ISL12026_08 Datasheet, PDF (20/24 Pages) Intersil Corporation – Real Time Clock/Calendar with I2C Bus™ and EEPROM

ISL12026, ISL12026A
TABLE 8. CRYSTAL MANUFACTURERS
MANUFACTURER
PART
NUMBER
TEMP +25°C FREQ.
RANGE TOLERANCE
(°C)
(ppm)
Citizen
CM201, CM202, CM200S -40 to +85 ±20ppm
Epson
MC-405, MC-406
-40 to +85 ±20ppm
Raltron
RSM-200S-A or B
-40 to +85 ±20ppm
SaRonix
32S12A or B
-40 to +85 ±20ppm
Ecliptek
ECPSM29T-32.768K
-10 to +60 ±20ppm
ECS
ECX-306/ECX-306I
-10 to +60 ±20ppm
Fox
FSM-327
-40 to +85 ±20ppm
The turnover temperature in Table 7 describes the
temperature where the apex of the of the drift vs temperature
curve occurs. This curve is parabolic with the drift increasing
as (T - T0)². For an Epson MC-405 device, for example, the
turnover temperature is typically +25°C, and a peak drift of
>110ppm occurs at the temperature extremes of -40°C and
+85°C. It is possible to address this variable drift by adjusting
the load capacitance of the crystal, which will result in
predictable change to the crystal frequency. The Intersil RTC
family allows this adjustment over temperature since the
devices include on-chip load capacitor trimming. This control
is handled by the Analog Trimming Register, or ATR, which
has 6-bits of control. The load capacitance range covered by
the ATR circuit is approximately 3.25pF to 18.75pF, in
0.25pF increments. Note that actual capacitance would also
include about 2pF of package related capacitance. In-circuit
tests with commercially available crystals demonstrate that
this range of capacitance allows frequency control from
+80ppm to -34ppm, using a 12.5pF load crystal.
In addition to the analog compensation afforded by the
adjustable load capacitance, a digital compensation feature
is available for the Intersil RTC family. There are 3-bits
known as the Digital Trimming Register or DTR, and they
operate by adding or skipping pulses in the clock signal. The
range provided is ±30ppm in increments of 10ppm. The
default setting is 0ppm. The DTR control can be used for
coarse adjustments of frequency drift over-temperature or
for crystal initial accuracy correction.
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, the IRQ/F_OUT output is set at 32.768kHz 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.
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. In Figure 23 is a suggested layout for the
ISL12026 or ISL12027 devices in 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 ISL12026 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 slightly adjusted 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
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FN8231.8
October 28, 2008