ISL12025 Datasheet, PDF (22/27 Pages) Intersil Corporation – Real-Time Clock/Calendar with I2C Bus and EEPROM

ISL12025
PARAMETER
Frequency
Frequency Tolerance
Turnover Temperature
Operating Temperature Range
Parallel Load Capacitance
Equivalent Series Resistance
TABLE 7. CRYSTAL PARAMETERS REQUIRED FOR INTERSIL RTC’S
MIN
TYP
MAX
UNITS
NOTES
32.768
kHz
±100
ppm
Down to 20ppm if desired
20
25
30
°C
Typically the value used for most crystals
-40
85
°C
12.5
pF
50
kΩ
For best oscillator performance
MANUFACTURER
Citizen
Epson
Raltron
SaRonix
Ecliptek
ECS
Fox
TABLE 8. CRYSTAL MANUFACTURERS
PART NUMBER
TEMP RANGE
CM201, CM202, CM200S
-40 to +85°C
MC-405, MC-406
-40 to +85°C
RSM-200S-A or B
-40 to +85°C
32S12A or B
-40 to +85°C
ECPSM29T-32.768K
-10 to +60°C
ECX-306/ECX-306I
-10 to +60°C
FSM-327
-40 to +85°C
+25°C FREQUENCY TOLERANCE
±20ppm
±20ppm
±20ppm
±20ppm
±20ppm
±20ppm
±20ppm
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.
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).
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.
XTAL1
32.768KH_Z
U₁
X1226SOIC
C₁
01µF
R₁ 10k
FIGURE 27. SUGGESTED LAYOUT FOR INTERSIL RTC IN SO-8
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.
22
FN6371.3
August 13, 2008