DS1672 Datasheet, PDF (3/13 Pages) Dallas Semiconductor – Low Voltage Serial Timekeeping Chip

DS1672
OSCILLATOR CONTROL
The EOSC bit (bit 7 of the control register) controls the oscillator when in back-up mode. This bit when
set to logic 0 will start the oscillator. When this bit is set to a logic 1, the oscillator is stopped and the
DS1672 is placed into a low-power standby mode with a current drain of less than 200 nanoamps when in
back-up mode. When the DS1672 is powered by V_CC, the oscillator is always on regardless of the status
of the EOSC bit; however, the counter is incremented only when EOSC is a logic 0.
CRYSTAL SELECTION
A standard 32.768 kHz quartz crystal should be directly connected to the X1 and X2 oscillator pins. The
crystal selected for use should have a specified load capacitance (C_L) of 6 pF. For more information on
crystal selection and crystal layout considerations, please consult Application Note 58, “Crystal
Considerations with Dallas Real Time Clocks.”
MICROPROCESSOR MONITOR
A temperature-compensated comparator circuit monitors the level of V_CC. When V_CC falls to the power-
fail trip point, the RST signal (open drain) is pulled active. When V_CC returns to nominal levels, the RST
signal is kept in the active state for 250 ms (typically) to allow the power supply and microprocessor to
stabilize. Note, however, that if the EOSC bit is set to a logic 1 (to disable the oscillator during write
protection), the reset signal will be kept in an active state for 250 ms plus the start-up time of the
oscillator.
TRICKLE CHARGER
The trickle charger is controlled by the trickle charge register. The simplified schematic of Figure 3
shows the basic components of the trickle charger. The trickle charge select (TCS) bit (bits 4-7) controls
the selection of the trickle charger. In order to prevent accidental enabling, only a pattern on 1010 will
enable the trickle charger. All other patterns will disable the trickle charger. The DS1672 powers up
with the trickle charger disabled. The diode select (DS) bits (bits 2-3) select whether or not a diode is
connected between V_CC and V_BACKUP. If DS is 01, no diode is selected or if DS is 10, a diode is selected.
The RS bits (bits 0-1) select whether a resistor is connected between V_CC and V_BACKUP and what the value
of the resistor is. The resistor selected by the resistor select (RS) bits and the diode selected by the diode
select (DS) bits are as follows:
TCS TCS TCS TCS DS DS RS RS Function
X XXX
0
0
X X Disabled
X XXX
1
1
X X Disabled
X XXX
XX0
0
Disabled
1
0
1
0
0 1 0 1 No diode, 100Ω resistor
1
0
1
0
1 0 0 1 One diode, 100Ω resistor
1
0
1
0
0 1 1 0 No diode, 2 kΩ resistor
1
0
1
0
1 0 1 0 One diode, 2 kΩ resistor
1
0
1
0
0 1 1 1 No diode, 4 kΩ resistor
1
0
1
0
1 0 1 1 One diode, 4 kΩ resistor
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