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GENERAL DESCRIPTION
The DS1338 serial real-time clock (RTC) is a low-
power, full binary-coded decimal (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
AM/PM indicator. The DS1338 has a built-in power-
sense circuit that detects power failures and
automatically switches to the backup supply,
maintaining time and date operation
APPLICATIONS
Handhelds (GPS, POS Terminal)
Consumer Electronics (Set-Top Box, Digital
Recording, Network Appliance)
Office Equipment (Fax/Printer, Copier)
Medical (Glucometer, Medicine Dispenser)
Telecommunications (Router, Switcher, Server)
Other (Utility Meter, Vending Machine, Thermostat,
Modem)
TYPICAL OPERATING CIRCUIT
FEATURES
RTC Counts Seconds, Minutes, Hours, Date
of the Month, Month, Day of the Week, and
Year with Leap-Year Compensation Valid Up
to 2100
Available in a Surface-Mount Package with an
Integrated Crystal (DS1338C)
56-Byte Battery-Backed General-Purpose
RAM with Unlimited Writes
I2C Serial Interface
Programmable Square-Wave Output Signal
Automatic Power-Fail Detect and Switch
Circuitry
-40°C to +85°C Operating Temperature Range
Underwriters Laboratories (UL) Recognized
ORDERING INFORMATION
PART TEMP RANGE PIN-PACKAGE TOP MARK†
DS1338Z-18+
-40°C to +85°C
8 SO (0.150″)
DS1338-18
DS1338Z-3+
-40°C to +85°C
8 SO (0.150
″
)
DS1338-3
DS1338Z-33+
-40°C to +85°C
8 SO (0.150″)
DS133833
DS1338U-18+ -40°C to +85°C 8 µSOP
1338
rr-18
DS1338U-3+ -40°C to +85°C 8 µSOP
1338
rr-3
DS1338U-33+ -40°C to +85°C 8 µSOP
1338
rr-33
DS1338C-18# -40°C to +85°C 16 SO (0.300″) DS1338C-18
DS1338C-3#
-40°C to +85°C
16 SO (0.300″)
DS1338C-3
DS1338C-33#
-40°C to +85°C
16 SO (0.300″)
DS1338C-33
rr = second line, revision level
+ Denotes a lead(Pb)-free/RoHS-compliant device.
# Denotes a RoHS-compliant device that may include lead that is
exempt under the RoHS requirements. The lead finish is JESD97
category e3, and is compatible with both lead-based and lead-free
soldering processes.
† A “+” anywhere on the top mark denotes a lead-free device. A “#”
denotes a RoHS-compliant device.
Pin Configurations appear at end of data sheet.
DS1338
CPU
V
V
CC
V
SDA
SCL
GND
X2
X1
V
R
PU
R
PU
CRYSTAL
SQW /OUT
V
BAT
i
R
PU
= t
r
/C
b
DS1338
I2C RTC with 56-
Byte NV RAM
19-6019; Rev 3/12
DS1338 I2C RTC with 56-Byte NV RAM
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ABSOLUTE MAXIMUM RATINGS
Voltage Range on Any Pin Relative to Ground………………………………………………………..……..-0.3V to +6.0V
Operating Temperature Range…………………………………………………………………………..……-40°C to +85°C
Storage Temperature Range………………………………………………………………………………...-55°C to +125°C
Lead Temperature (soldering, 10s) ……….………………………………………………………………………. +260°C
Soldering Temperature (reflow) ……………………………………………………………………………………. +260°C
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only,
and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is
not implied. Exposure to the absolute maximum rating conditions for extended periods may affect device reliability.
RECOMMENDED DC OPERATING CONDITIONS
(VCC = VCC(MIN) to VCC(MAX), TA = -40°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C, unless
otherwise noted.) (Note 1)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
Supply Voltage VCC
DS1338-18
1.71
1.8
5.5
V
DS1338-3
2.7
3.0
5.5
DS1338-33
3.0
3.3
5.5
Logic 1 VIH (Note 2)
0.7 x
VCC
V
CC
+
0.3
V
Logic 0 VIL (Note 2) -0.3
+0.3 x
VCC
V
Power-Fail Voltage VPF
DS1338-18
1.51
1.62
1.71
V
DS1338-3
2.45
2.59
2.70
DS1338-33
2.70
2.82
2.97
VBAT Input Voltage VBAT (Note 2) 1.3 3.0 3.7 V
DC ELECTRICAL CHARACTERISTICS
(VCC = VCC(MIN) to VCC(MAX), TA = -40°C to +85°C, unless otherwise noted. Typical values are at VCC = TYP,
TA = +25°C, unless otherwise noted.) (Note 1)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
Input Leakage ILI (Note 3) 1 µA
I/O Leakage ILO (Note 4) 1 µA
SDA Logic 0 Output IOLSDA
VCC > 2V; VOL = 0.4V
3.0
mA
VCC < 2V; VOL = 0.2 x VCC
3.0
SQW/OUT Logic 0 Output IOLSQW
V
CC
> 2V; V
OL
= 0.4V
3.0
mA
1.71V < V
CC
< 2V;
VOL = 0.2 x VCC
3.0
1.3V < V
CC
< 1.71V;
VOL = 0.2 x VCC
250 µA
Active Supply Current
(Note 5) ICCA
DS1338-18: VCC = 1.89V
75
150
µA
DS1338-3: VCC = 3.30V
110
200
DS1338-33
VCC = 3.63V
120
200
VCC = 5.5V
325
Standby Current (Note 6) ICCS
DS1338-18: VCC = 1.89V
60
100
µA
DS1338-3: VCC = 3.30V
80
125
DS1338-33
VCC = 3.63V
85
125
VCC = 5.5V
200
VBAT Leakage Current
(VCC Active) IBATLKG 25 100 nA
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DC ELECTRICAL CHARACTERISTICS
(VCC = 0V, TA = -40°C to +85°C, unless otherwise noted. Typical values are at VBAT = 3.0V, TA = +25°C, unless
otherwise noted.) (Note 1)
PARAMETER SYMBOL MIN TYP MAX UNITS
VBAT Current (OSC ON); VBAT = 3.7V, SQW/OUT OFF (Note 7) IBATOSC1 800 1200 nA
VBAT Current (OSC ON); VBAT = 3.7V, SQW/OUT ON (32kHz)
(Note 7) IBATOSC2 1025 1400 nA
VBAT Data-Retention Current (Osc Off); VBAT = 3.7V (Note 7) IBATDAT 10 100 nA
AC ELECTRICAL CHARACTERISTICS
(VCC = VCC(MIN) to VCC(MAX), TA = -40°C to +85°C) (Note 1)
PARAMETER SYMBOL CONDITION MIN TYP MAX UNITS
SCL Clock Frequency fSCL
Fast mode
100
400
kHz
Standard mode 0 100
Bus Free Time Between STOP
and START Condition tBUF Fast mode 1.3 µs
Standard mode 4.7
Hold Time (Repeated) START
Condition (Note 8) tHD:STA
Fast mode 0.6
µs
Standard mode 4.0
LOW Period of SCL Clock tLOW Fast mode 1.3
µs
Standard mode 4.7
HIGH Period of SCL Clock tHIGH Fast mode 0.6
µs
Standard mode 4.0
Setup Time for Repeated
START Condition tSU:STA Fast mode 0.6
µs
Standard mode 4.7
Data Hold Time (Notes 9, 10) tHD:DAT Fast mode 0 0.9 µs
Standard mode 0
Data Setup Time (Note 11) tSU:DAT Fast mode 100 ns
Standard mode 250
Rise Time of Both SDA and
SCL Signals (Note 12) tR
Fast mode 20 + 0.1CB 300 ns
Standard mode 20 + 0.1CB 1000
Fall Time of Both SDA and
SCL Signals (Note 12) tF Fast mode 20 + 0.1CB 300 ns
Standard mode 20 + 0.1CB 300
Setup Time for STOP
Condition tSU:STO
Fast mode
0.6
µs
Standard mode 4.0
Capacitive Load for Each Bus
Line CB (Note 12) 400 pF
I/O Capacitance (SDA, SCL) CI/O (Note 13) 10 pF
Oscillator Stop Flag (OSF)
Delay tOSF (Note 14) 100 ms
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POWER-UP/POWER-DOWN CHARACTERISTICS
(TA = -40°C to +85°C) (Note 1, Figure 1)
PARAMETER SYMBOL MIN TYP MAX UNITS
Recovery at Power-Up (Note 15) tREC 2 ms
VCC Fall Time; VPF(MAX) to VPF(MIN) tVCCF 300 µs
VCC Rise Time; VPF(MIN) to VPF(MAX) tVCCR 0 µs
Warning: Negative undershoots below -0.3V while the part is in battery-backed mode may cause
loss of data.
Note 1:
Limits at -40°C are guaranteed by design and not production tested.
Note 2:
All voltages are referenced to ground.
Note 3:
SCL only.
Note 4:
SDA and SQW/OUT.
Note 5:
ICCA—SCL clocking at max frequency = 400kHz.
Note 6:
Specified with the I2C bus inactive.
Note 7:
Measured with a 32.768kHz crystal attached to X1 and X2.
Note 8:
After this period, the first clock pulse is generated.
Note 9: A device must internally provide a hold time of at least 300ns for the SDA signal (referred to the VI H(MI N) of the SCL signal) to
bridge the undefined region of the falling edge of SCL.
Note 10:
The maximum tHD:DAT need only be met if the device does not stretch the LOW period (tLOW) of the SCL signal.
Note 11: A fast-mode device can be used in a standard-mode system, but the requirement tS U:DAT ≥ to 250ns must then be met. This is
automatically the case if the device does not stretch the LOW period of the SCL signal. If such a device does stretch the LOW
period of the SCL signal, it must output the next data bit to the SDA line tR(MAX) + tS U:DAT = 1000 + 250 = 1250ns before the SCL line
is released.
Note 12:
CB—total capacitance of one bus line in pF.
Note 13:
Guaranteed by design. Not production tested.
Note 14: The parameter tOSF is the time period the oscillator must be stopped for the OSF flag to be set over the voltage range of
0.0V ≤ VCC ≤ VCC(MAX) and 1.3V ≤ VBAT ≤ 3.7V.
Note 15: This delay applies only if the oscillator is enabled and running. If the oscillator is disabled or stopped, no power-up delay occurs.
Figure 1. Power-Up/Power-Down Timing
OUTPUTS
V
CC
V
PF(MAX)
V
PF(MIN)
INPUTS
HIGH-Z
DON'T CARE
VALID
RECOGNIZED
RECOGNIZED
VALID
t
VCCF
t
VCCR
t
REC
DS1338 I2C RTC with 56-Byte NV RAM
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Figure 2. Timing Diagram
Figure 3. Block Diagram
RAM
(56 X 8)
SERIAL BUS
INTERFACE
AND ADDRESS
REGISTER
CONTROL
LOGIC
1Hz
1Hz/4.096kHz/8.192kHz/32.768kHz MUX/
BUFFER
USER BUFFER
(7 BYTES)
CLOCK,
CALENDAR,
AND CONTROL
REGISTERS
"C" VERSION ONLY
POWER
CONTROL
DS1338
X1
C
L
C
L
X2
SDA
SCL
SQW/OUT
V
CC
GND
V
BAT
Oscillator
and divider
N
DS1338 I2C RTC with 56-Byte NV RAM
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TYPICAL OPERATING CHARACTERISTICS
I
BAT
vs. V
BAT
550
600
650
700
750
800
850
900
950
1000
1050
1100
1150
1200
1250
1.3 1.8 2.3 2.8 3.3 3.8 4.3 4.8 5.3
V
BAT
(V)
SUPPLY CURRENT (nA
VCC=0V
RS1 =RS0=1
IBATOSC2
(SQ W E = 1)
IBATOSC1
( SQWE = 0 )
I
BAT
vs. Temperature
V
BAT
= 3.0V
600
650
700
750
800
850
900
950
1000
-40 -20 020 40 60 80
TEMPERATURE (°C)
SUPPLY CURRENT (nA
VCC=0V
SQWE=1
SQWE=0
I
CC
vs. V
CC
50
75
100
125
150
175
200
225
250
1.8 2.3 2.8 3.3 3.8 4.3 4.8 5.3
V
CC
(V)
SUPPLY CURRENT (uA
SCL=400kHz
SCL =S DA =0Hz
Oscillator Frequency vs. Supply Voltage
32768.0
32768.1
32768.2
32768.3
32768.4
32768.5
1.3 1.8 2.3 2.8 3.3 3.8 4.3 4.8
Oscillator Supply Voltage (V)
FREQUENCY (Hz)
DS1338 I2C RTC with 56-Byte NV RAM
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PIN DESCRIPTION
PIN
NAME FUNCTION
8
16
1 — X1
32.768kHz Crystal Connections. The internal oscillator circuitry is designed for
operation with a crystal having a specified load capacitance (CL) 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 left
unconnected.
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 VBAT
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 VBAT pin may
prevent proper operation. If a backup supply is not required, VBAT must be
grounded. UL recognized to ensure against reverse charging when used with a
lithium cell. For more information, visit www.maxim-ic.com/qa/info/ul.
4 15 GND
Ground. DC power is provided to the device on these pins. VCC 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 VCC is below VPF, reads and writes are inhibited.
However, the timekeeping function continues unaffected by the lower input
voltage.
5 16 SDA
Serial Data. Input/output pin for the I2C serial interface. It is an open drain
output and requires an external pullup resistor. The pull up voltage may be up
to 5.5V regardless of the voltage on VCC.
6 1 SCL
Serial Clock. Input pin for the I2C serial interface. Used to synchronize data
movement on the serial interface. The pull up voltage may be up to 5.5V
regardless of the voltage on VCC.
7 2 SQW/OUT
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 an open drain output and requires an external pullup resistor.
Operates with either VCC or VBAT applied. The pull up voltage may be up to
5.5V regardless of the voltage on VCC. If not used, this pin may be left
unconnected.
8 3 VCC
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 VCC is below VPF, reads and writes are inhibited.
The backup supply maintains the timekeeping function while VCC is absent.
— 4–13 N.C. No Connection. These pins are not connected internally, but must be grounded
for proper operation.
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 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 AM/PM
indicator. The DS1338 has a built-in power-sense circuit that detects power failures and automatically switches to
the VBAT supply.
DS1338 I2C RTC with 56-Byte NV RAM
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OPERATION
The DS1338 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 sequentially
until a STOP condition is executed. The device is fully accessible and data can be written and read when VCC is
greater than VPF. However, when VCC falls below VPF, the internal clock registers are blocked from any access. If
VPF is less than VBAT, the device power is switched from VCC to VBAT when VCC drops below VPF. If VPF is greater
than VBAT, the device power is switched from VCC to VBAT when VCC drops below VBAT. The oscillator and
timekeeping functions are maintained from the VBAT source until VCC is returned to nominal levels. The block
diagram (Figure 3) shows the main elements of the DS1338.
An enable bit in the seconds register controls the oscillator. 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 start-up times. A circuit using a crystal with the recommended characteristics and proper layout
usually starts within 1 second.
POWER CONTROL
The power-control function is provided by a precise, temperature-compensated voltage reference and a
comparator circuit that monitors the VCC level. The device is fully accessible and data can be written and read when
VCC is greater than VPF. However, when VCC falls below VPF, the internal clock registers are blocked from any
access. If VPF is less than VBAT, the device power is switched from VCC to VBAT when VCC drops below VPF. If VPF is
greater than VBAT, the device power is switched from VCC to VBAT when VCC drops below VBAT. The registers are
maintained from the VBAT source until VCC is returned to nominal levels (Table 1). After VCC returns above VPF, read
and write access is allowed after tREC (Figure 1). 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). The CH bit in the seconds register will be set
to a 0.
Table 1. Power Control
SUPPLY
CONDITION
READ/WRITE
ACCESS
POWERED
BY
VCC < VPF, VCC < VBAT
No
VBAT
VCC < VPF, VCC > VBAT
No
VCC
VCC > VPF, VCC < VBAT
Yes
VCC
VCC > VPF, VCC > VBAT
Yes
VCC
OSCILLATOR CIRCUIT
The DS1338 uses an external 32.768kHz crystal. The oscillator circuit does not require any external resistors or
capacitors to operate. Table 2 specifies several crystal parameters for the external crystal. Figure 3 shows a
functional schematic of the oscillator circuit. The startup time is usually less than 1 second when using a crystal
with the specified characteristics.
Table 2. Crystal Specifications*
PARAMETER SYMBOL MIN TYP MAX UNITS
Nominal Frequency fO 32.768 kHz
Series Resistance ESR 50 kΩ
Load Capacitance CL 12.5 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.
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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. Crystal
frequency drift caused by temperature shifts creates additional error. External circuit noise coupled into the
oscillator circuit can result in the clock running fast. Figure 4 shows a typical PC board layout for isolating the
crystal and oscillator from noise. Refer to Application Note 58: Crystal Considerations with Dallas Real-Time Clocks
for detailed information.
DS1338C ONLY
The DS1338C integrates a standard 32,768Hz crystal in the package. Typical accuracy at nominal VCC and +25°C
is approximately 10ppm. Refer to Application Note 58 for information about crystal accuracy vs. temperature.
Figure 4. Typical PC Board Layout for Crystal
RTC AND RAM ADDRESS MAP
Table 3 shows the address map for the RTC and RAM registers. The RTC registers and control register are located
in address locations 00h to 07h. The RAM registers are located in address locations 08h to 3Fh. During a multibyte
access, when the register pointer reaches 3Fh (the end of RAM space) it wraps around to location 00h (the
beginning of the clock space). On an I2C START, STOP, or register pointer incrementing to location 00h, the
current time and date is transferred to a second set of registers. The time and date in the secondary registers are
read in a multibyte data transfer, while the clock continues to run. This eliminates the need to re-read the registers
in case of an update of the main registers during a read.
CLOCK AND CALENDAR
The time and calendar information is obtained by reading the appropriate register bytes. See Figure 6 for the RTC
registers. The time and calendar are set or initialized by writing the appropriate register bytes. The contents of the
time and calendar registers are in the BCD format. Bit 7 of Register 0 is the clock halt (CH) bit. When this bit is set
to 1, the oscillator is disabled. When cleared to 0, the oscillator is enabled. The clock can be halted whenever the
timekeeping functions are not required, which minimizes VBAT current (IBATDAT) when VCC is not applied.
The day-of-week register increments at midnight. Values that correspond to the day of week are user-defined but
must be sequential (i.e., if 1 equals Sunday, then 2 equals Monday, and so on). Illogical time and date entries
result in undefined operation.
When reading or writing the time and date registers, secondary (user) buffers are used to prevent errors when the
internal registers update. When reading the time and date registers, the user buffers are synchronized to the
internal registers on any start or stop and when the register pointer rolls over to zero. The countdown chain is reset
LOCAL GROUND PLANE (LAYER 2)
CRYSTAL
X1
X2
GND
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.
DS1338 I2C RTC with 56-Byte NV RAM
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whenever the seconds register is written. Write transfers occur on the acknowledge from the DS1338. Once the
countdown chain is reset, to avoid rollover issues the remaining time and date registers must be written within
1 second. The 1Hz square-wave output, if enabled, transitions high 500ms after the seconds data transfer,
provided the oscillator is already running.
The DS1338 runs in either 12-hour or 24-hour mode. Bit 6 of the hours register is defined as the 12-hour or 24-hour
mode-select bit. When high, the 12-hour mode is selected. In the 12-hour mode, bit 5 is the AM/PM bit, with logic
high being PM. In the 24-hour mode, bit 5 is the 20-hour bit (20–23 hours). If the 12/24-hour mode select is
changed, the hours register must be re-initialized to the new format.
On an I2C START, the current time is transferred to a second set of registers. The time information is read from
these secondary registers, while the clock continues to run. This eliminates the need to re-read the registers in
case of an update of the main registers during a read.
Table 3. RTC and RAM Address Map
ADDRESS BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0 FUNCTION RANGE
00H
CH
10 Seconds
Seconds
Seconds
00–59
01H
0
10 Minutes
Minutes
Minutes
00–59
02H 0 12/24
AM/PM 10
Hour Hour Hours
1–12
+AM/PM
00–23
20 Hour
03H
0
0
0
0
0
Day
Day
1–7
04H
0
0
10 Date
Date
Date
01–31
05H 0 0 0
10
Month
Month Month 01–12
06H
10 Year
Year
Year
00–99
07H
OUT
0
OSF
SQWE
0
0
RS1
RS0
Control
08H–3FH RAM 56 x 8 00H–FFH
Note: Bits listed as “0” always read as a 0.
DS1338 I2C RTC with 56-Byte NV RAM
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CONTROL REGISTER (07H)
The control register controls the operation of the SQW/OUT pin and provides oscillator status.
Bit #
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
BIT 2
BIT 1
BIT 0
Name
OUT
0
OSF
SQWE
0
0
RS1
RS0
POR
1
0
1
1
0
0
1
1
Bit 7: Output Control (OUT). Controls the output level of the SQW/OUT pin when the square-wave output is
disabled. If SQWE = 0, the logic level on the SQW/OUT pin is 1 if OUT = 1; it is 0 if OUT = 0.
Bit 5: Oscillator Stop Flag (OSF). A logic 1 in this bit indicates that the oscillator has stopped or was stopped for
some time period and can be used to judge the validity of the clock and calendar data. This bit is edge triggered,
and is set to logic 1 when the internal circuitry senses the oscillator has transitioned from a normal run state to a
STOP condition. The following are examples of conditions that may cause the OSF bit to be set:
1) The first time power is applied.
2) The voltage present on VCC and VBAT are insufficient to support oscillation.
3) The CH bit is set to 1, disabling the oscillator.
4) External influences on the crystal (i.e., noise, leakage, etc.).
This bit remains at logic 1 until written to logic 0. This bit can only be written to logic 0. Attempting to write OSF to
logic 1 leaves the value unchanged.
Bit 4: Square-Wave Enable (SQWE). When set to logic 1, this bit enables the oscillator output to operate with
either VCC or VBAT applied. The frequency of the square-wave output depends upon the value of the RS0 and RS1
bits.
Bits 1 and 0: Rate Select (RS1 and RS0). These bits control the frequency of the square-wave output when the
square-wave output has been enabled. The table below lists the square-wave frequencies that can be selected
with the RS bits.
Square-Wave Output
OUT
RS1
RS0
SQW OUTPUT
SQWE
X
0
0
1Hz
1
X
0
1
4.096kHz
1
X
1
0
8.192kHz
1
X
1
1
32.768kHz
1
0
X
X
0
0
1
X
X
1
0
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I2C SERIAL DATA BUS
The DS1338 supports the I2C protocol. A device that sends data onto the bus is defined as a transmitter and a
device receiving data is a receiver. The device that controls the message is called a master. The devices that are
controlled by the master are referred to as slaves. The bus must be controlled by a master device, which generates
the serial clock (SCL), controls the bus access, and generates the START and STOP conditions. The DS1338
operates as a slave on the I2C bus. Within the bus specifications, a standard mode (100kHz maximum clock rate)
and a fast mode (400kHz maximum clock rate) are defined. The DS1338 works in both modes. Connections to the
bus are made through the open-drain I/O lines SDA and SCL.
The following bus protocol has been defined (Figure 5).
Data transfer can be initiated only when the bus is not busy.
During data transfer, the data line must remain stable whenever the clock line is HIGH. Changes in the data
line while the clock line is HIGH are interpreted as control signals.
Accordingly, the following bus conditions have been defined:
Bus not busy: Both data and clock lines remain HIGH.
Start data transfer: A change in the state of the data line, from HIGH to LOW, while the clock is HIGH, defines a
START condition.
Stop data transfer: A change in the state of the data line, from LOW to HIGH, while the clock line is HIGH, defines
the STOP condition.
Data valid: The state of the data line represents valid data when, after a START condition, the data line is stable
for the duration of the HIGH period of the clock signal. The data on the line must be changed during the LOW
period of the clock signal. There is one clock pulse per bit of data.
Each data transfer is initiated with a START condition and terminated with a STOP condition. The number of data
bytes transferred between START and STOP conditions is not limited and is determined by the master device. The
information is transferred byte-wise and each receiver acknowledges with a ninth bit.
Acknowledge: Each receiving device, when addressed, is obliged to generate an acknowledge after the reception
of each byte. The master device must generate an extra clock pulse that is associated with this acknowledge bit.
A device that acknowledges must pull down the SDA line during the acknowledge clock pulse in such a way that
the SDA line is stable LOW during the HIGH period of the acknowledge-related clock pulse. Of course, setup and
hold times must be taken into account. A master must signal an end of data to the slave by not generating an
acknowledge bit on the last byte that has been clocked out of the slave. In this case, the slave must leave the data
line HIGH to enable the master to generate the STOP condition.
Figure 5. Data Transfer on I2C Serial Bus
DS1338 I2C RTC with 56-Byte NV RAM
13 of 16
Depending upon the state of the R/W bit, two types of data transfer are possible:
1) Data transfer from a master transmitter to a slave receiver. The master transmits the first byte (the slave
address). Next follows a number of data bytes. The slave returns an acknowledge bit after each received byte.
Data is transferred with the most significant bit (MSB) first.
2) Data transfer from a slave transmitter to a master receiver. The master transmits the first byte (the slave
address). The slave then returns an acknowledge bit, which is followed by the slave transmitting a number of
data bytes. The master returns an acknowledge bit after all received bytes other than the last byte. At the end
of the last received byte, a “not acknowledge” is returned. The master device generates all of the serial clock
pulses and the START and STOP conditions. A transfer is ended with a STOP condition or with a repeated
START condition. Since a repeated START condition is also the beginning of the next serial transfer, the bus is
not released. Data is transferred with the most significant bit (MSB) first.
The DS1338 can operate in the following two modes:
1) Slave receiver mode (write mode): Serial data and clock are received through SDA and SCL. An
acknowledge bit is transmitted after each byte is received. START and STOP conditions are recognized as the
beginning and end of a serial transfer. Hardware performs address recognition after reception of the slave
address and direction bit (Figure 6). The slave address byte is the first byte received after the master
generates the START condition. The slave address byte contains the 7-bit DS1338 address—1101000—
followed by the direction bit (R/W), which, for a write, is 0. After receiving and decoding the slave address byte,
the slave outputs an acknowledge on the SDA line. After the DS1338 acknowledges the slave address and
write bit, the master transmits a register address to the DS1338. This sets the register pointer on the DS1338,
with DS1338 acknowledging the transfer. The master may then transmit zero or more bytes of data, with the
DS1338 acknowledging each byte received. The register pointer increments after each data byte is transferred.
The master generates a STOP condition to terminate the data write.
2) Slave transmitter mode (read mode): The first byte is received and handled as in the slave receiver mode.
However, in this mode, the direction bit indicates that the transfer direction is reversed. The DS1338 transmits
serial data on SDA while the serial clock is input on SCL. START and STOP conditions are recognized as the
beginning and end of a serial transfer (Figure 7). The slave address byte is the first byte received after the
master generates the START condition. The slave address byte contains the 7-bit DS1338 address—
1101000—followed by the direction bit (R/W), which, for a read, is 1. After receiving and decoding the slave
address byte, the slave outputs an acknowledge on the SDA line. The DS1338 then starts transmitting data
using the register address pointed to by the register pointer. If the register pointer is not set before the initiation
of a read mode, the first address that is read is the last one stored in the register pointer. The register pointer is
incremented after each byte is transferred. The DS1338 must receive a “not acknowledge” to end a read.
DS1338 I2C RTC with 56-Byte NV RAM
14 of 16
Figure 6. Data Write—Slave Receiver Mode
Figure 7. Data Read (From Current Pointer Location)—Slave Transmitter Mode
Figure 8. Data Read (Write Pointer, Then Read—Slave Receive and Transmit
...
A
XXXXXXXX
A
1101000
S
1
XXXXXXXX
A
XXXXXXXX
XXXXXXXX
A
P
<Data (n+2)> <Data (n+X)>
A
S - START
A - ACKNOWLEDGE (ACK)
P - STOP
A
- NOT ACKNOWLEDGE (NACK)
<R/W>
DATA TRANSFERRED
(X+1 BYTES + ACKNOWLEDGE)
NOTE: LAST DATA BYTE IS FOLLOWED BY A NACK
<Slave Address>
<Data (n)>
<Data (n+1)>
MASTER TO SLAVE
SLAVE TO MASTER
...
A
XXXXXXXX
XXXXXXXX
A
XXXXXXXX
A
XXXXXXXX
A
P
S - START
SR - REPEATED START
A - ACKNOWLEDGE (ACK)
P - STOP
A - NOT ACKNOWLEDGE (NACK)
DATA TRANSFERRED
(X+1 BYTES + ACKNOWLEDGE)
NOTE: LAST DATA BYTE IS FOLLOWED BY A NACK
A
XXXXXXXX
A
1101000
S
0
<R/W>
<Word Address (n)>
A
1101000
Sr
1
<R/W>
<Slave Address>
<Data (n)>
<Data (n+1)>
<Data (n+2)>
<Data (n+X)>
MASTER TO SLAVE
SLAVE TO MASTER
...
A
XXXXXXXX
A
S
0
XXXXXXXX
A
XXXXXXXX
A
XXXXXXXX
A
P
S - START
A - ACKNOWLEDGE (ACK)
P - STOP
<R/W>
DATA TRANSFERRED
(X+1 BYTES + ACKNOWLEDGE)
1101000
<Slave Address>
<Word Address (n)>
<Data (n)>
<Data (n+1)>
<Data (n+X)>
MASTER TO SLAVE
SLAVE TO MASTER
DS1338 I2C RTC with 56-Byte NV RAM
15 of 16
HANDLING, PCB LAYOUT, AND ASSEMBLY
The DS1338C package contains a quartz tuning-fork crystal. Pick-and-place equipment may be used, but
precautions should be taken to ensure that excessive shocks are avoided. Ultrasonic cleaning should be avoided to
prevent damage to the crystal. Exposure to reflow is limited to 2 times maximum.
Avoid running signal traces under the package, unless a ground plane is placed between the package and the
signal line. All N.C. (no connect) pins must be connected to ground.
The RoHS and lead-free/RoHS packages may be reflowed using a reflow profile that complies with JEDEC J-STD-
020.
Moisture-sensitive packages are shipped from the factory dry-packed. Handling instructions listed on the package
label must be followed to prevent damage during reflow. Refer to the IPC/JEDEC J-STD-020 standard for moisture-
sensitive device (MSD) classifications.
PIN CONFIGURATIONS
CHIP INFORMATION
TRANSISTOR COUNT: 12,231
PROCESS: CMOS
THERMAL INFORMATION
PART THETA-JA
(°C/W)
THETA-JC
(°C/W)
8 SO
132
38
8 μSOP
206.3
42
16 SO
73
23
PACKAGE INFORMATION
For the latest package outline information and land patterns (footprints), go to www.maxim-ic.com/packages.
Note that a “+”, “#”, or “-“ in the package code indicates RoHS status only. Package drawings may show a different
suffix character, but the drawing pertains to the package regardless of RoHS status.
PACKAGE TYPE
PACKAGE CODE
OUTLINE NO.
LAND PATTERN NO.
8 SO
S8+4
21-0041
90-0096
8
µ
MAX
U8+1 21-0036 90-0092
16 SO
W16#H2
21-0042
90-0107
SO, μSOP
SQW /OUT
1
2
3
4
8
7
6
5
X1
X2
V
BAT
GND
V
CC
SCL
SDA
TOP VIEW
SCL
SDA
GND
V
BAT
SQW /OUT
Vcc
N.C.
N.C.
N.C.
N.C.
N.C.
N.C.
N.C.
N.C.
N.C.
N.C.
DS1338C
SO (300 mils)
TOP VIEW
DS1338
DS1338 I2C RTC with 56-Byte NV RAM
16 of 16
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim
reserves the right to change the circuitry and specifications without notice at any time.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
© 2012 Maxim Integrated Products Maxim is a registered trademark of Maxim Integrated Products, Inc.
REVISION HISTORY
REVISION
DATE
DESCRIPTION
PAGES
CHANGED
100108
Modified the Features bullet to indicate that battery-backed RAM has unlimited
writes.
1
Removed leaded part numbers from the Ordering Information table. 1
Removed the pullup resistor voltage spec from the Recommended DC
Operating Conditions table and added it to the pin descriptions.
2, 7
Updated the block diagram (Figure 3) to show that SQW is open drain.
5
Added the initial POR state for time and date registers in the Power Control
section.
8
Added text to explain the use of the oscillator bit to control battery current in the
Clock and Calendar section.
9
9/11
Updated the Absolute Maximum Ratings, Recommended DC Operating
Conditions, DC Electrical Characteristics, Power Control, Oscillator Circuit,
Table 3, Handling, PBB Layout, and Assembly, Thermal Information, and
Package Information.
2, 8, 10, 15
3/12 Corrected the CH bit POR condition from 1 to 0 in the Power Control section 8