ISL12024 Intersil Corporation, ISL12024 Datasheet - Page 21

• Manufacturer Part Number: ISL12024
• Description: Real-Time Clock/Calendar
• Manufacturer: Intersil Corporation

For more detailed operation, see Intersil’s application note
AN154 on Intersil’s website at www.intersil.com.
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. Below in Figure 23 is a suggested layout for
the ISL12024 or ISL12026 devices in 8 pin SO package.
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
with a solid connection to ground.
The ISL12024 product has a special consideration. The
IRQ/F
the X2 pin. When this pin is used as a frequency output
(IRQ/F
X1 or X2 pins and cause double-clocking. The layout in
Figure 23 minimizes this by running the IRQ/F
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
For other RTC products, the same rules stated above should
be observed, but adjusted slightly since the packages and
pinouts are different.
FIGURE 23. SUGGESTED LAYOUT FOR INTERSIL RTC IN SO-8
OUT
OUT
pin on the 8-lead SOIC package is located next to
) and is set to 32.768kHz noise can couple to the
OUT
outputs.
DD
21
pin of the chip is mandatory,
OUT
output
ISL12024
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 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 an open-drain output).
Alternatively, the ISL12024 device has an IRQ/F
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
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 is predictable. In Legacy Mode, the
V
less than V
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
BAT
pin can power the chip if the voltage is above V
OUT
TRIP
output, look at the output of that pin on an
. In this mode, it is possible to generate alarm
DD
may disappear intermittently for
OUT
pull up resistor must be
DD
to charge the
BAT
µA
pin. Try to
OUT
October 18, 2006
desirable.
DD
DD
output
FN6370.1
and