ISL12026_08 Datasheet, PDF(21/24 Page) Intersil Corporation – Real Time Clock/Calendar with I2C Bus™ and EEPROM

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ISL12026_08 Datasheet, PDF (21/24 Pages) Intersil Corporation – Real Time Clock/Calendar with I2C Bus™ and EEPROM

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ISL12026, ISL12026A

circuits in less than one second. Some circuits may take

slightly longer, but start-up 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

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.

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 start-up 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 SuperCap for

applications where VDD may disappear intermittently for

short periods of time. Depending on the value of SuperCap

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 SuperCap, 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 the

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.

2.7V TO 5.5V

VDD

VBAT

VSS

SUPERCAP

FIGURE 24. SUPERCAPACITOR CHARGING CIRCUIT

Alarm Operation Examples

Following are examples of both Single Event and periodic

Interrupt Mode alarms.

EXAMPLE 1

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

seconds changes from 59 to 00) by setting the AL0 bit in the

status register to â1â and also bringing the IRQ/FOUT output low.

EXAMPLE 2

Pulsed interrupt once per minute (IM = â1â)

Interrupts at one minute intervals when the seconds register

is at 30 seconds.

A. Set Alarm 0 registers as follows:

B. Set the Interrupt register as follows:

FN8231.8

October 28, 2008

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