Rainbow Electronics DS1342 - Manual (Page 7)

Low-Current I 2 C RTCs for High-ESR Crystals
DS1341/DS1342
7
External Synchronization
When an external clock reference is used, the input
from CLKIN/INTA is divided down to 1Hz by the divi-
sor selected by the CLKSEL[2:1] bits. The 1Hz from the
divider (Ext-1Hz, see the Functional Diagram) is used to
correct the 1Hz that is derived from the 32.768kHz oscil-
lator (Osc-1Hz). As Osc-1Hz drifts in relation to Ext-1Hz,
Osc-1Hz is digitally adjusted.
As shown in the Functional Diagram, the three high-
est frequencies driving the SQW/INTB pin are derived
from the uncorrected oscillator, while the 1Hz output is
derived from the adjusted Osc-1Hz signal.
Conceptually, the circuit can be thought of as two 1Hz
signals, one derived from the internal oscillator and the
other derived from the external reference clock, with the
oscillator-derived 1Hz signal being locked to the 1Hz
signal derived from the external reference clock. The
edges of the 1Hz signals do not need to be aligned with
each other. While the external clock source is present
and within tolerance, the Ext-1Hz and Osc-1Hz maintain
their existing lock, regardless of their edge alignment,
with periodic correction of the Osc-1Hz signal. If the
external signal is lost and then regained sometime later,
the signals relock with whatever new alignment exists
(see Figure 3).
The Ext-1Hz is used by the device as long as it is within
tolerance, which is about 0.8% of Osc-1Hz. While Ext-
1Hz is within tolerance, the skew between the two sig-
nals could shift until a change of approximately 7.8ms
accumulates, after which the Osc-1Hz signal is adjusted
(see Figure 4). The adjustment is accomplished by digi-
tally adjusting the 32kHz oscillator divider chain.
If the difference between Ext-1Hz and Osc-1Hz is greater
than approximately 0.8%, Osc-1Hz runs unadjusted (see
Figure 3) and the loss of signal (LOS) is set, provided the
ECLK bit is set.
Figure 3. Loss and Reacquisition of External Reference Clock
Figure 4. Drift and Adjustment of Internal 1Hz to External Reference Clock
OSC-1Hz
FROM OSCILLATOR
EXT-1Hz
FROM EXTERNAL REFERENCE
SKEW SKEW
BREAK IN EXTERNAL REFERENCE SIGNAL
CURRENT LOCK SHIFTED BACK TO CURRENT LOCK DRIFT AFTER N CYCLES
OSC-1Hz
FROM OSCILLATOR
EXT-1Hz
FROM EXTERNAL REFERENCE