X1227_06 Datasheet, PDF (24/28 Pages) Intersil Corporation – 2-Wire™ RTC Real TimeClock/Calendar/ CPU Supervisor with EEPROM

X1227
A final application for the ATR control is in-circuit cali-
bration for high accuracy applications, along with a
temperature sensor chip. Once the RTC circuit is pow-
ered up with battery backup, the frequency drift is
measured. The ATR control is then adjusted to a set-
ting which minimizes drift. Once adjusted at a particu-
lar 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 cir-
cuitry 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 accu-
racy of the RTC. Care needs to be taken in layout of the
RTC circuit to avoid noise pickup. Below in Figure 19 is
a suggested layout for the X1226 or X1227 devices.
Figure 19. 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 capaci-
tor at the Vcc 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.
Assembly
Most electronic circuits do not have to deal with
assembly issues, but with the RTC devices assembly
includes insertion or soldering of a live battery into an
unpowered circuit. If a socket is soldered to the board,
and a battery is inserted in final assembly, then there
are no issues with operation of the RTC. If the battery
is soldered to the board directly, then the RTC device
Vback pin will see some transient upset from either
soldering tools or intermittent battery connections
which can stop the circuit from oscillating. Once the
battery is soldered to the board, the only way to assure
the circuit will start up is to momentarily (very short
period of time!) short the Vback pin to ground and the
circuit will begin to oscillate.
Oscillator Measurements
When a proper crystal is selected and the layout guide-
lines 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 cir-
cuits, 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 oscillat-
ing. 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.
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 Vcc
may disappear intermittently for short periods of time.
Depending on the value of supercapacitor used,
backup time can last from a few days to two weeks
(with >1F). A simple silicon or Schottky barrier diode
can be used in series with Vcc to charge the superca-
pacitor, which is connected to the Vback pin. Do not
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FN8099.2
May 8, 2006