ISL12027 Datasheet, PDF (24/28 Pages) Intersil Corporation – Real Time Clock/Calendar with EEPROM

ISL12027
Oscillator Measurements
When a proper crystal is selected and the layout guidelines
above are observed, the oscillator should start up in most
circuits in less than one second. Some circuits may take
slightly longer, but startup should definitely occur in less than
5s. When testing RTC circuits, the most common impulse is
to apply a scope probe to the circuit at the X2 pin (oscillator
output) and observe the waveform. DO NOT DO THIS!
Although in some cases you may see a usable waveform,
due to the parasitics (usually 10pF to ground) applied with
the scope probe, there will be no useful information in that
waveform other than the fact that the circuit is oscillating.
The X2 output is sensitive to capacitive impedance so the
voltage levels and the frequency will be affected by the
parasitic elements in the scope probe. Applying a scope
probe can possibly cause a faulty oscillator to start up, hiding
other issues (although in the Intersil RTC’s, the internal
circuitry assures startup when using the proper crystal and
layout).
The best way to analyze the RTC circuit is to power it up and
read the real time clock as time advances. Alternatively the
frequency can be checked by setting an alarm for each
minute. Using the pulse interrupt mode setting, the once-per-
minute interrupt functions as an indication of proper
oscillation.
Backup Battery Operation
Many types of batteries can be used with the Intersil RTC
products. 3.0V or 3.6V Lithium batteries are appropriate, and
sizes are available that can power a Intersil RTC device for
up to 10 years. Another option is to use a supercapacitor for
applications where V_DD may disappear intermittently for
short periods of time. Depending on the value of
supercapacitor used, backup time can last from a few days
to two weeks (with >1F). A simple silicon or Schottky barrier
diode can be used in series with V_DD to charge the
supercapacitor, which is connected to the V_BAT pin. Try to
use Schottky diodes with very low leakages, <1µA desirable.
Do not use the diode to charge a battery (especially lithium
batteries!).
There are two possible modes for battery backup operation,
Standard and Legacy mode. In Standard mode, there are no
operational concerns when switching over to battery backup
since all other devices functions are disabled. Battery drain
is minimal in Standard mode, and return to Normal V_DD
powered operations predictable. In Legacy modes the V_BAT
pin can power the chip if the voltage is above V_DD and
V_TRIP. This makes it possible to generate alarms and
communicate with the device under battery backup, but the
supply current drain is much higher than the Standard mode
and backup time is reduced. During initial power-up, the
default mode is the Legacy mode.
2.7-5.5V
V_CC
V_back
V_SS
Supercapacitor
FIGURE 28. SUPERCAPACITOR CHARGING CIRCUIT
I²C Communications During Battery Backup and
LVR Operation
Operation in Battery Backup mode and LVR is affected by
the BSW and SBIB bits as described earlier. These bits allow
flexible operation of the serial bus and EEPROM in battery
backup mode, but certain operational details need to be
clear before utilizing the different modes. The most
significant detail is that once V_DD goes below V_RESET, then
I²C communications cease regardless of whether the device
is programmed for I²C operation in battery backup mode.
Table 9 describes 4 different modes possible with using the
BSW and SBIB bits, and how they are affect LVR and battery
backup operation.
• Mode A - In this mode selection bits indicate a low V_DD
switchover combined with I²C operation in battery backup
mode. In actuality the V_DD will go below V_RESET before
switching to battery backup, which will disable I²C
ANYTIME the device goes into battery backup mode.
Regardless of the battery voltage, the I²C will work down
to the V_RESET voltage (See Figure 29).
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FN8232.4
October 18, 2006