SA.45s Chip-Scale Atomic Clock
SA.45s CSAC User Guide Revision D
39
y(t): Fractional frequency of the clock at time t, approximated as
y(t) = y + at + y (t)
0
e
y : Fractional frequency offset at t = 0
0
a: Clock aging rate
y (t): Fractional frequency offset due to environmental effects (that is, temperature)
e
ε(t): Random fractional frequency fluctuations
ε(t) = τσ(τ)
σ(τ): Allan deviation at sampling rate (τ)
3.7.6
Writes to NVRAM
CSAC has a physical limit to the number of writes to NVRAM <20,000. To maximize the lifetime of the
CSAC, restrict the number of NVRAM writes accordingly.
The following are the scenarios of a write to NVRAM:
Customer issuing a Frequency Latch command (format
) causes an NVRAM write.
!FL
Customer issuing a PPS cable length compensation latch command (format
) causes an
!DCL
NVRAM write.
Customer issuing a PPS disciplining tau set command that changes discipline tau (format
where
!Dn
n is a decimal number) causes an NVRAM write.
Customer issuing a mode command that changes mode register (format
where x is an
!Mx
alphabetical character) in list below causes an NVRAM write:
RS232 Communications Checksum on/off (
vs
)
!MC
!Mc
External oscillator on/off (
vs
)
!MZ
!Mz
PPS auto-sync on/off (
vs
)
!MS
!Ms
PPS disciplining on/off (
vs
)
!MD
!Md
PPS measure on/off (
vs
)
!MM
!Mm
Analog tuning on/off (
vs
)
!MA
!Ma
ULP on/off (
vs
)
!MU
!Mu
Customer issuing a ULP configuration command (format
where n and p are decimal
!Un,p
numbers) causes an NVRAM write.
Every ULP cycle, firmware does an automatic NVRAM write.
Every auto-reset (status 0->8) due to alarm where status reverts from lock state, firmware does an
automatic NVRAM write.
Periodically every 30 days in clock-lock state, firmware does an automatic NVRAM write.
Customer issuing a PPS pulse width set command that changes pulse width (format
where N is
!>N
a decimal number) will cause an NVRAM write. (FW1.08 and later.)
Customer issuing a PPS disciplining status threshold phase command that changes phase threshold
(format
where N is a decimal number) will cause an NVRAM write. (FW1.08 and later.)
!mN
Lock set points are saved after each acquisition of a lock state. This write occurs ~102 seconds after
Lock is achieved.
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