Preliminary Information
X1288
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).
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 23. Supercapactor charging circuit
2.7-5.5V
VCC
Vback
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). 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.
Supercapacitor
VSS
Since the battery switchover occurs at Vcc=Vback-
0.1V (see Figure 20), 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:
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
V
V
V
V
= 4.63V 3%
= 4.38V 3%
= 2.85V 3%
= 2.65V 3%
TRIP
TRIP
TRIP
TRIP
The summary of conditions for backup battery opera-
tion is given in Table 9:
Table 9. Battery Backup Operation
1. Example Application, Vcc=5V, Vback=±.3V
Condition
a. Normal Operation
Vcc
5.00
Vback
3.00
Vtrip
4.38
4.38
4.38
Iback
<<1µA
0
Reset
Notes
H
H
L
b. Vcc on with no battery
c. Backup Mode
5.00
0
0–1.8
1.8-3.0
<2µA
Timekeeping
only
2. Example Application, Vcc=±.±V,Vback=±.3V
Condition
a. Normal Operation
Vcc
3.30
Vback
3.00
Vtrip
2.65
2.65
2.65
Iback
<<1µA
0
Reset
H
H
L
b. Vcc on with no battery
c. Backup Mode
3.30
0
0–1.8
1.8–3.0*
<2µA*
Timekeeping
only
d. UNWANTED - Vcc ON, Vback
powering
2.65 - 3.30
> Vcc
2.65
up to 3mA
H
Internal
Vcc=Vback
*since Vback>2.65V is higher than Vtrip, the battery is powering the entire device
27 of 31
REV 1.1.30 3/24/04
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