17
FN8085.3
July 29, 2005
Application Section
Oscillator Crystal Requirements
The ISL1208 uses a standard 32.768kHz crystal. Either
through hole or surface mount crystals can be used. Table 6
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 ISL1208 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 less than 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.
Crystal Oscillator Frequency Adjustment
The ISL1208 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 six 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 6-10 ppm/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 -34 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 ISL1208. There are 3 bits known as the
Digital Trimming Register (DTR). The range provided is
±60ppm in increments of 20ppm. 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. 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
(T
0
) 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.
If full industrial temperature compensation is desired in an
ISL1208 circuit, then both the DTR and ATR registers will
need to be utilized (total correction range = -94 to +140ppm).
TABLE 6. 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
TEMPERATURE (°C)
-160.0
-140.0
-120.0
-100.0
-80.0
-60.0
-40.0
-20.0
0.0
-40 -30 -20 -10 0
10 20 30 40 50 60 70 80
PP
M
FIGURE 17. RTC CRYSTAL TEMPERATURE DRIFT
ISL1208
