Microchip Technology SA5 Series, User Manual

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Operation

2019 Microchip Technology Inc. DS50002938A-page 23
3.4 1PPS DISCIPLINING
For further reduction of phase and frequency errors, disciplining can be enabled/dis-
abled with the
Disciplining
parameter. The algorithm implements a high-resolution
phase meter within the MAC to automatically correct the phase and frequency relative
to a reference 1PPS input once per second with a resolution of 450
ps. The algorithm
will simultaneously steer the phase and frequency to that of the external reference
(1PPS input), ultimately achieving accuracies of <1
ns and 1×10
-13
, respectively,
depending on the stability of the 1PPS input and ignoring external cabling delays, etc
(See
Section 3.4.5 “Cable Length Compensation” ).
3.4.1 Theory
The 1PPS input is user selectable from pins J1-5,7 or J1-1,3 via the
PpsSource
parameter. If a valid 1PPS is present on the selected input,
PpsInDetected
= 1 and
the user will notice the
DigitalTuning
parameter automatically adjust, once
Dis-
ciplining
is enabled. The
LastCorrection
parameter will tell the user how much
the MAC was digitally steered (in frequency) since its last correction; it is simply the dif-
ference between the last two successive
DigitalTuning
values. Similarly, the
Phase
parameter will report the most recent phase meter measurement to indicate the
time-difference between input and output 1PPS signals.
The speed and effectiveness of the disciplining algorithm can be adjusted by the time
constant “Tau”, which is user selectable through the
TauPps0
(or
TauPps1
) parame-
ter. See Section 3.4.2 “Selection of Disciplining Time Constant, “Tau”” for advice
on choosing an appropriate time constant.
When disciplining is turned on, it will make a correction once per second based on its
most recent internal phase meter measurement. The disciplining algorithm will attempt
to dampen an initial phase and/or frequency error E
0
by ~63% after an elapsed time of
one Tau, 99% after five Tau per the equation:
EQUATION 3-1:
E E
0
1 e
t

-- –
–
 
 
 

=
Where:
E = The instantaneous error.
E
0
= The initial error calculated from the moving average of the previous errors observed over
time duration τ , updated once per second.
τ = The user-selectable time constant, tau, in seconds.
t = Elapsed time, in seconds.
Any subsequent additional errors after E
0
will prolong the settling time of the algorithm.
In the event that the reference 1PPS input is removed from the selected input (pins
J1-5,7 or J1-1,3) while disciplining, the MAC remains in holdover and preserves the
most recent
DigitalTuning
value. If the 1PPS reference subsequently reappears,
disciplining continues where it left off. The exception being if the measured instanta-
neous
Phase
>
PhaseLimit
, whereby a JamSync will be implemented and the Disci-
plining algorithm will start anew. (Phase outliers are ignored. See
Section 3.4.3 “JamSync” )
The status of disciplining is indicated by the
DisciplineLocked
parameter in the
telemetry.
DisciplineLocked
= 0 upon algorithm startup or when a valid PPS input
signal is not detected.
DisciplineLocked
= 1 when magnitude of the phase mea-