16
FN6690.1
September 26, 2008
Application Section
Oscillator Crystal Requirements
The ISL12008 uses a standard 32.768kHz crystal. Either
through hole or surface mount crystals can be used. Table 7
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 ISL12008 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 ISL12008 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. 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 ISL12008. There are 6 bits known as the
Digital Trimming Register (DTR). The range provided is
-63.0695ppm 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 512Hz
frequency output (using the FT bit, address 08h bit 6), and
monitoring the FT/OUT pin with a calibrated frequency
counter. The gating time should be set long enough to
ensure accuracy to at least 1ppm. To calculate the ppm on
the measured 512Hz, simply divide the measured 512Hz by
512, then subtract 1 from the result and mulitple by
1,000,000. Please see Equation 3 for the formula:
The ATR should be set to the center position, or 00000b, to
begin with. Once the initial measurement is made, then the
ATR register can be changed to adjust the frequency. Note
for a range of 0 to 31 for the ATR register will increased
capacitance and lower the frequency with 31 for the
maximum negative correction, and for a range of 32 to 63 for
the ATR register will decreased capacitance and increase
the frequency with 32 for the maximum positive correction. 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 15. 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.
TABLE 7. 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
ppm = (FT/512 - 1)*1E6
(EQ. 3)
TEMPERATURE (°C)
-160
-140
-120
-100
-80
-60
-40
-20
0
-40-30-20-100 1020304050607080
PPM
FIGURE 15. RTC CRYSTAL TEMPERATURE DRIFT
ISL12008
