ISL12025 Intersil Corporation, ISL12025 Datasheet - Page 22

• Manufacturer Part Number: ISL12025
• Description: Real-Time Clock/Calendar
• Manufacturer: Intersil Corporation

A final application for the ATR control is in-circuit calibration
for high accuracy applications, along with a temperature
sensor chip. Once the RTC circuit is powered up with battery
backup, and frequency drift is measured. The ATR control is
then adjusted to a setting which minimizes drift. Once
adjusted at a particular temperature, it is possible to adjust at
other discrete temperatures for minimal overall drift, and
store the resulting settings in the EEPROM. Extremely low
overall temperature drift is possible with this method. The
Intersil evaluation board contains the circuitry necessary to
implement this control.
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. Figure 27 shows a suggested layout for the
ISL12025 or ISL12027 devices.
FIGURE 27. SUGGESTED LAYOUT FOR INTERSIL RTC IN SO-8
Citizen
Epson
Raltron
SaRonix
Ecliptek
ECS
Fox
MANUFACTURER
CM201, CM202, CM200S
MC-405, MC-406
RSM-200S-A or B
32S12A or B
ECPSM29T-32.768K
ECX-306/ECX-306I
FSM-327
22
PART NUMBER
TABLE 8. CRYSTAL MANUFACTURERS
ISL12025
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 (Figure 27).
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
short periods of time. Depending on the value of
supercapacitor used, backup time can last from a few days
TEMP RANGE
-40 to +85°C
-40 to +85°C
-40 to +85°C
-40 to +85°C
-10 to +60°C
-10 to +60°C
-40 to +85°C
DD
may disappear intermittently for
+25°C FREQUENCY TOLERANCE
DD
pin of the chip is mandatory,
±20ppm
±20ppm
±20ppm
±20ppm
±20ppm
±20ppm
±20ppm
October 18, 2006
FN6371.1