ISL12024 Datasheet, PDF (21/25 Pages) Intersil Corporation – Real-Time Clock/Calendar with Embedded Unique ID

ISL12024
Layout Considerations
The crystal input at X1 has a very high impedance and will
pick up high frequency signals from other circuits on the
board. Since the X2 pin is tied to the other side of the crystal,
it is also a sensitive node. These signals can couple into the
oscillator circuit and produce double clocking or
mis-clocking, seriously affecting the accuracy of the RTC.
Care needs to be taken in layout of the RTC circuit to avoid
noise pickup. Figure 23 is a suggested layout for the
ISL12024 or ISL12026 devices in an 8 Ld SO package.
U1
R5
X1
47k
FIGURE 23. SUGGESTED LAYOUT FOR INTERSIL RTC IN SO-8
The X1 and X2 connections to the crystal are to be kept as
short as possible. A thick ground trace around the crystal is
advised to minimize noise intrusion, but ground near the X1
and X2 pins should be avoided as it will add to the load
capacitance at those pins. Keep in mind these guidelines for
other PCB layers in the vicinity of the RTC device. A small
decoupling capacitor at the V_DD pin of the chip is mandatory,
with a solid connection to ground.
The ISL12024 product has a special consideration. The
IRQ/F_OUT pin on the 8 Ld SOIC package is located next to
the X2 pin. When this pin is used as a frequency output
(IRQ/F_OUT) and is set to 32.768kHz, noise can couple to the
X1 or X2 pins and cause double-clocking. The layout in
Figure 23 minimizes this by running the IRQ/F_OUT output
away from the X1 and X2 pins. Also, reducing the switching
current at this pin by careful selection of the pull-up resistor
value will reduce noise. Intersil suggests a minimum value of
5.1kΩ for 32.768kHz, and higher values (up to 20kΩ) for
lower frequency IRQ/F_OUT outputs.
For other RTC products, the same rules previously stated
should be observed, but adjusted slightly since the packages
and pinouts are different.
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 useable
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 RTCs,
the internal circuitry assures start-up 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_OUT output, look at the output of that pin on an
oscilloscope (after enabling it with the control register, and
using a pull-up resistor for an open-drain output).
Alternatively, the ISL12024 device has an IRQ/F_OUT output
which 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 the Super
Cap 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 Super Cap,
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 is predictable. In Legacy Mode, the
V_BAT pin can power the chip if the voltage is above V_DD and
less than V_TRIP. In this mode, it is possible to generate alarm
and communicate with the device, unless SBI = 1, but the
supply current drain is much higher than the Standard Mode
and backup time is reduced. In this case if alarms are used
in backup mode, the IRQ/F_OUT pull-up resistor must be
connected to V_BAT voltage source. During initial power-up
the default mode is the Standard Mode.
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FN6370.3
August 18, 2008