Datasheet
DS1685/DS1687 3V/5V Real-Time Clocks
6 of 34
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
V
BAT
GND
OSC
BUS
Interface
V
CC
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
V
BAUX
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