7
FN8085.3
July 29, 2005
General Description
The ISL1208 device is a low power real time clock with
timing and crystal compensation, clock/calendar, power fail
indicator, periodic or polled alarm, intelligent battery backup
switching, and battery-backed user SRAM.
The oscillator uses an external, low-cost 32.768kHz crystal.
The real time clock tracks time with separate registers for
hours, minutes, and seconds. The device has calendar
registers for date, month, year and day of the week. The
calendar is accurate through 2099, with automatic leap year
correction.
The ISL1208's powerful alarm can be set to any
clock/calendar value for a match. For example, every
minute, every Tuesday or at 5:23 AM on March 21. The
alarm status is available by checking the Status Register, or
the device can be configured to provide a hardware interrupt
via the IRQ pin. There is a repeat mode for the alarm
allowing a periodic interrupt every minute, every hour, every
day, etc.
The device also offers a backup power input pin. This V
BAT
pin allows the device to be backed up by battery or
SuperCap with automatic switchover from V
DD
to V
BAT
. The
entire ISL1208 device is fully operational from 2.0V to 5.5V
and the clock/calendar portion of the device remains fully
operational down to 1.8V (Standby Mode).
Pin Description
X1, X2
The X1 and X2 pins are the input and output, respectively, of
an inverting amplifier. An external 32.768kHz quartz crystal
is used with the ISL1208 to supply a timebase for the real
time clock. Internal compensation circuitry provides high
accuracy over the operating temperature range from
-40°C to +85°C. This oscillator compensation network can
be used to calibrate the crystal timing accuracy over
temperature either during manufacturing or with an external
temperature sensor and microcontroller for active
compensation. The device can also be driven directly from a
32.768kHz source at pin X1.
V
BAT
This input provides a backup supply voltage to the device.
V
BAT
supplies power to the device in the event that the V
DD
supply fails. This pin can be connected to a battery, a Super
Cap or tied to ground if not used.
IRQ/F
OUT
(Interrupt Output/Frequency Output)
This dual function pin can be used as an interrupt or
frequency output pin. The IRQ/F
OUT
mode is selected via
the frequency out control bits of the control/status register.
•
Interrupt Mode.
The pin provides an interrupt signal
output. This signal notifies a host processor that an alarm
has occurred and requests action. It is an open drain
active low output.
•
Frequency Output Mode.
The pin outputs a clock signal
which is related to the crystal frequency. The frequency
output is user selectable and enabled via the I
2
C bus. It is
an open drain active low output.
Serial Clock (SCL)
The SCL input is used to clock all serial data into and out of
the device. The input buffer on this pin is always active (not
gated). It is disabled when the backup power supply on the
V
BAT
pin is activated to minimize power consumption.
Serial Data (SDA)
SDA is a bidirectional pin used to transfer data into and out
of the device. It has an open drain output and may be ORed
with other open drain or open collector outputs. The input
buffer is always active (not gated) in normal mode.
An open drain output requires the use of a pull-up resistor.
The output circuitry controls the fall time of the output signal
with the use of a slope controlled pull-down. The circuit is
designed for 400kHz I
2
C interface speeds. It is disabled
when the backup power supply on the V
BAT
pin is activated.
V
DD
, GND
Chip power supply and ground pins. The device will operate
with a power supply from 2.0V to 5.5VDC. A 0.1µF capacitor
is recommended on the V
DD
pin to ground.
Functional Description
Power Control Operation
The power control circuit accepts a V
DD
and a V
BAT
input.
Many types of batteries can be used with Intersil RTC
products. For example, 3.0V or 3.6V Lithium batteries are
appropriate, and battery sizes are available that can power
FIGURE 7. STANDARD OUTPUT LOAD FOR TESTING THE
DEVICE WITH V
DD
= 5.0V
SDA
AND
IRQ/FOUT
1533
Ω
100pF
5.0V
FOR V
OL
= 0.4V
AND I
OL
= 3mA
EQUIVALENT AC OUTPUT LOAD CIRCUIT FOR V
DD
= 5V
FIGURE 8. RECOMMENDED CRYSTAL CONNECTION
X1
X2
ISL1208
