X1286 Intersil Corporation, X1286 Datasheet - Page 22

• Manufacturer Part Number: X1286
• Description: Intersil Real Time Clock/Calendar/CPU Supervisor with EEPROM X1286
• Manufacturer: Intersil Corporation

Table 9. Battery Backup Operation
*since Vback>2.65V is higher than Vtrip, the battery is powering the entire device
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 Intersil
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). Alternatively, the X1226/X1286/1288 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 Intersil
RTC products. 3.0V or 3.6V Lithium batteries are
appropriate, and sizes are available that can power a
Intersil 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,
REV 1.1 7/8/04
1. Example Application, Vcc=5V, Vback=3.0V
a. Normal Operation
b. Vcc on with no battery
c. Backup Mode
2. Example Application, Vcc=3.3V,Vback=3.0V
a. Normal Operation
b. Vcc on with no battery
c. Backup Mode
d. UNWANTED - Vcc ON, Vback
powering
Condition
Condition
2.65 - 3.30
0–1.8
0–1.8
5.00
5.00
3.30
3.30
Vcc
Vcc
www.intersil.com
1.8–3.0*
Vback
1.8-3.0
Vback
> Vcc
3.00
3.00
0
0
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 9:
2.7-5.5V
Vtrip
Vtrip
4.38
4.38
4.38
2.65
2.65
2.65
2.65
V
CC
V
up to 3mA
SS
<<1µA
<<1µA
<2µA*
Iback
Iback
<2µA
V
back
0
0
Internal Vcc=Vback
Timekeeping only
Timekeeping only
Supercapacitor
Notes
X1286
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