SA.45s Chip-Scale Atomic Clock
SA.45s CSAC User Guide Revision D
16
Figure 10 • Frequency Record of CSAC in ULP Mode
The green arrows indicate the on time calibrations. Note the relatively poor TCXO drift and temperature
behavior between calibrations.
ULP is an unusual operating mode for an atomic clock and it is important for the user to understand
exactly how the clock is behaving to effectively implement this feature in a system. In particular, note
the following:
When operating in ULP mode, the Status register indicates Status = 9 (asleep) when the atomic clock
portion of the CSAC is asleep. Each wake cycle is indicated by the usual lock process (Status = 8, 7, 6,
…) followed by wake-time seconds of operation at Status = 0 before the cycle repeats. This cycle is
also reflected on the BITE pin, which is 1 (high) whenever the CSAC is unlocked (or asleep) and only
0 (low) during the locked periods.
When using Disciplining (see
) in conjunction with ULP, disciplining
"1PPS Disciplining" (see page 12)
functionality is disabled during sleep and unlocked cycles, though steering information is preserved
and updated across wake cycles.
Frequency Steering commands may be entered when the CSAC is asleep or unlocked but do not
affect the output frequency until lock is achieved, typically on the next wake cycle (see
). Also, the Latch command is only valid when the CSAC is locked
(Status = 0).
If enabled, Analog Tuning (see
) is only active during wake cycles.
ULP is enabled through bit 5 (0x0020) in the mode register (see
"Set/Clear Operating Modes (M)" (see
) and the sleep-time and wake-time are set by the
command (see
!U
). These values are non-volatile; they persist across power cycles.
Mode Parameters (U)" (see page 26)
Note that the wake-time begins counting after the CSAC achieves lock, so the actual time that the
atomic clock portion of the CSAC is powered on is the sum of the time to lock and the user-configured
wake-time. The minimum allowed values of wake-time and sleep-time are 10 seconds and 1800
seconds, respectively.
To configure ULP parameters using CSACdemo, select
from the View menu to
Ultra-Low Power Mode
access the panel shown in the following screen shot.
