Datasheet
DS1672
8 of 15
Detailed Description
The DS1672 provides a 32-bit counter that increments once-per-second. The counter data is accessible
via an I
2
C serial interface. A precision, temperature-compensated, voltage reference and comparator
circuit monitors V
CC
. When V
CC
drops below V
PF
, RST becomes active and the interface is disabled to
prevent data corruption. The device switches to the backup supply input, which maintains oscillator and
counter operation while V
CC
is absent. When V
CC
rises above V
PF
, RST remains low for a period of time
(t
RPU
) to allow V
CC
to stabilize.
The block diagram in Figure 4 shows the main elements of the DS1672. As shown, communications to
and from the DS1672 occur serially over a I
2
C, bidirectional bus. The DS1672 operates as a slave device
on the I
2
C bus. Access is obtained by implementing a START condition and providing a device
identification code followed by a register address. Subsequent registers can be accessed sequentially until
a STOP condition is executed.
Figure 4. Block Diagram
Oscillator Circuit
The DS1672 uses an external 32.768kHz crystal. The oscillator circuit does not require any external
resistors or capacitors to operate. Table 1 specifies several crystal parameters for the external crystal.
Figure 4 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.
Table 1. Crystal Specifications*
PARAMETER
SYMBOL
MIN
TYP
MAX
UNITS
NOTES
Nominal Frequency
F
O
32.768
kHz
Series Resistance
ESR
45
kΩ
Load Capacitance
C
L
6
pF
* The crystal, traces, and crystal input pins should be isolated from RF generating signals. Refer to Application Note 58:
Clock Accuracy
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
32-Bit Counter
(4 Bytes)
I
2
C Interface
Power Control
Address Register
Control Logic
1Hz
V
CC
V
BACKUP
GND
SCL
SDA
Control
Trickle Charger
X1 X2
RST
Dallas
Semiconductor
DS1672
C
L
C
L
N
Oscillator
and
divider