X1226 Datasheet, PDF (20/24 Pages) Xicor Inc. – Real Time Clock/Calendar with EEPROM

X1226
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
2.7-5.5V
V_CC
V_back
V_SS
Supercapacitor
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
Table 8. Battery Backup Operation
1. Example Application, Vcc=5V, Vback=3.0V
Condition
Vcc
a. Normal Operation
5.00
b. Vcc on with no battery
5.00
c. Backup Mode
0–1.8
Vback
3.00
0
1.8-3.0
Vtrip
4.38
4.38
4.38
Iback
<<1µA
0
<2µA
Notes
Timekeeping only
2. Example Application, Vcc=3.3V,Vback=3.0V
Condition
Vcc
Vback
a. Normal Operation
3.30
3.00
b. Vcc on with no battery
3.30
0
c. Backup Mode
0–1.8
1.8–3.0*
d. UNWANTED - Vcc ON, Vback
powering
2.65 - 3.30 > Vcc
*since Vback>2.65V is higher than Vtrip, the battery is powering the entire device
Vtrip
2.65
2.65
2.65
2.65
Iback
<<1µA
0
<2µA*
up to 3mA
Timekeeping only
Internal
Vcc=Vback
REV 1.1.24 1/13/03
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Characteristics subject to change without notice. 20 of 24