ISL12028 Intersil Corporation, ISL12028 Datasheet - Page 24

• Manufacturer Part Number: ISL12028
• Manufacturer: Intersil Corporation

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
5 seconds. 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, or if the chip has
the IRQ/F
oscilloscope (after enabling it with the control register, and
using a pull-up resistor for the open-drain output).
Alternatively, the ISL12028 IRQ/F
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
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
supercapacitor, which is connected to the V
use Schottky diodes with very low leakages, <1
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
powered operations predictable. In Legacy modes the V
pin can power the chip if the voltage is above V
V
communicate with the device under battery backup, but the
supply current drain is much higher than the Standard mode
TRIP
. This makes it possible to generate alarms and
OUT
output, look at the output of that pin on an
DD
may disappear intermittently for
24
OUT
DD
- output can be
to charge the
BAT
µA
DD
pin. Try to
desirable.
and
DD
BAT
ISL12028
and backup time is reduced. In this case if alarms are used
in backup mode, the IRQ/F
connected to V
the default mode is the Standard mode.
I
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
I
is programmed for I
Table 10 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
2
2
C communications cease regardless of whether the device
C Communications During Battery backup and
switchover combined with I
mode. In actuality the V
switching to battery backup, which will disable I
ANYTIME the device goes into battery backup mode.
Regardless of the battery voltage, the I
to the V
2.7-5.5V
FIGURE 28. SUPERCAPACITOR CHARGING CIRCUIT
RESET
BAT
voltage. See Figure 29.
V
voltage source. During initial power up
CC
2
C operation in battery backup mode.
V
SS
DD
OUT
V
back
2
will go below V
DD
C operation in battery backup
pull up resistor must be
goes below VRESET, then
2
C will work down
RESET
Supercapacitor
2
C
April 17, 2006
before
FN8233.3
DD