ISL12082 Datasheet, PDF (22/26 Pages) Intersil Corporation – I2C-Bus™ Real Time Clock with Two Interrupts, Alarm, and Timer

ISL12082
Application Section
Oscillator Crystal Requirements
The ISL12082 uses a standard 32.768kHz crystal. Either
through hole or surface mount crystals can be used. Table
15 lists some recommended surface mount crystals and the
parameters of each. This list is not exhaustive and other
surface mount devices can be used with the ISL12082 if
their specifications are very similar to the devices listed.
The crystal should have a required parallel load capacitance
of 12.5pF and an equivalent series resistance of <50k. The
crystal’s temperature range specification should match the
application. Many crystals are rated for -10°C to +60°C
(especially through-hole and tuning fork types), so an
appropriate crystal should be selected if extended
temperature range is required.
TABLE 15. SUGGESTED SURFACE MOUNT CRYSTALS
MANUFACTURER
PART NUMBER
Citizen
CM200S
Epson
MC-405, MC-406
Raltron
RSM-200S
SaRonix
32S12
Ecliptek
ECPSM29T-32.768K
ECS
ECX-306
Fox
FSM-327
Crystal Oscillator Frequency Adjustment
The ISL12082 device contains circuitry for adjusting the
frequency of the crystal oscillator. This circuitry can be used
to trim oscillator initial accuracy as well as adjust the
frequency to compensate for temperature changes.
The Analog Trimming Register (ATR) is used to adjust the
load capacitance seen by the crystal. There are 6 bits of ATR
control, with linear capacitance increments available for
adjustment. Since the ATR adjustment is essentially “pulling”
the frequency of the oscillator, the resulting frequency
changes will not be linear with incremental capacitance
changes. The equations which govern pulling show that
lower capacitor values of ATR adjustment will provide larger
increments. Also, the higher values of ATR adjustment will
produce smaller incremental frequency changes. These
values typically vary from 6ppm to 10ppm/bit at the low end
to <1ppm/bit at the highest capacitance settings. The range
afforded by the ATR adjustment with a typical surface mount
crystal is typically -34ppm to +80ppm around the ATR = 0
default setting because of this property. The user should
note this when using the ATR for calibration. The
temperature drift of the capacitance used in the ATR control
is extremely low, so this feature can be used for temperature
compensation with good accuracy.
In addition to the analog compensation afforded by the
adjustable load capacitance, a digital compensation feature is
available for the ISL12082. There are 6 bits known as the
Digital Trimming Register (DTR). The range provided is
63.0696ppm to +126.139ppm. DTR operates by adding or
skipping pulses in the clock counter. It is very useful for coarse
adjustments of frequency drift over-temperature or extending
the adjustment range available with the ATR register.
Initial accuracy is best adjusted by enabling the frequency
output (using the INT register, address 08h), and monitoring
the ~IRQ/f_OUT pin with a calibrated frequency counter. The
frequency used is unimportant, although 1Hz is the easiest
to monitor. The gating time should be set long enough to
ensure accuracy to at least 1ppm. The ATR should be set to
the center position, or 100000Bh, to begin with. Once the
initial measurement is made, then the ATR register can be
changed to adjust the frequency. Note that increasing the
ATR register for increased capacitance will lower the
frequency, and vice-versa. If the initial measurement shows
the frequency is far off, it will be necessary to use the DTR
register to do a coarse adjustment. Also, note that most all
crystals will have tight enough initial accuracy at room
temperature so that a small ATR register adjustment should
be all that is needed.
Temperature Compensation
The ATR and DTR controls can be combined to provide
crystal drift temperature compensation. The typical
32.768kHz crystal has a drift characteristic that is similar to
that shown in Figure 17. There is a turnover-temperature
(T₀) where the drift is very near zero. The shape is parabolic
as it varies with the square of the difference between the
actual temperature and the turnover-temperature.
0
-20
-40
-60
-80
-100
-120
-140
-160
-40 -30 -20 -10 0 10 20 30 40 50 60 70 80
TEMPERATURE (°C)
FIGURE 17. RTC CRYSTAL TEMPERATURE DRIFT
If full industrial temperature compensation is desired in an
ISL12082 circuit, then both the DTR and ATR registers will
need to be utilized (total correction range = -97ppm to
+206ppm).
A system to implement temperature compensation would
consist of the ISL12082, a temperature sensor, and a micro
controller. These devices may already be in the system so
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FN6731.3
November 24, 2008