Tickle Charger and Reset Input/Output

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13

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.

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

no diode is selected; if DS is 10, a diode is selected.

The ROUT bits (bits 0, 1) select the value of the resistor

the resistor selected by the resistor select (ROUT) bits

and the diode selected by the diode select (DS) bits.

Warning: The ROUT value of 250

Ω must not be select-

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

assume the trickle charger has been enabled with a

as follows:

≈ (3.3V - 0.7V) / 2kΩ≈

1.3mA

As the super cap changes, the voltage drop between

current decreases.

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

OSF

0

0

00

00

AF

TCS3

TCS2

TCS1

TCS0

DS1

DS0

ROUT1

ROUT0

FUNCTION

XX

XX

00

XX

Disabled

XX

XX

11

XX

Disabled

XX

X

XXX0

0

Disabled

10

1

001

01

No diode, 250

Ω resistor

10

1

010

01

One diode, 250

Ω resistor

10

1

001

10

No diode, 2k

Ω resistor

10

1

010

10

One diode, 2k

Ω resistor

10

1

001

11

No diode, 4k

Ω resistor

10

1

010

11

One diode, 4k

Ω resistor

00

0

000

00

Power-on reset value

Table 5. Trickle Charge Register