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NOTICE
Caution should be used when using a longer time constant. Initial frequency errors at
the start of Disciplining produce phase errors that will grow proportionally with elapsed
time. For slow (long Tau) Disciplining algorithms, the initial errors could grow outside
the
DisciplineThreshold
or
PhaseLimit
settings before an adequate correction can
be made by the algorithm, resulting in a JamSync and
DisciplineLocked
= 0. Gener-
ally, a good Discipline approach is to use a short Tau to make quick coarse corrections,
then apply a longer Tau to further reduce the MAC’s phase and frequency error.
MAC-SA5X User’s Guide
DS50002938A-page 26

2019 Microchip Technology Inc.
3.4.3 JamSync
When Disciplining is first enabled, the algorithm will implement a JamSync (Observable
with the
JamSyncing
parameter). It will also occur whenever the
Phase
is beyond the
PhaseLimit
parameter. A JamSync has the benefit of speeding up the disciplining
routine by quickly synchronizing the output 1PPS after one clock cycle (phase is briefly
aligned, but frequency is not). The disciplining algorithm can then continue onward by
further refining the frequency and phase errors. However, the drawback of a JamSync
is that the user will observe a phase “jump” during the JamSync, rather than a slow
steer as predicated by disciplining alone. To avoid said “jump”, the user can adjust the
PhaseLimit
parameter. This is particularly useful for long Disciplining time constants.
However, the initial JamSync (when disciplining is initialized) cannot be avoided.
3.4.4 1PPS Phase Measurement Mode
Phase measurement mode does not steer or Discipline the MAC’s output frequency.
Rather, it makes use of the MAC’s internal phase meter by reporting the time difference
between the MAC’s internally-generated 1PPS output (pins J1-17,19) and the exter-
nally applied reference 1PPS active input (pins J1-5,7 or J1-1,3) once per second.
Measurement resolution is approximately 450
ps. The mode is enabled with the
PhaseMetering
parameter and the internal phase difference can be read via the
Phase
parameter.
Note: Tau has no effect on the
Phase
parameter.
3.4.5 Cable Length Compensation
The zero point of disciplining can be adjusted to accommodate cable and other instru-
mentation delays (or advances) that impact the arrival time of the 1PPS at the MAC
1PPS input pin. The compensation value can be adjusted with the
CableDelay
parameter.
The maximum compensation adjustment is ±0.5 seconds, where the positive sign indi-
cates phase advancement of the input 1PPS. For example, if there is 45
ns of delay
(approximately 33 feet of RG-58 coaxial cable) between the on-time point and the MAC
1PPS input then the compensation value would be +45.
Note: Compensation is implemented in the disciplining algorithm, not in the phase
measurement itself. The phase measurement, as reported through teleme-
try, reports the actual phase measurement. That is, if the MAC is disciplined
with +50
ns of compensation, the phase meter reports –50 ns of phase dif-
ference when disciplining is settled.