DS1302

Trickle Charge Timekeeping Chip

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4

CLOCK/CALENDAR

The clock/calendar is contained in seven write/read registers as shown in Figure 4. Data contained in the clock/calendar registers

is in binary coded decimal format (BCD).

CLOCK HALT FLAG

Bit 7 of the seconds register is defined as the clock halt 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 100 nanoamps. When this bit is written to logic

0, the clock will start. The initial power on state is not defined.

AM-PM/12-14 MODE

Bit 7 of the hours register is defined as the 12- or 24-hour mode select bit. When high, the 12-hour mode is selected. In the

12-hour mode, bit 5 is the AM/PM bit with logic high being PM. In the 24-hour mode, bit 5 is the second 10-hour bit (20-23 hours).

WRITE PROTECT BIT

Bit 7 of the control register is the write-protect bit. The first seven bits (bits 0-6) are forced to 0 and will always read a 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 4-7) control the selection of the trickle charger. In order

to prevent accidental enabling, only a pattern of 1010 will enable the trickle charger. All other patterns will disable the trickle

charger. The DS1302 powers up with the trickle charger disabled. The diode select (DS) bits (bits 2-3) select whether one diode or

two diodes are connected between Vcc2 and Vcc1. If DS is 01, one diode is selected or if DS is 10, two diodes are selected. If DS

is 00 or 11, the trickle charger is disabled independently of TCS. The RS bits (bits 0-1) select the resistor that is connected

between Vcc2 and Vcc1. The resistor selected by the resistor select (RS) bit is as follows.

RS Bits

Resistor

Typical Value

00

None

None

01

R1

2K

Ω

10

R2

4K

Ω

11

R3

8K

Ω

If RS is 00, the trickle charger is disabled independently of TCS.

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. Assume that a system power supply of 5

volt is applied to Vcc2 and a super cap is connected to Vcc1. Also assume that the trickle charger has been enabled with one

diode and resistor R1 between Vcc2 and Vcc1. The maximum current Imax would therefore be calculated as follows:

Imax =(5.0V – diode drop)/R1

~(5.0V -0.7V)/2K

Ω

~2.2mA

Obviously, as the super cap charges, the voltage drop between Vcc2 and Vcc1 will decrease and therefore the charge current will

decrease.