ISL12026 Datasheet, PDF(21/24 Page) Intersil Corporation – Real Time Clock/Calendar with EEPROM

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ISL12026 Datasheet, PDF (21/24 Pages) Intersil Corporation – Real Time Clock/Calendar with EEPROM

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ISL12026

minimizes this by running the IRQ/FOUT output away from

the X1 and X2 pins. Also, reducing the switching current at

this pin by careful selection of the pull-up resistor value will

reduce noise. Intersil suggests a minimum value of 5.1kâ¦ for

32.768kHz, and higher values (up to 20kâ¦) for lower

frequency IRQ/FOUT outputs.

For other RTC products, the same rules stated above should

be observed, but adjusted slightly since the

packages and pinouts are different.

Oscillator Measurements

When a proper crystal is selected and the layout guidelines

above are observed, the oscillator should start up in most

circuits in less than one second. Some circuits may take

slightly longer, but startup should definitely occur in less than

5 seconds. When testing RTC circuits, the most 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 useable

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.

Applying a scope probe can possibly cause a faulty oscillator

to start up, hiding other issues (although in the Intersil RTCs,

the internal circuitry assures startup when using the proper

crystal and layout).

The best way to analyze the RTC circuit is to power it up and

read the real time clock as time advances, or if the chip has

the IRQ/FOUT output, look at the output of that pin on an

oscilloscope (after enabling it with the control register, and

using a pull-up resistor for an open-drain output).

Alternatively, the ISL12026 device has an IRQ/FOUT output

which can be checked by setting an alarm for each minute.

Using the pulse interrupt mode setting, the once-per-minute

interrupt functions as an indication of proper oscillation.

Backup Battery Operation

Many types of batteries can be used with the Intersil RTC

products. 3.0V or 3.6V Lithium batteries are appropriate, and

sizes are available that can power a Intersil RTC device for

up to 10 years. Another option is to use a supercapacitor for

applications where VDD may disappear intermittently for

short periods of time. Depending on the value of

supercapacitor used, backup time can last from a few days

to two weeks (with >1F). A simple silicon or Schottky barrier

diode can be used in series with VDD to charge the

supercapacitor, which is connected to the VBAT pin. Try to

use Schottky diodes with very low leakages, <1ÂµA desirable.

Do not use the diode to charge a battery (especially lithium

batteries!).

There are two possible modes for battery backup operation,

Standard and Legacy mode. In Standard mode, there are no

operational concerns when switching over to battery backup

since all other devices functions are disabled. Battery drain

is minimal in Standard mode, and return to Normal VDD

powered operations is predictable. In Legacy mode the VBAT

pin can power the chip if the voltage is above VDD and less

than VTRIP., In this mode, it is possible to generate alarm and

communicate with the device, unless SBI = 1, but the supply

current drain is much higher than the Standard mode and

backup time is reduced. In this case if alarms are used in

backup mode, the IRQ/FOUT pull up resistor must be

connected to VBAT voltage source. During initial power up

the default mode is the Standard mode.

2.7-5.5V

VCC

Vback

VSS

Supercapacitor

FIGURE 24. SUPERCAPACITOR CHARGING CIRCUIT

Alarm Operation Examples

Below are examples of both Single Event and periodic

Interrupt Mode alarms.

Example 1 â Alarm 0 set with single interrupt (IM = â0â)

A single alarm will occur on January 1 at 11:30am.

A. Set Alarm 0 registers as follows:

BIT

ALARM0

SCA0 0 0 0 0 0 0 0 0 00h Seconds disabled

MNA0 1 0 1 1 0 0 0 0 B0h Minutes set to 30,

enabled

HRA0 1 0 0 1 0 0 0 1 91h Hours set to 11,

enabled

DTA0

1 0 0 0 0 0 0 1 81h Date set to 1,

enabled

MOA0 1 0 0 0 0 0 0 1 81h Month set to 1,

enabled

DWA0 0 0 0 0 0 0 0 0 00h Day of week

disabled

B. Also the AL0E bit must be set as follows:

BIT

CONTROL

INT

0 0 1 0 0 0 0 0 x0h Enable Alarm

After these registers are set, an alarm will be generated when

the RTC advances to exactly 11:30am on January 1 (after

FN8231.5

October 23, 2006

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