ISL1208IU8 View Datasheet(PDF) - Intersil

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ISL1208IU8

I2C® Real Time Clock/Calendar

Intersil 

ISL1208

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.

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

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 6ppm to 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 -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 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/fOUT 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

(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

ISL1208 circuit, then both the DTR and ATR registers will

need to be utilized (total correction range = -94ppm to

+140ppm).

18

FN8085.8

September 12, 2008

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