ISL12026 Intersil Corporation, ISL12026 Datasheet - Page 20

• Manufacturer Part Number: ISL12026
• 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, the IRQ/F
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 23 is a suggested layout for the
ISL12026 or ISL12027 devices in 8 pin SO package.
FIGURE 23. SUGGESTED LAYOUT FOR INTERSIL RTC IN SO-8
Citizen
Epson
Raltron
SaRonix
Ecliptek
ECS
Fox
MANUFACTURER
OUT
output is set at 32.768kHz and
CM201, CM202, CM200S
MC-405, MC-406
RSM-200S-A or B
32S12A or B
ECPSM29T-32.768K
ECX-306/ECX-306I
FSM-327
20
PART NUMBER
TABLE 7. CRYSTAL MANUFACTURERS
ISL12026
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.
The ISL12026 product has a special consideration. The IRQ/
F
X2 pin. When this pin is used as a frequency output (IRQ/
F
X2 pins and cause double-clocking. The layout in Figure 23
minimizes this by running the IRQ/F
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
For other RTC products, the same rules stated above should
be observed, but adjusted slightly 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
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 useable
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 RTCs,
the internal circuitry assures startup when using the proper
crystal and layout).
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
OUT
OUT
- pin on the 8-lead SOIC package is located next to the
) and is set to 32.768kHz noise can couple to the X1 or
OUT
outputs.
+25°C FREQUENCY TOLERANCE
DD
pin of the chip is mandatory,
OUT
±20ppm
±20ppm
±20ppm
±20ppm
±20ppm
±20ppm
±20ppm
output away from
April 13, 2006
FN8231.3