DS1302 Trickle-Charge Timekeeping Chip

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CLOCK HALT FLAG

Bit 7 of the seconds register is defined as the clock halt (CH) flag. When this bit is set to logic 1, the clock oscillator

is stopped and the DS1302 is placed into a low-power standby mode with a current drain of less than 100nA. When

this bit is written to logic 0, the clock will start. The initial power-on state is not defined.

WRITE-PROTECT BIT

Bit 7 of the control register is the write-protect bit. The first seven bits (bits 0 to 6) are forced to 0 and always read 0

when read. Before any write operation to the clock or RAM, bit 7 must be 0. When high, the write-protect bit

prevents a write operation to any other register. The initial power-on state is not defined. Therefore, the WP bit

should be cleared before attempting to write to the device.

TRICKLE-CHARGE REGISTER

This register controls the trickle-charge characteristics of the DS1302. The simplified schematic of Figure 5 shows

the basic components of the trickle charger. The trickle-charge select (TCS) bits (bits 4 to 7) control the selection of

the trickle charger. To prevent accidental enabling, only a pattern of 1010 enables the trickle charger. All other

patterns will disable the trickle charger. The DS1302 powers up with the trickle charger disabled. The diode select

one diode is selected or if DS is 10, two diodes are selected. If DS is 00 or 11, the trickle charger is disabled

resistor and diodes are selected by the RS and DS bits as shown in Table 2.

Table 2. Trickle Charger Resistor and Diode Select

TCS

BIT 7

TCS

BIT 6

TCS

BIT 5

TCS

BIT 4

DS

BIT 3

DS

BIT 2

RS

BIT 1

RS

BIT 0

FUNCTION

X

X

X

X

X

X

0

0

Disabled

X

X

X

X

0

0

X

X

Disabled

X

X

X

X

1

1

X

X

Disabled

1

0

1

0

0

1

0

1

1 Diode, 2k

Ω

1

0

1

0

0

1

1

0

1 Diode, 4k

Ω

1

0

1

0

0

1

1

1

1 Diode, 8k

Ω

1

0

1

0

1

0

0

1

2 Diodes, 2k

Ω

1

0

1

0

1

0

1

0

2 Diodes, 4k

Ω

1

0

1

0

1

0

1

1

2 Diodes, 8k

Ω

0

1

0

1

1

1

0

0

Initial power-on state

Diode and resistor selection is determined by the user 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.

decreases.