization of dividers using a SYSREF clock, or SYSREF clock
pulse. The SYSREF clock pulse resets dividers internal to the data converters and logic devices to alignthe LMFC (local multi-frame clock) deterministically between all devices. Because there may be several
dividers in series, multiple SYSREF pulses may be required to fully synchronize the system. In a
JESD204B system with different LMFCs, it is possible to use a SYSREF frequency, the GCD of all
LMFCs, to produce a SYSREF clock valid for all devices. The SYSREF frequency is required to be an
integer multiple of the LMFC period.

3

PLL Phase Uncertainty
An issue with PLLs and clock synchronization is they typically also contain dividers. An analog PLL isillustrated in Figure 3. The PLL forces the outputs of the R divider and N divider to a fixed phasealignment. When the phase between the R divider output and N divider output is fixed and no longer
changing, frequency lock is achieved. However, the forward division of the reference frequency by thereference divider R may introduce phase uncertainty between the reference input clock and VCO output
clock.

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PLL Phase Uncertainty

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Phase certainty or a deterministic phase relationship from reference input clock to the VCO output clock
can be achieved with the R divider equal to one. The N divider behaves as a multiplier to the reference
frequency, so there is always a corresponding output clock edge for every input clock edge. Determinism
of an input clock edge to an output clock edge is therefore achieved. It is not strictly required that R = 1 tohave deterministic phase between reference input and VCO output. The frequency of the VCO =Reference Frequency * (N / R). Anytime the fraction N / R reduces to a lowest terms fraction of N' / R'such that R' = 1, then deterministic phase from input to VCO occurs. If the reduced R' value is still greaterthan one, then only every R'th reference clock edge aligns with a VCO clock edge.
Input
Clock

/R

/N

Phase
Detector&
ChargePump

Loop
Filter

VCO

OutputClock

Figure 3. Basic PLL Block Diagram

NOTE: The mathematical maximum phase detector frequency for an integer PLL is the GCD of thereference frequency and the VCO frequency.

3.1

Example 1: All Reference Clocks have Deterministic Phase Alignment to VCOGiven the following conditions for system use case 1A:• Reference frequency = 40 MHz• VCO frequency = 160 MHz• Every reference clock edge has a corresponding VCO clock edge
To lock phase with a 10-MHz phase detector frequency with the given inputs, R = 4 and N = 16. Because
N / R = 16 / 4 and reduces to 4 / 1, the reference always has a VCO clock in phase with it. While
instantaneously it is possible to have one of four different phases ® = 4), the PLL drives the output of the
feedback divider to phase lock with the output of the reference divider.
Reference Frequency
40 MHzPhase Detector
Frequency
10 MHz
VCO Frequency
160 MHz

Figure 4. Timing Plot for Deterministic PLL Case

NOTE: The ability for the PLL to drive the phase of the output of the R and N dividers to a fixed
relationship is how zero delay mode (ZDM) can synchronize output clocks with dividers. ZDM
includes an output divider in the feedback path so the divider output is synchronized to the
input clock.

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3.2

Example 2: Reference Clock with Non-Deterministic Phase Alignment to VCOGiven the following conditions for system use case 2A:• Reference frequency = 60 MHz• VCO frequency = 160 MHz• Every R'th reference clock edge has a corresponding VCO clock edge.
To lock phase with a 10-MHz phase detector frequency, R = 6 and N = 16. Because N / R = 16 / 6
reduces to 8 / 3, therefore R' = 3 and the reference has one of three different phase relationships to the
VCO clock. This configuration is non-deterministic.
Because the GCD of 60 MHz and 160 MHz is 20 MHz, the highest phase detector useable for an integer
PLL is 20 MHz. With a 20-MHz phase detector frequency, the N / R fraction would already have been inlowest terms and non-determinism would still exist.
Reference Frequency
60 MHzPhase Detector Frequency
10 MHz
VCO Frequency
160 MHz

Figure 5. Timing Plot for Non-Deterministic PLL Case
To summarize, the requirement for the reference frequency (input clock) of a PLL to have a deterministicphase relationship to the VCO frequency (output clock) is that the GCD of the reference frequency and the
VCO frequency must equal the reference frequency. In the case of non-determinism, the number ofpossible phase relationships between the reference frequency and VCO is the reference frequency / GCD(reference frequency, VCO frequency).
If the least terms fraction of N / R results in an R' value greater than one, it is still possible todeterministically synchronize the PLL if the R divider of the PLL supports synchronization. While the input
clock to output clock does not always align, the input clock and output clock always has the same phaserelationship with respect to the synchronization signal, which allows multiple PLLs in parallel to have their
output clocks phase aligned.
NOTE: Devices including but not limited to LMK04832 and LMX2594 support synchronization of PLL
R dividers.

NOTE: At the falling edge of the "Phase Detector Frequency" waveform in Figure 5, the reference
clock and the VCO clock share common rising edges, because the GCD(60, 160) = 20 MHz.
With GCD of 20 MHz, the clock edges align at a rate of 20 MHz. A phase detector as high as
20 MHz can be used to lock the 60-MHz reference to the 160-MHz VCO.

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Zero Delay Mode Synchronization

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Zero Delay Mode Synchronization
Clocking ICs typically integrate a PLL and a clock distribution block with division. Figure 6 illustrates a PLL
with two outputs, each output with its own output clock divider.
Input
Clock

/R

/N

Phase
Detector&
ChargePump

Loop
Filter

/D1

OutputClock 1

/D2

OutputClock 2

VCO

Figure 6. PLL Block Diagram with Clock Distribution
Most clock generators can synchronize the phase of the dividers D1 and D2 using the synchronization by
divider reset technique described previously. Many clocking devices have digital or analog delays, which
let the user specify a fixed delay between the clock distribution dividers. For many clock generation
applications, the alignment of the phases of each individual output is sufficient.
If phase determinism between reference input and clock output is required, the zero delay mode (ZDM)
method achieve this is by including the divided clock output in the feedback path from VCO to phasedetector. This eliminates the many possible phases of the clock output divider relative to the reference by“disciplining” the clock output phase to the reference phase through the phase detector. When the output
clock divider is not in the feedback path, the output clock divider introduces phase uncertainty between thereference input and output with phases spaced at the period of the VCO frequency.
Figure 7 illustrates the output of divider D2 in the feedback path with PLL N divider to the phase detector.
Only one output must be fed back to the phase detector because all the other outputs can share thephase determinism by synchronization with the D2 divider.
Input
Clock

/R

/N

Phase
Detector&
ChargePump

Loop
Filter

/D1

OutputClock 1

/D2

OutputClock 2

VCO

Figure 7. PLL Block Diagram with Clock Distribution and ZDMThis feedback can assure deterministic phase between input and output clock, but it does not alone
assure a true “zero delay” from input waveform to output waveform. To achieve a delay of zero seconds
from rising edge of input clock to rising edge of output clock, use delays on the PLL R and N paths or on
the feedback output or non-feedback outputs. This may result in the feedback clock having a different
phase than the other output clocks if adjustment is done in the clock output.
When using ZDM and including an output clock divider in the feedback loop, the VCO frequency equation
also includes the output clock divider, for Figure 7 VCO frequency = reference frequency * (N * D2) / R.

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4.1

ZDM Rules for Determinism
Select the input clock frequency and the feedback clock frequency to ensure ZDM results in deterministic
clock phase. For a clock output to have deterministic phase to the clock input, it must meet the following
two requirements:

4.1.1

First Rule, the GCD Between Input and Output Frequency must Equal Input Frequency
In the case of multiple output frequencies, calculate the GCD of all output frequencies and use this for theclock output frequency in the equation:
GCD(input clock frequency, output clock frequency) == input clock frequency.
If the output clock frequency is less than the input clock frequency, then a lower clock input frequency
must be presented to ensure deterministic phase. Otherwise there is an unsynchronized divide between
the input and the output.
If the output clock frequency is greater than the clock input frequency, but the result of the GCD of clock
input and output frequency is still less than clock input frequency, the frequencies are poorly related andthere is an unsynchronized divide between them. Change output frequencies to eliminate the poor
relationship to the input or reduce the input clock frequency.
NOTE: Reducing the input frequency of a PLL may reduce the phase detector frequency of the PLL,
which could reduce the in-band performance of the PLL. For PLLs with narrow loop
bandwidths as commonly designed for PLL1 of the LMK0482x and LMK04832, reduced inband PLL performance is of little concern because these PLLs are operating in a jittercleaning mode.

4.1.2

Second Rule, Lowest Output Frequency Requiring Determinism used for ZDM Feedback
The feedback frequency of a clocking device with multiple clock output frequencies all requiring
deterministic phase with the input shall be the lowest frequency.

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Zero Delay Mode Synchronization

4.2

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ZDM Examples
The following are three cases using ZDM to achieve deterministic phase between input and output of aclocking device. Only the first and third case achieves zero delay without modification. The first two cases
reuse the examples from the previous PLL section. The third case highlights complications which canoccur when more than one output frequency must have phase determinism with the reference clock.

4.2.1

Case 1: ZDM Aligning 40-MHz Reference with 40-MHz Clock Output
Given the following conditions for system use case 1B, which are the same as system use case 1A but
with an output divider:• Reference frequency = 40 MHz• VCO frequency = 160 MHz• Output frequency = 40 MHz
It is possible to achieve deterministic phase between the 40-MHz reference input and 40-MHz clock
output.
Setting PLL R = 4 and using a total PLL feedback divide value of 16 configures the PLL to lock phase at a10-MHz phase detector frequency. Setting the output divider = 4 configures the device to divide 160-MHz
VCO to a 40-MHz output clock frequency. Because this 40-MHz clock frequency drives the PLL N divider
in ZDM, the PLL N divider must be programmed to "total PLL feedback divide value" / "output divide
value." Thus, PLL N equals 4 which is 16 / 4. The total feedback divide is still 16 as in system use case
1A, Section 3.1, but the divide-by-16 is distributed between the clock divider and the PLL N divider.
Figure 8 illustrates the waveforms produced in this example. The reference frequency, phase detector
frequency, and VCO frequency are as in Section 3.1. The clock output frequency has been added andupon lock is shown to have matching phase with the reference frequency. The PLL N = /4 does produce
four possible phases of output from the PLL N divider in combination with 4 possible phases of output
from the clock output divider, but no matter which edge occurs, the PLL drives the phase error to a
deterministic minimum value to phase lock the other possibilities to all have the same phase as the
waveform "Output of PLL N: #1" shown in Figure 8, which is phase aligned (by the PLL) to the "10-MHz
Phase Detector Frequency" waveform above.
If ZDM was not used, the bottom of Figure 8 illustrates 3 other possible phase relationships the output
clock could have to the input clock. Any number of possible phase relationships greater than one results inphase uncertainty from input to output.
Reference Frequency
40 MHzPhase Detector Frequency
10 MHz
VCO Frequency
160 MHz
Clock Output Frequency
40 MHz1
Output of PLL N
from one of four
possible divides
phases (PLL N = /4).

2
3
4

Other possible phases ofclock output frequency
without 0-delay
40 MHzFigure 8. Timing Plot for ZDM with Phase determinism

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4.2.2

Case 2: ZDM Aligning 60-MHz Reference with 40-MHz Clock Output
Given the following conditions for system use case 2B, which are the same as system use case 2A but
with an output divider:• Reference frequency = 60 MHz• VCO frequency = 160 MHz• Output frequency = 40 MHz
It is possible for every R'th 60-MHz reference clock edge to have a corresponding 40-MHz clock output
edge.
Setting PLL R = 6 and using a total PLL feedback divide value of 16 configures the PLL to lock phase at a10-MHz phase detector frequency. Setting the output divider = 4 configures the device to divide the 160MHz VCO to a 40-MHz output clock frequency. Because this 40-MHz clock frequency drives the PLL N
divider in ZDM, the PLL N divider must be programmed to "total PLL feedback divide value" / "output
divide value." Thus, PLL N equals 4 which is 16 / 4. The total feedback divide is still 16 as in system use
case 2A, Section 3.2, but the divide-by-16 is distributed between the clock divider and the PLL N divider.
Reducing 16 / 6 to a least terms fraction results in N' / R' = 8 / 3, where R' = 3 indicates the number ofpossible phase relationships to the reference input clock. Any number of possible phase relationships
greater than one results in phase uncertainty from input to output. However, if multiple PLLs are used inparallel, a divider reset to all R dividers would result in the deterministic phase between output clocks ofthe different PLLs.
Figure 9 illustrates the waveforms consistent with this example. The reference frequency, phase detector
frequency, and VCO frequency are as before in system use case 2A, Section 3.2. However the clock
output frequency has been added and has matching phase with the reference frequency. The PLL N / 4
does produce four possible phase of output from the PLL N divider, but no matter which edge occurs, the
PLL phase locks the other possibilities to have the same phase. Refer to Figure 8, which illustrates this in
more detail. Finally, if ZDM was not used, the bottom of Figure 9 illustrates the other three possible phaserelationships the output clock could have to the input clock, making a total of four possible phaserelationships. Any number of possible phase relationships greater than one results in phase uncertainty
from input to output.
Reference Frequency
60 MHzPhase Detector Frequency
10 MHz
VCO Frequency
160 MHz
Clock Output Frequency
40 MHz
Other possible phases ofclock output frequency
without 0-delay
40 MHzFigure 9. Timing Plot for ZDM with Least Terms R' > 1

4.2.2.1

Modification for Phase Certainty

If phase determinism from input to output is required, change the reference frequency or output
frequencies so the two rules for ZDM determinism are true. Another option may be to synchronize the R
divider of the PLL to allow multiple parallel PLLs to produce synchronized outputs.

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Zero Delay Mode Synchronization

4.2.3

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Case 3: ZDM Aligning a Reference Frequency to Two Clock Output Frequencies
Given the following conditions for system use case 3:• 25-MHz reference frequency• 100-MHz and 125-MHz output frequencies• 500-MHz VCO
It is possible to achieve deterministic phase between the 25-MHz reference input and the 100-MHz and
125-MHz clock outputs.
When multiple outputs must have deterministic phase with the input, first find the GCD of the output
frequencies for which deterministic phase must be achieved, in this case GCD (100 MHz, 125 MHz) = 25
MHz. Thus the highest frequency that may be used for ZDM feedback is 25 MHz. Because the GCD of thereference frequency of 25 MHz and the feedback frequency of 25 MHz is also 25 MHz, the phase detector
may be programmed to 25 MHz.
Because neither output clock is 25 MHz, a 3rd output clock must be created at 25 MHz for ZDM feedback.
Using a 500-MHz VCO, the clock divider D1 is calculated as /5 for the 100-MHz clock output 1, D2 is /4
for the 125-MHz clock output 2, and D3 is /20 for the 25-MHz feedback clock. To achieve a 25-MHz phasedetector frequency using a 25-MHz reference PLL R = 1. The Total PLL N = 20, which is 500-MHz VCO /25-MHz phase detector. However, because a 25-MHz clock is generated and the phase detector
frequency is 25 MHz, the Total PLL N divide is implemented in the clock divider and thus PLL N = 1.
Figure 10 illustrates the waveforms produced in this example when all outputs have their dividers
synchronized together. Because the Total PLL N divider is 20, there could be 20 different phases of the
25-MHz feedback clock from the 500-MHz VCO before the PLL phase locks. After lock, the waveforms
look as shown below.
When an extra frequency is required for alignment, the LMK0482x and LMK04832 require only the clock
divider to operate while the output driver may be powered down to reduce power consumption. In manycases, the feedback clock is the same frequency as the lowest output clock. For instance, if 50 MHz and
100 MHz are generated, the GCD (50 MHz, 100 MHz) = 50 MHz and the 50-MHz clock would have beenused as the feedback clock, but could have also been used as an output clock. This case illustrates thenecessity to add another feedback clock.
Reference
25 MHzPhase Detector
25 MHz
VCO
500 MHz
Clock Output 1 (/10)
100 MHz
Clock Output 2 (/8)
125 MHz
Clock Output 3 (/40)
25 MHz (Feedback Clock)Figure 10. Timing Plot for ZDM with Phase Determinism for Multiple Frequencies
If only the 100-MHz or 125-MHz clock required synchronization, then a 3rd clock would not be needed.
Also, phase detector frequencies of higher than 25 MHz would be possible if a higher input clock was alsoavailable and only the 100-MHz or 125-MHz clock required synchronization. It is best practice to operate
PLLs with phase detector frequencies as high as possible to improve the PLL noise performance.

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4.3

ZDM and SYSREF with LMK0482x and LMK04832
One simple method to align SYSREF clocks between multiple devices is to use the SYSREF clock as
reference frequency and the SYSREF clock as the ZDM feedback clock. Using SYSREF as a reference
clock or for ZDM feedback may reduce the performance of the PLL, because SYSREF frequencies are
typically low. In the dual loop LMK0482x and LMK04832 PLLs, this ZDM method can be used while
retaining high performance due to the dual loop architecture. PLL1 may receive the low frequency
feedback clock of SYSREF while PLL2 receives a higher frequency reference from the selected VCXO
frequency.

5Synchronization using ZDM and Divider Reset
The LMK0482x and LMK04832 SYSREF divider clocks a D flip-flop that accepts an input from CLKin0 or
the SYNC pin with the proper mux settings. Refer to Figure 11. The output of this D flip-flop can beselected through the re-clocked mode of the SYSREF_MUX to drive the SYNC/SYSREF distribution path.
When the reference input to the LMK is the same frequency as the output of the SYSREF divider whichclocks D flip-flop, and the PLL is using a ZDM feedback clock at the same frequency as the output of the
SYSREF divider, then a CLKin0 or SYNC signal meeting setup and hold times to the reference input canbe used to perform (1) a deterministic divider reset and/or (2) re-clock SYSREF clock onto the SYSREF
outputs of the LMK.
Figure 11 shows CLKin0 being re-clocked by the SYSREF divider. It also shows the SYSREF divider
configured as the feedback clock, which is not required but the typical use case for this mode of operation.
There are two benefits of the SYSREF re-clocking configuration:
First, it is possible to reset clock dividers in the LMK from an external signal while meeting the setup andhold time to the relatively low frequency reference versus the high frequency VCO clock. For example, a250-MHz reference has a period of 4 ns while a 3-GHz VCO has a period of approximately 333 ps. The
SYSREF divider and feedback divider (if different than SYSREF divider) should be synchronized together
by a local device synchronization at the start-up of the LMK, then have the SYNC input disabled to these
dividers. ZDM then causes these dividers to be synchronized with the reference. Then the other clock
dividers may be synchronized by an external source attached to CLKin0 or SYNC pin. Typically resetting
the clock dividers from an external source is not required for determinism of the output clocks, as
determinism of output clocks would normally be achieved through the ZDM. However, as illustrated in
Section 4.2.3, the feedback frequency required for synchronization may be lower than the used output
clock frequencies. Using divider synchronization eliminates the need for a lower ZDM feedback frequency
or a lower input frequency.
Second, it is possible to operate in a SYSREF or SYNC repeating mode. If multiple LMK devices accept acommon reference to a PLL and a SYSREF clock to CLKin0 or SYNC, the LMK04828 re-clocks this
SYSREF input to the output. In addition to the input SYSREF being re-clocked onto the phase domain of
the LMK's internal VCO or external clock if in distribution mode, the digital and analog delays on the LMK
output path are available to further optimize the SYSREF placement against the paired device clock fromthe LMK. This method allows multiple LMK devices to output simultaneous SYSREF clock pulses provided
the SYSREF input is re-clocked by the same SYSREF divider edge at the D flip-flop. Because the
SYSREF divider is used for the re-clocking operation at the reference frequency, an upstream device
provides the SYSREF pulses or continuous frequency, which is fanned out to all downstream devices
operating in this SYSREF repeating mode.

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Synchronization using ZDM and Divider Reset

CLKin0

CLKin0_OUT_MUX

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Note: The SYNC/CLKin0 input isreclocked to the Dist Path
PLL1
SYSREF_REQ_EN

Off

SYNC

SYNC
_POL

SYNC
_MODE

SYSREF_MUX

D

PULSER MODE
SYSREF_PULSE_CNT

VCO0

VCO1

VCO
_MUX

Dist. Path

External
VCO

SYSREFDDLY

SYSREF
Divider

SYSREF_
CLKin0_SYNC/SYSREF
MUX

DCLKout6

SYNC_
DISSYSREF
CLKin1_OUT_MUX

SYSREF_PLSR_PD

SYSREF_PD
SYSREF_DDLY_PD

CLKin1

PulserOSCin

DCLKout8
FB_MUX

OSCout_MUXOSCout

CLKin1FB_MUXTo PLL N
PLL1
DCLKout0, 2, 4, 8, 10, 12

Clock Distribution
Path

DDLY
(4 to 32)

Divider(1 to 32)

AnalogDLY

DCC

OutputBufferDigitalDLY

AnalogDLY

OutputBufferSYNC_
DISX

SYSREF/SYNC
LegendSYSREF/SYNC Clock
VCO/Distribution ClockSPI Register: SYNC_EN
Must be set to enable any SYNC/
SYSREF functionality

SYSREF_CLR

SPI Register

SDCLKout1, 3, 5, 9, 11, 13

Figure 11. Functional Block Diagram of LMK0482x Configured to Re-clock CLKin0 for JESD204B SYSREF
Both LMK0482x and LMK04832 can be used as illustrated in Figure 11.
In all cases, setup and hold time of the CLKin0 or SYNC pin to the reference of the PLL must be met.
Because CLKin0 is a high performance clock input path, TI recommends using CLKin0 over SYNC assetup and hold times are larger.
NOTE: On LMK0482x and LMK04832, it is possible to configure the device to cause the SYSREF
divider to reset itself. If this occurs, the output of the SYSREF will not be well behaved.
Especially when SYSREF is operating continuously, disable the SYNC to the clock output
dividers or the clock outputs will not be well behaved. Be aware of and use the
SYNC_DISSYSREF and SYNC_DIS# for each output to disable reset of the SYSREF divider
and clock dividers as needed.

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6

Conclusion
The primary reason for synchronization of multiple clocks is to fan-out large clocking trees which havedeterministic phase relationships to one another. For example, deterministic phase between clocks isuseful in phased array antenna systems or in JESD204B systems which distribute a SYSREF to multiple
data converters or logic devices.
This application note has discussed achieving phase determinism from the input to output of a clocking
device by divider reset, zero delay mode (ZDM), and a combination of ZDM and divider reset. Eachmethod has advantages and disadvantages listed in Table 1.
Table 1. Synchronization Methods Pros and Cons
Synchronization
methodPro

Con

Divider reset
Section 2• All dividers synchronize to a single event orseries of events. From the moment of divider
reset, the phase of all output clocks is known.

• It can be hard or impossible to meet the setup
and hold times to high frequency signals, suchas multi-gigahertz VCOs.
• Input and output frequencies must be chosen
according to ZDM rules for determinism.
–

Zero delay mode
Section 4• No timing requirements to align periodic clock
outputs.
• The PLL automatically aligns the phase of
input and output clocks.

Zero Delay Mode +Re-clocked divider reset
or SYSREF
Section 5• ZDM with re-clocked divider reset reduces the
setup and hold timing requirements externalsignals must meet for divider reset.
• ZDM with re-clocked SYSREF allows anexternal SYSREF to be re-clocked onto the
VCO phase domain for distribution to a target
JESD204B device.

Some cases require divider reset of
PLL R divider.
• If a pulsed SYSREF output is to occur at thesame moment across multiple clocking
devices for reasons beyond JESD204B
synchronization, setup and hold times must be
met for requesting SYSREF pulses.
–

JESD204B does not require SYSREF
to be output at the same moment toalign the LMFC.
• A low input frequency to the PLL may berequired when using ZDM to synchronize the
SYSREF frequency. Use dual loop mode,
which requires external an VCXO on
LMK0482x and LMK04832 to retain highperformance.

• JESD204B SYSREF generation requires an
upstream device to produce the SYSREF
clock frequency. This may be another
LMK0482x or LMK04832.

SNAA294 – December 2019
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Multi-Clock Synchronization
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