DS1374_10 Datasheet, PDF (13/18 Pages) Maxim Integrated Products – I2C, 32-Bit Binary Counter Watchdog RTC with Trickle Charger and Reset Input/Output

I²C, 32-Bit Binary Counter Watchdog RTC with
Trickle Charger and Reset Input/Output
Bit 7
OSF
Bit 6
0
Bit 5
0
Bit 4
0
Bit 3
0
Bit 2
0
Bit 1
0
Bit 0
AF
Status Register (08h)
Bit 7/Oscillator Stop Flag (OSF). A logic 1 in this bit
indicates that the oscillator either is stopped or was
stopped for some period of time and can be used to
judge the validity of the timekeeping data. This bit is set
to logic 1 any time the oscillator stops. The following
are examples of conditions that can cause the OSF bit
to be set:
1) The first time power is applied.
2) The voltage present on both V_CC and V_BACKUP are
insufficient to support oscillation.
3) The EOSC bit is turned off.
4) External influences on the crystal (i.e., noise, leak-
age, etc.).
This bit remains at logic 1 until written to logic 0.
Bit 0/Alarm Flag (AF). A logic 1 in the alarm flag bit
indicates that the WD/ALM counter reached zero. If
WD/ALM is set to zero and the AIE bit = 1, the INT pin
goes low and stays low until AF is cleared. AF is
cleared when written to logic 0. This bit can only be
written to logic 0. Attempting to write logic 1 leaves the
value unchanged. If WD/ALM is set to 1 and the AIE
bit = 1, the INT pin pulses low for 250ms when the
WD/ALM counter reaches zero and sets AF = 1. At the
pulse completion, the DS1374 clears the AF bit to zero.
If the 250ms pulse is active, writing AF to zero does not
truncate the pulse.
Trickle-Charge Register (10h)
The simplified schematic in Figure 7 shows the basic
components of the trickle charger. The trickle-charge
select (TCS) bits (bits 4–7) control the selection of the
trickle charger. To prevent accidental enabling, only a
pattern of 1010 enables the trickle charger. All other
patterns disable the trickle charger. The trickle charger
is disabled when power is first applied. 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; if DS is 10, a diode is selected.
The ROUT bits (bits 0, 1) select the value of the resistor
connected between V_CC and V_BACKUP. Table 5 shows
the resistor selected by the resistor select (ROUT) bits
and the diode selected by the diode select (DS) bits.
Warning: The ROUT value of 250Ω must not be select-
ed whenever V_CC is greater than 3.63V.
The user determines diode and resistor selection
according to the maximum current desired for battery or
super cap charging. The maximum charging current can
be calculated as illustrated in the following example.
Assume that a system power supply of 3.3V is applied
to V_CC and a super cap is connected to V_BACKUP. Also
assume the trickle charger has been enabled with a
diode and resistor R2 between V_CC and V_BACKUP. The
maximum current I_MAX would therefore be calculated
as follows:
I_MAX = (3.3V - diode drop) / R2 ≈ (3.3V - 0.7V) / 2kΩ ≈
1.3mA
As the super cap changes, the voltage drop between
V_CC and V_BACKUP decreases and therefore the charge
current decreases.
Table 5. Trickle Charge Register
TCS3
TCS2
TCS1
TCS0
DS1
X
X
X
X
0
X
X
X
X
1
X
X
X
X
X
1
0
1
0
0
1
0
1
0
1
1
0
1
0
0
1
0
1
0
1
1
0
1
0
0
1
0
1
0
1
0
0
0
0
0
DS0
ROUT1 ROUT0
FUNCTION
0
X
X
Disabled
1
X
X
Disabled
X
0
0
Disabled
1
0
1
No diode, 250Ω resistor
0
0
1
One diode, 250Ω resistor
1
1
0
No diode, 2kΩ resistor
0
1
0
One diode, 2kΩ resistor
1
1
1
No diode, 4kΩ resistor
0
1
1
One diode, 4kΩ resistor
0
0
0
Power-on reset value
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