DS1685/DS1687 3V/5V Real-Time Clocks

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Figure 1. Block Diagram

OSCILLATOR CIRCUIT

The DS1685 uses an external 32.768kHz crystal. The oscillator circuit does not require any external resistors or

capacitors to operate. Table 1 specifies several crystal parameters for the external crystal, and Figure 2 shows a

functional schematic of the oscillator circuit. The oscillator is controlled by an enable bit in the control register.

Oscillator startup times are highly dependent upon crystal characteristics, PC board leakage, and layout. High ESR

and excessive capacitive loads are the major contributors to long startup times. A circuit using a crystal with the

recommended characteristics and proper layout usually starts within one second.

Power

Control

GND

OSC

BUS

Interface

X1

X2

CS

WR

ALE

RD

AD0 - AD7

Divide

by 8

Divide by

64

Divide by

64

16:1 Mux

Square

Wave

Generator

Registers A, B,C,D

Clock/Calendar and

Alarm Registers

User Ram

114 Bytes

Clock/Calender

Update Logic

IRQ

SQW

RLCR

IRQ

Generator

Buffered Clock/

Calendar and Alarm

Registers

Extended RAM Addr/

Data Registers

Extended Control/

Status Registers

64-Bit Serial Number

Century Counter

Date Alarm

RTC Address -2

RTC Address -3

Extended

User RAM

128 Bytes

RAM

Clear

Logic

DS1687 only

DS1687 only

Select

PWR

KS

DS1685/DS1687