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Application Note 1549
Addressing Power Issues in Real Time Clock
Applications
Introduction
Intersil Real Time Clock (RTC) products now include
many products with a variety of functions. Common
functions include a low power 32,768kHz oscillator and
also the ability to keep time in SRAM registers after initial
time and date is set.
The power for RTC devices includes a V
DD
source, and
can also include a backup source of either a battery or
large storage capacitor (super capacitor). In many
applications the RTC device is the only device in a system
that has an alternative power source such as the backup
battery, so there can be unique demands placed on the
device compared to other semiconductor products.
In addition, the RTC device has a communications bus,
usually I
2
C, that has pull-up resistors which can go to a
different power source than the RTC. The interrupt or
frequency output pull-up can be connected to yet
another voltage.
Some RTC devices contain non-volatile EEPROM storage,
both for device control registers and for general purpose
storage. The EEPROM is a robust storage method that
can survive temperature extremes, millions of write
cycles and up to 10 years of endurance. Despite the
robust design, there are power and ground voltage
transients that, although they exceed data sheet limits,
are present in systems and may upset EEPROM memory
cells. In addition, some applications may have I
2
C
communication during power-down, another possible
source of upset.
RTC Device Power Connections
The typical applications circuit for the RTC is shown in
Figure 1. This seemingly simple circuit has no less than 4
possible power connections:
•V
DD
power (V
POWER
)
•I
2
C bus open drain pull-up power (V
BUS
)
• Battery backup (V
BAT
)
• Frequency output or interrupt open drain pull-up
(V
FOUT
)
Having four different power sources raises the issue of
power sequencing. Ideally, the RTC circuit will have 3 of
the four power sources in common. The V
CC
, V
BUS
and
V
FOUT
should all connect to the main V
POWER
source, and
when that is the case there are virtually no concerns with
power sequencing.
Many applications prefer to have these connections
powered separately. Examples include:
• RTC device is powered down while the I
2
C bus and
microcontroller are active (not recommended since it
will violate RTC data sheet, but RTC devices are OK
in this state).
• RTC device is powered down while the F
OUT
or IRQ is
pulled up to V
BAT
to allow a wake-up interrupt or
clock while other circuits are asleep (this is OK, but
the RTC data sheet absolute maximum ratings need
to be followed).
• RTC device is powered from a higher or lower voltage
than the I
2
C pull-ups (this may or may not be
allowed, possible unreliable operation).
• RTC device operates with one supply voltage and the
V
BAT
pin is grounded (this is OK, and is covered later
in this document).
Table 1 provides some guidance when designing an RTC
applications circuit.
V
BAT
V
DD
SCL
SDA
X1
X2
GND
F
OUT
10k
V
BUS
C
1
I
2
C
Bus
V
POWER
2.2k
2.2k
V
FOUT
C
2
BT
1
Y
1
FIGURE 1. TYPICAL RTC APPLICATION CIRCUIT
March 1, 2010
AN1549.0
