ISL12028 Datasheet, PDF (23/28 Pages) Intersil Corporation – Real Time Clock/Calendar with EEPROM

ISL12028
MANUFACTURER
Citizen
Epson
Raltron
SaRonix
Ecliptek
ECS
Fox
TABLE 9. 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
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.
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 27 is a suggested layout for the
ISL12029 or ISL12028 devices.
C1
0.1µF
XTAL1
3³²2^..⁷7⁶6⁸8^kk^HG^zz
R1 10k
U1
IS_XL₁1₂2₂₈028
FIGURE 27. SUGGESTED LAYOUT FOR INTERSIL RTC IN SO-14
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.
For other RTC products, the same rules stated above should
be observed, but adjusted slightly since the packages and
pinouts are slightly different.
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 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, or if the chip has
the IRQ/F_OUT output, look at the output of that pin on an
oscilloscope (after enabling it with the control register, and
using a pull-up resistor for the open-drain output).
Alternatively, the ISL12028 IRQ/F_OUT- output 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.
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
23
FN8233.5
October 18, 2006