ISL12027_10 Datasheet, PDF (23/28 Pages) Intersil Corporation – Real Time Clock/Calendar with EEPROM

ISL12027, ISL12027A
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 ISL12027 or ISL12026 devices.
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 (see Figure 27).
TABLE 8. CRYSTAL PARAMETERS REQUIRED FOR INTERSIL RTC’S
PARAMETER
MIN
TYP
MAX
UNITS
NOTES
Frequency
32.768
kHz
Frequency Tolerance
±100
ppm
Down to 20ppm if desired
Turnover Temperature
20
25
30
°C
Typically the value used for most crystals
Operating Temperature Range
-40
85
°C
Parallel Load Capacitance
12.5
pF
Equivalent Series Resistance
50
kΩ
For best oscillator performance
MANUFACTURER
Citizen
Epson
Raltron
SaRonix
Ecliptek
ECS
Fox
TABLE 9. CRYSTAL MANUFACTURERS
PART NUMBER
TEMP RANGE (°C)
CM201, CM202, CM200S
-40 to +85
MC-405, MC-406
-40 to +85
RSM-200S-A or B
-40 to +85
32S12A or B
-40 to +85
ECPSM29T-32.768K
-10 to +60
ECX-306/ECX-306I
-10 to +60
FSM-327
-40 to +85
+25°C FREQUENCY TOLERANCE (ppm)
±20
±20
±20
±20
±20
±20
±20
XTAL1
32.768kH_Z
U₁
ISL12027
C₁
01µF
R₁ 10k
FIGURE 27. SUGGESTED LAYOUT FOR INTERSIL RTC IN SO-8
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 start-up 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
23
FN8232.8
August 12, 2010