DS1308 Datasheet, PDF (8/16 Pages) Dallas Semiconductor – 64 X 8 Serial Real Time Clock

DS1308
Low-Current I²C RTC with 56-Byte NV RAM
Functional Diagram
X1
X2
VCC
V_BAT
SCL
SDA
N
32.768kHz
8.192kHz
/4
/2
4.096kHz
128Hz
/32
OSC-1Hz
EXTSYNC
MUX/
BUFFER
EXT-1Hz
DIVIDER
N
POWER CONTROL
CONTROL LOGIC
SERIAL BUS
INTERFACE AND
ADDRESS
REGISTER
OSC-1Hz
RAM
CLOCK AND
CALENDAR REGISTERS
DS1308
SQW/CLKIN
Detailed Description
The DS1308 serial RTC is a low-power, full BCD clock/
calendar plus 56 bytes of NV SRAM. Address and data
are transferred serially through an I²C interface. The
clock/calendar provides seconds, minutes, hours, day,
date, month, and year information. The end of the month
date is automatically adjusted for months with fewer than
31 days, including corrections for leap year. The clock
operates in either the 24-hour or 12-hour format with an
AM/PM indicator. The DS1308 has a built-in power-sense
circuit that detects power failures and automatically
switches to the V_BAT supply.
Operation
The DS1308 operates as a slave device on the serial bus.
Access is obtained by implementing a START condition
and providing a device identification code, followed by
data. Subsequent registers can be accessed sequen-
tially until a STOP condition is executed. The device is
fully accessible and data can be written and read when
V_CC is greater than V_PF. However, when V_CC falls below
V_PF, the internal clock registers are blocked from any
access. If V_BAT is greater than V_BAT, the device power is
switched from V_CC to V_BAT when V_CC drops below V_PF.
If V_BAT is less than V_PF, the device power is switched
from V_CC to V_BAT when V_CC drops below V_BAT. The
oscillator and timekeeping functions are maintained
from the V_BAT source until V_CC returns above V_PF, read
and write access is allowed after t_REC. The Functional
Diagram shows the main elements of the DS1308.
An enable bit in the seconds register (CH) controls the
oscillator. Oscillator startup times are highly dependent
upon crystal characteristics, PCB 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 1 second.
On the first application of power to the device, the time
and date registers are reset to 01/01/00 01 00:00:00 (DD/
MM/YY DOW HH:MM:SS), and CH bit in the seconds
register is set to 0.
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