FN6690 datasheet(17/19 Pages) RENESAS

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ISL12008 I2C Real Time Clock with Battery Backup

FN6690 Rev 1.00

Page 17 of 19

Sep 26, 2008

need to be utilized (total correction range = -97.0695ppm to

+206.139ppm).

A system to implement temperature compensation would

consist of the ISL12008, a temperature sensor, and a

microcontroller. These devices may already be in the system

so the function will just be a matter of implementing software

and performing some calculations. Fairly accurate temperature

compensation can be implemented just by using the crystal

manufacturer’s specifications for the turnover temperature T0

and the drift coefficient (

). The formula for calculating the

oscillator adjustment necessary is Equation 4:

Once the temperature curve for a crystal is established, then

the designer should decide at what discrete temperatures the

compensation will change. Since drift is higher at extreme

temperatures, the compensation may not be needed until the

temperature is greater than +20°C from T0.

A sample curve of the ATR setting vs Frequency Adjustment

for the ISL12008 and a typical RTC crystal is given in

Figure 16. This curve may vary with different crystals, so it is

good practice to evaluate a given crystal in an ISL12008 circuit

before establishing the adjustment values.

This curve is then used to figure what ATR and DTR settings

are used for compensation. The results would be placed in a

lookup table for the microcontroller to access.

Layout Considerations

The crystal input at X1 has a very high impedance, and

oscillator circuits operating at low frequencies (such as

32.768kHz) are known to pick up noise very easily if layout

precautions are not followed. Most instances of erratic clocking

or large accuracy errors can be traced to the susceptibility of

the oscillator circuit to interference from adjacent high speed

clock or data lines. Careful layout of the RTC circuit will avoid

noise pickup and insure accurate clocking.

Figure 17 shows a suggested layout for the ISL12008 device

using a surface mount crystal. Two main precautions should be

followed:

1. Do not run the serial bus lines or any high speed logic lines

in the vicinity of the crystal. These logic level lines can

induce noise in the oscillator circuit to cause misclocking.

2. Add a ground trace around the crystal with one end

terminated at the chip ground. This will provide termination

for emitted noise in the vicinity of the RTC device.

In addition, it is a good idea to avoid a ground plane under the

X1 and X2 pins and the crystal, as this will affect the load

capacitance and therefore the oscillator accuracy of the circuit.

If the FT/OUT pin is used as a clock, it should be routed away

from the RTC device as well. The traces for the VBAT and VCC

pins can be treated as a ground, and should be routed around

the crystal.

Super Capacitor Backup

The ISL12008 device provides a VBAT pin which is used for a

battery backup input. A super capacitor can be used as an

alternative to a battery in cases where shorter backup times

are required. Since the battery backup supply current required

by the ISL12008 is extremely low, it is possible to get months

of backup operation using a super capacitor. Typical capacitor

values are a few µF to 1F or more, depending on the

application.

If backup is only needed for a few minutes, then a small

inexpensive electrolytic capacitor can be used. For extended

periods, a low leakage, high capacity super capacitor is the

best choice. These devices are available from such vendors as

Panasonic and Murata. The main specifications include

working voltage and leakage current. If the application is for

charging the capacitor from a +5V ±5% supply with a signal

diode, then the voltage on the capacitor can vary from ~4.5V to

slightly over 5.0V. A capacitor with a rated WV of 5.0V may

have a reduced lifetime if the supply voltage is slightly high.

The leakage current should be as small as possible. For

example, a super capacitor should be specified with leakage of

well below 1µA. A standard electrolytic capacitor with DC

leakage current in the microamps will have a severely

shortened backup time.

Following are some examples with equations to assist with

calculating backup times and required capacitance for the

ISL12008 device. The backup supply current plays a major

part in these equations, and a typical value was chosen for

example purposes. For a robust design, a margin of 30%

should be included to cover supply current and capacitance

tolerances over the results of the calculations. Even more

Adjustment(ppm)

T T0

–

2



(EQ. 4)

-40

-30

-20

-10

5 10 15 20 25 30 35 40 45 50 55 60

ATR SETTING

FIGURE 16. ATR SETTING vs OSCILLATOR FREQUENCY

ADJUSTMENT

FIGURE 17. SUGGESTED LAYOUT FOR ISL12008 AND

CRYSTAL

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