DS1341_12 Datasheet, PDF (7/16 Pages) Maxim Integrated Products – Low-Current I2C RTCs for High-ESR Crystals

Low-Current I²C RTCs for High-ESR Crystals
Detailed Description
The DS1341/DS1342 low-current RTCs are timekeeping
devices that consume an extremely low timekeeping cur-
rent, which permits longer life from a power supply. The
clock/calendar provides seconds, minutes, hours, day,
date, month, and year information. The date at the end of
the month is automatically adjusted for months with fewer
than 31 days, including corrections for leap year through
2099. The clock operates in either a 24hr or 12hr format
with an AM/PM indicator.
The DS1341/DS1342 use an external 32.768kHz crys-
tal. The oscillator circuit does not require any external
resistors or capacitors to operate. The devices support
a high-ESR crystal, which broadens the pool of usable
crystals for the device. The DS1342 uses a 12.5pF crys-
tal. The DS1341 uses a 6pF crystal, which decreases
oscillator current draw, but is less commonly available
than the 12.5pF crystals.
The DS1341/DS1342 also accept an external clock
reference for synchronization. The external clock can
be a 32.768kHz, 50Hz, 60Hz, or 1Hz source. When the
enable oscillator bit (EOSC) is a 0, the DS1341/DS1342
use the oscillator for timekeeping. If the enable external
clock input bit (ECLK) is set to 1, the time base derived
from the oscillator is compared to the 1Hz signal that is
derived from the CLKIN signal. The conditioned signal
drives the RTC time and date counters. If the oscillator
is disabled and the CLKIN signal is absent, the time and
date values remain static, provided that VCC remains at
a valid level.
When the external clock is lost or when the frequency
differs more than Q0.8% from the crystal frequency, the
signal derived from the crystal oscillator drives the RTC
counter.
When ECLK is set to 0, the RTC counter is always driven
with the signal derived from the crystal oscillator. When
the EOSC bit is a 1 and the external clock source is
selected, the RTC counter is always clocked by the sig-
nal from the CLKIN pin.
Address and data are transferred serially through an I²C
serial interface. Other features include two time-of-day
alarms, two interrupts, a programmable square-wave
output, and a bus timeout mechanism that resets the I²C
bus if it remains inactive for a minimum of tTIMEOUT.
Both devices are available in lead(Pb)-free/RoHS-
compliant, 8-pin FSOP or TDFN packages, and support
the -40NC to +85NC extended temperature range.
Oscillator Circuit
The DS1341/DS1342 use an external 32.768kHz crys-
tal. The oscillator circuit does not require any external
resistors or capacitors to operate. The DS1341 includes
integrated capacitive loading for a 6pF CL crystal, and
the DS1342 includes integrated capacitive loading for a
12.5pF CL crystal. See the Crystal Parameters table for
the external crystal parameters. The Functional Diagram
shows a simplified schematic of the oscillator circuit. The
startup time is usually less than 1 second when using a
crystal with the specified characteristics.
Clock Accuracy
When running from the internal oscillator, the accuracy of
the clock is dependent upon the accuracy of the crystal
and the accuracy of the match between the capacitive
load of the oscillator circuit and the capacitive load for
which the crystal was trimmed. Additional error is added
by crystal frequency drift caused by temperature shifts.
External circuit noise coupled into the oscillator circuit
can result in the clock running fast. Figure 2 shows a
typical PCB layout for isolation of the crystal and oscil-
lator from noise. Refer to Application Note 58: Crystal
Considerations with Dallas Real-Time Clocks for detailed
information.
LOCAL GROUND PLANE (LAYER 2)
X1
CRYSTAL
X2
GND
NOTE: AVOID ROUTING SIGNALS IN THE CROSSHATCHED AREA (UPPER LEFT-HAND
QUADRANT) OF THE PACKAGE UNLESS THERE IS A GROUND PLANE BETWEEN THE
SIGNAL LINE AND THE PACKAGE.
Figure 2. Layout Example
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