X1227 Datasheet, PDF (23/28 Pages) Xicor Inc. – Real Time Clock/Calendar/CPU Supervisor with EEPROM

X1227
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.
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. Supercapacitor 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. 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:
V_TRIP = 4.63V ± 3%
V_TRIP = 4.38V ± 3%
V_TRIP = 2.85V ± 3%
V_TRIP = 2.65V ± 3%
The summary of conditions for backup battery opera-
tion is given in Table 8:
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
Reset
H
H
L
Notes
Timekeeping
only
2. Example Application, Vcc=3.3V,Vback=3.0V
Condition
Vcc
a. Normal Operation
3.30
b. Vcc on with no battery
3.30
Vback
3.00
0
Vtrip
2.65
2.65
c. Backup Mode
0–1.8 1.8–3.0*
2.65
d. UNWANTED - Vcc ON, Vback
powering
2.65 - 3.30 > Vcc
2.65
*since Vback>2.65V is higher than Vtrip, the battery is powering the entire device
Iback
<<1µA
0
<2µA*
Reset
H
H
L
up to 3mA
H
Timekeeping
only
Internal
Vcc=Vback
REV 1.1.20 1/13/03
www.xicor.com
Characteristics subject to change without notice. 23 of 28