2
I C RTC with Trickle Charger
Oscillator Circuit
Operation
The DS1340 uses an external 32.768kHz crystal. The
oscillator circuit does not require any external resistors
or capacitors to operate. Table 2 specifies several crys-
tal parameters for the external crystal. Figure 3 shows a
functional schematic of the oscillator circuit. If using a
crystal with the specified characteristics, the startup
time is usually less than one second.
The DS1340 operates as a slave device on the serial
bus. Access is obtained by implementing a START
condition and providing a device identification code fol-
lowed by data. Subsequent registers can be accessed
sequentially until a STOP condition is executed. The
device is fully accessible and data can be written and
read when V
CC
is greater than V . However, when
PF
falls below V , the internal clock registers are
CC
V
PF
Clock Accuracy
blocked from any access. If V is less than V
,
BACKUP
PF
The initial clock accuracy depends on the accuracy of
the crystal and the accuracy of the match between the
capacitive load of the oscillator circuit and the capaci-
tive 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 4 shows a typical PC board layout for isolating
the crystal and oscillator from noise. Refer to
Application Note 58: Crystal Considerations with Dallas
Real-Time Clocks (www.maxim-ic.com/RTCapps) for
detailed information.
the device power is switched from V
to V
BACKUP
CC
when V
drops below V . If V
is greater than
CC
PF
PF
V
V
, the device power is switched from V
to
CC
BACKUP
BACKUP
ters are maintained from the V
when V
drops below V
. The regis-
CC
BACKUP
source until V
CC
BACKUP
is returned to nominal levels. The functional diagram
(Figure 5) shows the main elements of the serial RTC.
LOCAL GROUND PLANE (LAYER 2)
X1
DS1340C Only
The DS1340C integrates a standard 32,768Hz crystal
CRYSTAL
into the package. Typical accuracy with nominal V
CC
X2
and +25°C is approximately +15ppm. Refer to
Application Note 58 for information about crystal accu-
racy vs. temperature.
Table 2. Crystal Specifications*
PARAMETER
Nominal
Frequency
SYMBOL MIN
TYP
MAX UNITS
GND
f
32.768
kHz
O
Series Resistance
Load Capacitance
ESR
45,60**
kΩ
pF
Figure 4. Layout Example
C
12.5
L
*The crystal, traces, and crystal input pins should be isolated
from RF generating signals. Refer to Application Note 58:
Crystal Considerations for Dallas Real-Time Clocks for addi-
tional specifications.
X1
X2
FT/OUT
32,768Hz
512Hz
MUX/BUFFER
**A crystal with up to 60kΩ ESR can be used if the minimum
OSCILLATOR
operating voltages on both V
and V
are at least 2.0V.
CC
BACKUP
DIVIDER AND
CALIBRATION
CIRCUIT
"C" VERSION ONLY
RTC
V
CC
POWER
CONTROL
CLOCK AND
CALENDAR
REGISTERS
1Hz
V
BACKUP
COUNTDOWN
CHAIN
CONTROL
LOGIC
SCL
SDA
SERIAL BUS
INTERFACE
AND ADDRESS
REGISTER
USER BUFFER
(7 BYTES)
RTC
REGISTERS
C 1
L
C 2
L
DS1340
X2
X1
CRYSTAL
Figure 5. Functional Diagram
_____________________________________________________________________
Figure 3. Oscillator Circuit Showing Internal Bias Network
7
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