X1226

REV 1.1.24 1/13/03

Characteristics subject to change without notice.

20 of 24

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scope probe, there will be no useful information in that

waveform other than the fact that the circuit is oscillat-

ing. 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 Xicor

RTC’s, 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 PHZ output, look at the output of that

pin on an oscilloscope (after enabling it with the control

register, and using a pullup resistor for an open-drain

output). Alternaltively, the X1226 device has an IRQ-

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 indica-

tion of proper oscillation.

Backup Battery Operation

Many types of batteries can be used with the Xicor

RTC products. 3.0V or 3.6V Lithium batteries are

appropriate, and sizes are available that can power a

Xicor RTC device for up to 10 years. Another option is

to use a supercapacitor for applications where Vcc 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 Vcc to charge the superca-

pacitor, which is connected to the Vback pin. Do not

use the diode to charge a battery (especially lithium

batteries!).

Figure 16. Supercapactor charging circuit

Since the battery switchover occurs at Vcc=Vback-

0.1V (see Figure 16), the battery voltage must always

be lower than the Vcc voltage during normal operation

or the battery will be drained.

The summary of conditions for backup battery opera-

tion is given in Table 8:

Referring to Figure 16, Vtrip applies to the “Internal

Vcc” node which powers the entire device. This means

that if Vcc is powered down and the battery voltage at

Vback is higher than the Vtrip voltage, then the entire

2.7-5.5V

Supercapacitor

Table 8. Battery Backup Operation

*since Vback>2.65V is higher than Vtrip, the battery is powering the entire device

1. Example Application, Vcc=5V, Vback=3.0V

Condition

Vcc

Vback

Vtrip

Iback

Notes

a. Normal Operation

5.00

3.00

4.38

<<1µA

b. Vcc on with no battery

5.00

0

4.38

0

c. Backup Mode

0–1.8

1.8-3.0

4.38

<2µA

Timekeeping only

2. Example Application, Vcc=3.3V,Vback=3.0V

Condition

Vcc

Vback

Vtrip

Iback

a. Normal Operation

3.30

3.00

2.65

<<1µA

b. Vcc on with no battery

3.30

0

2.65

0

c. Backup Mode

0–1.8

1.8–3.0*

2.65

<2µA*

Timekeeping only

d. UNWANTED - Vcc ON, Vback

powering

2.65 - 3.30

> Vcc

2.65

up to 3mA

Internal

Vcc=Vback