X1228 datasheet(26/31 Pages) XICOR

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X1228

REV 1.3 3/24/04

Characteristics subject to change without notice.

26 of 31

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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

output). Alternatively, the X1226/1286/1288 devices

have 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 indication 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. A second consideration

is the trip point setting for the system RESET- function,

known as Vtrip. Vtrip is set at the factory at levels for

systems with either Vcc = 5V or 3.3V operation, with

the following standard options:

VTRIP = 4.63V ± 3%

VTRIP = 4.38V ± 3%

VTRIP = 2.85V ± 3%

VTRIP = 2.65V ± 3%

The summary of conditions for backup battery opera-

tion is given in Table 8:

2.7-5.5V

Supercapacitor

VSS

VCC

Vback

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

Reset

Notes

a. Normal Operation

5.00

3.00

4.38

<<1µA

b. Vcc on with no battery

5.00

4.38

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

Reset

a. Normal Operation

3.30

3.00

2.65

<<1µA

b. Vcc on with no battery

3.30

2.65

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

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