FN6659 datasheet(28/32 Pages) RENESAS

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ISL12022

FN6659 Rev.3.01

Page 28 of 32

Oct 24, 2019

Applications Information

Crystal Oscillator Frequency Compensation

CRYSTAL CHARACTERISTICS

The ISL12022 device contains a complete system for adjusting

the frequency of the crystal oscillator to compensate for

temperature variation. A typical 32.768kHz crystal used with RTC

devices has a temperature versus frequency curve, as shown in

Figure 22.

The curve in Figure 22 follows Equation 7:

Where

 is the temperature constant, with a typical value of

0.034 ppm/°C.

T0 is the turnover temperature of the crystal, which is the apex of

the parabolic curve. If the two factors

 and T0 are known, it is

possible to correct for crystal temperature error to very high

accuracy.

The crystal will have an initial accuracy error at room temperature,

typically specified at ±20°C. The other important characteristic is

the capacitances associated with the crystal. The load capacitance

is normally specified at 12.5pF, although it can be lower in some

cases. There is also a motional capacitance which affects the

ability of the load capacitance to pull the oscillation frequency, and

it is usually in the range of 2.2fF to 4.0fF.

RTC CLOCK CONTROL

The ISL12022 uses two mechanisms to adjust the RTC clock and

correct for the temperature error of the external crystal.

The Analog Trimming (AT) adjusts the load capacitance seen by

the crystal. Analog switches connect the appropriate capacitance

to change the frequency in increments of 1ppm. The adjustment

range for the ISL12022 is +32/-31ppm.

The AT can be further refined using the BETA register. the BETA

capacitance) which will affect the incremental frequency change

of the AT adjustment. A simple test procedure uses the BETA

Normally, the crystal frequency is adjusted at room temperature

to zero out the frequency error using the IATRxx register bits

(initial Analog Trimming). In addition, the IATRxx setting is varied

up and down to record the variation in oscillator frequency

compared to the step change in IATRxx. Once that value is known

then the BETA register is used to adjust the step size to be as

close to 1ppm per IATRxx step as possible. After that adjustment

is made, then any ISL12022 temperature compensation

adjustments will use a 1ppm change for each bit change in the

internal AT adjustment.

The Digital Trimming (DT) uses clock pulse add/subtract logic to

change the RTC timing during temperature compensation. The

DT steps are much coarser than the AT steps and are therefore

used for large adjustments. The DT steps are 30.5ppm, and the

range is from -305ppm to +305ppm. The Frequency Output

function will show the clock variation with DT settings, except for

the 32,768Hz setting which only shows the AT control.

ACTIVE TEMPERATURE COMPENSATION

The ISL12022 contains an intelligent logic circuit which takes the

temperature sensor digital value as the only input variable. It

then uses the register values for the crystal variables

 and T0,

and combines those with calibration from the BETA and ITR0

registers to produce “Final” values for the AT and DT, known as

FATR (Final AT Register) and FDTR (Final DT Register). Those AT

and DT values combine to directly compensate for the

temperature error shown in Figure 22.

The temperature sensor produces a new value every 60s (or up to

10 minutes in battery mode), which triggers the logic to calculate

a new AT/DT value set. For every temperature calculation result,

there can only be one corresponding AT/DT correction value.

Measuring Oscillator Accuracy

The best way to analyze the ISL12022 frequency accuracy is to

set the IRQ/FOUT pin for a specific frequency, and look at the

output of that pin on a high accuracy frequency counter (at least

7 digits accuracy). Note that the IRQ/FOUT is a drain output and

will require a pull-up resistor.

Using the 1.0Hz output frequency is the most convenient as the

ppm error is as expressed in Equation 8:

Other frequencies may be used for measurement but the error

calculation becomes more complex.

When the proper layout guidelines are observed, the oscillator

should start up in most circuits in less than 1s. When testing RTC

circuits, a 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 usable

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 oscillating. 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. Use the FOUT output and a frequency counter for the

most accurate results.

TEMPERATURE (°C)

-160

-140

-120

-100

-80

-60

-40

-20

-40 -30 -20 -10 0 10 203040506070 80

FIGURE 22. RTC CRYSTAL TEMPERATURE DRIFT

f





T0

–



(EQ. 7)

ppm error



OUT

 1e6



–

(EQ. 8)

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