DS1338 I
2
C RTC with 56-Byte NV RAM
PIN DESCRIPTION
PIN
8
1
16
—
NAME
X1
FUNCTION
32.768kHz Crystal Connections. The internal oscillator circuitry is designed for
operation with a crystal having a specified load capacitance (C
L
) of 12.5pF. An
external 32.768kHz oscillator can also drive the DS1338. In this configuration,
the X1 pin is connected to the external oscillator signal and the X2 pin is
floated.
Note:
For more information about crystal selection and crystal layout considerations,
refer to
Application Note 58: Crystal Considerations with Dallas Real-Time Clocks.
2
—
X2
3
14
V
BAT
4
15
GND
Backup Supply Input for Lithium Cell or Other Energy Source. Battery voltage
must be held between the minimum and maximum limits for proper operation.
Diodes placed in series between the backup source and the V
BAT
pin may
prevent proper operation. If a backup supply is not required, V
BAT
must be
grounded. UL recognized to ensure against reverse charging when used with a
lithium cell. For more information, visit
Ground. DC power is provided to the device on these pins. V
CC
is the primary
power input. When voltage is applied within normal limits, the device is fully
accessible and data can be written and read. When a backup supply is
connected to the device and V
CC
is below V
PF
, reads and writes are inhibited.
However, the timekeeping function continues unaffected by the lower input
voltage.
Serial Data. Input/output pin for the I
2
C serial interface. It is open drain and
requires an external pullup resistor.
Serial Clock. Used to synchronize data movement on the serial interface
Square-Wave/Output Driver. When enabled and the SQWE bit set to 1, the
SQW/OUT pin outputs one of four square-wave frequencies (1Hz, 4kHz, 8kHz,
32kHz). It is open drain and requires an external pullup resistor. Operates with
either V
CC
or V
BAT
applied.
Primary Power Supply. When voltage is applied within normal limits, the device
is fully accessible and data can be written and read. When a backup supply is
connected to the device and V
CC
is below V
PF
, reads and writes are inhibited.
The backup supply maintains the timekeeping function while V
CC
is absent.
No Connection. These pins are not connected internally, but must be grounded
for proper operation.
5
6
16
1
SDA
SCL
7
2
SQW/OUT
8
3
V
CC
—
4–13
N.C.
DETAILED DESCRIPTION
The DS1338 serial RTC is a low-power, full BCD clock/calendar plus 56 bytes of NV SRAM. Address and data are
transferred serially through an I
2
C 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 AM/PM
indicator. The DS1338 has a built-in power-sense circuit that detects power failures and automatically switches to
the V
BAT
supply.
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