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DS1302
Trickle-Charge Timekeeping Chip
www.maxim-ic.com
FEATURES
Real-Time Clock Counts Seconds, Minutes,
Hours, Date of the Month, Month, Day of the
Week, and Year with Leap-Year
Compensation Valid Up to 2100
31 x 8 Battery-Backed General-Purpose RAM
Serial I/O for Minimum Pin Count
2.0V to 5.5V Full Operation
Uses Less than 300nA at 2.0V
Single-Byte or Multiple-Byte (Burst Mode)
Data Transfer for Read or Write of Clock or
RAM Data
8-Pin DIP or Optional 8-Pin SO for Surface
Mount
Simple 3-Wire Interface
TTL-Compatible (V
CC
= 5V)
Optional Industrial Temperature Range:
-40°C to +85°C
DS1202 Compatible
Underwriters Laboratories (UL®)
Recognized
PIN CONFIGURATIONS
TOP VIEW
V
CC2
X1
X2
GND
1
8
V
CC1
SCLK
I/O
CE
3
4
DS1302
2
7
6
5
DIP (300 mils)
V
CC2
X1
X2
GND
1
DS1302
2
3
4
8
7
6
5
V
CC1
SCLK
I/O
CE
SO (208 mils/150 mils)
ORDERING INFORMATION
PART
DS1302+
DS1302N+
DS1302S+
DS1302SN+
DS1302Z+
DS1302ZN+
TEMP RANGE
0°C to +70°C
-40°C to +85°C
0°C to +70°C
-40°C to +85°C
0°C to +70°C
-40°C to +85°C
PIN-PACKAGE
8 PDIP (300 mils)
8 PDIP (300 mils)
8 SO (208 mils)
8 SO (208 mils)
8 SO (150 mils)
8 SO (150 mils)
TOP MARK*
DS1302
DS1302
DS1302S
DS1302S
DS1302Z
DS1302ZN
+Denotes a lead-free/RoHS-compliant package.
*An N anywhere on the top mark indicates an industrial temperature grade device. A + anywhere on the top mark indicates a lead-free device.
UL is a registered trademark of Underwriters Laboratories, Inc.
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REV: 120208
DS1302 Trickle-Charge Timekeeping Chip
DETAILED DESCRIPTION
The DS1302 trickle-charge timekeeping chip contains a real-time clock/calendar and 31 bytes of static RAM. It
communicates with a microprocessor via a simple serial interface. The real-time 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.
Interfacing the DS1302 with a microprocessor is simplified by using synchronous serial communication. Only three
wires are required to communicate with the clock/RAM: CE, I/O (data line), and SCLK (serial clock). Data can be
transferred to and from the clock/RAM 1 byte at a time or in a burst of up to 31 bytes. The DS1302 is designed to
operate on very low power and retain data and clock information on less than 1μW.
The DS1302 is the successor to the DS1202. In addition to the basic timekeeping functions of the DS1202, the
DS1302 has the additional features of dual power pins for primary and backup power supplies, programmable
trickle charger for V
CC1
, and seven additional bytes of scratchpad memory.
OPERATION
Figure 1 shows the main elements of the serial timekeeper: shift register, control logic, oscillator, real-time clock,
and RAM.
TYPICAL OPERATING CIRCUIT
V
CC
X1
CE
CPU
I/O
SCLK
GND
X2
V
CC2
V
CC
DS1302
V
CC1
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DS1302 Trickle-Charge Timekeeping Chip
Figure 1. Block Diagram
X2
X1
v
CC1
v
CC2
GND
POWER
CONTROL
DS1302
C
L
C
L
CE
1Hz
REAL TIME
CLOCK
31 X 8 RAM
I/O
INPUT SHIFT
REGISTERS
COMMAND AND
CONTROL LOGIC
SCLK
TYPICAL OPERATING CHARACTERISTICS
(V
CC
= 3.3V, T
A
= +25°C, unless otherwise noted.)
I
CC1T
vs. V
CC1T
400
30
I
CC2T
vs. V
CC2T
350
25
SUPPLY CURRENT (uA)
SUPPLY CURRENT (nA)
300
20
250
15
200
150
10
100
2.0
3.0
V
CC1
(V)
4.0
5.0
5
2.0
3.0
V
CC2
(V)
4.0
5.0
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DS1302 Trickle-Charge Timekeeping Chip
PIN DESCRIPTION
PIN
NAME
FUNCTION
Primary Power-Supply Pin in Dual Supply Configuration. V
CC1
is connected to a
backup source to maintain the time and date in the absence of primary power. The
DS1302 operates from the larger of V
CC1
or V
CC2
. When V
CC2
is greater than V
CC1
+
0.2V, V
CC2
powers the DS1302. When V
CC2
is less than V
CC1
, V
CC1
powers the
DS1302.
Connections for Standard 32.768kHz Quartz Crystal. The internal oscillator is
designed for operation with a crystal having a specified load capacitance of 6pF.
For more information on crystal selection and crystal layout considerations, refer to
Application Note 58: Crystal Considerations for Dallas Real-Time Clocks.
The
DS1302 can also be driven by an external 32.768kHz oscillator. In this
configuration, the X1 pin is connected to the external oscillator signal and the X2 pin
is floated.
Ground
Input. CE signal must be asserted high during a read or a write. This pin has an
internal 40kΩ (typ) pulldown resistor to ground. Note: Previous data sheet revisions
referred to CE as
RST.
The functionality of the pin has not changed.
Input/Push-Pull Output. The I/O pin is the bidirectional data pin for the 3-wire
interface. This pin has an internal 40kΩ (typ) pulldown resistor to ground.
Input. SCLK is used to synchronize data movement on the serial interface. This pin
has an internal 40kΩ (typ) pulldown resistor to ground.
Low-Power Operation in Single Supply and Battery-Operated Systems and Low-
Power Battery Backup. In systems using the trickle charger, the rechargeable
energy source is connected to this pin. UL recognized to ensure against reverse
charging current when used with a lithium battery. Go to
www.maxim-
ic.com/TechSupport/QA/ntrl.htm.
1
V
CC2
2
X1
3
4
5
6
7
X2
GND
CE
I/O
SCLK
8
V
CC1
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DS1302 Trickle-Charge Timekeeping Chip
OSCILLATOR CIRCUIT
The DS1302 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. Figure 1 shows a
functional schematic of the oscillator circuit. If using a crystal with the specified characteristics, the startup time is
usually less than one second.
CLOCK ACCURACY
The accuracy of the clock is dependent upon the accuracy of the crystal and the accuracy of the match between
the capacitive load of the oscillator circuit and the capacitive load for which the crystal was trimmed. Additional
error will be added by crystal frequency drift caused by temperature shifts. External circuit noise coupled into the
oscillator circuit may result in the clock running fast. Figure 2 shows a typical PC board layout for isolating the
crystal and oscillator from noise. Refer to
Application Note 58: Crystal Considerations for Dallas Real-Time Clocks
for detailed information.
Table 1. Crystal Specifications*
PARAMETER
Nominal Frequency
Series Resistance
Load Capacitance
SYMBOL
f
O
ESR
C
L
6
MIN
TYP
32.768
45
MAX
UNITS
kHz
kΩ
pF
*The crystal, traces, and crystal input pins should be isolated from RF generating signals. Refer to
Application Note 58: Crystal Considerations for Dallas Real-Time Clocks
for additional specifications.
Figure 2. Typical PC Board Layout for Crystal
LOCAL GROUND PLANE (LAYER 2)
X1
CRYSTAL
X2
NOTE:
AVOID ROUTING SIGNALS IN THE
CROSSHATCHED AREA (UPPER LEFT-
HAND QUADRANT) OF THE PACKAGE
UNLESS THERE IS A GROUND PLANE
BETWEEN THE SIGNAL LINE AND THE
PACKAGE.
GND
COMMAND BYTE
Figure 3 shows the command byte. A command byte initiates each data transfer. The MSB (bit 7) must be a logic
1. If it is 0, writes to the DS1302 will be disabled. Bit 6 specifies clock/calendar data if logic 0 or RAM data if logic 1.
Bits 1 to 5 specify the designated registers to be input or output, and the LSB (bit 0) specifies a write operation
(input) if logic 0 or read operation (output) if logic 1. The command byte is always input starting with the LSB (bit 0).
Figure 3. Address/Command Byte
7
1
6
RAM
CK
5
A4
4
A3
3
A2
2
A1
1
A0
0
RD
WR
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