General Description
The DS3231 is a low-cost, extremely accurate I2C real-
time clock (RTC) with an integrated temperature-
compensated crystal oscillator (TCXO) and crystal. The
device incorporates a battery input, and maintains accu-
rate timekeeping when main power to the device is inter-
rupted. The integration of the crystal resonator enhances
the long-term accuracy of the device as well as reduces
the piece-part count in a manufacturing line. The DS3231
is available in commercial and industrial temperature
ranges, and is offered in a 16-pin, 300-mil SO package.
The RTC maintains seconds, minutes, hours, day, date,
month, and year information. The date at the end of the
month 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. Two programmable time-of-
day alarms and a programmable square-wave output
are provided. Address and data are transferred serially
through an I2C bidirectional bus.
A precision temperature-compensated voltage refer-
ence and comparator circuit monitors the status of VCC
to detect power failures, to provide a reset output, and
to automatically switch to the backup supply when nec-
essary. Additionally, the RST pin is monitored as a
pushbutton input for generating a reset externally.
Applications
Servers Utility Power Meters
Telematics GPS
Features
♦Accuracy ±2ppm from 0°C to +40°C
♦Accuracy ±3.5ppm from -40°C to +85°C
♦Battery Backup Input for Continuous
Timekeeping
♦Operating Temperature Ranges
Commercial: 0°C to +70°C
Industrial: -40°C to +85°C
♦Low-Power Consumption
♦Real-Time Clock Counts Seconds, Minutes,
Hours, Day, Date, Month, and Year with Leap Year
Compensation Valid Up to 2100
♦Two Time-of-Day Alarms
♦Programmable Square-Wave Output
♦Fast (400kHz) I2C Interface
♦3.3V Operation
♦Digital Temp Sensor Output: ±3°C Accuracy
♦Register for Aging Trim
♦RST Input/Output
♦UL Recognized
DS3231
Extremely Accurate I2C-Integrated
RTC/TCXO/Crystal
______________________________________________ Maxim Integrated Products 1
Rev 2; 6/05
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.
Ordering Information
PART
TEMP RANGE
PIN-PACKAGE
TOP
MARK
DS3231S
0°C to +70°C
16 SO
DS3231
DS3231SN
-40°C to +85°C
16 SO
DS3231N
DS3231S+
0°C to +70°C
16 SO
DS3231+
DS3231SN+
-40°C to +85°C
16 SO
DS3231N+
Pin Configuration appears at end of data sheet.
Purchase of I2C components from Maxim Integrated Products, Inc., or one of its sublicensed Associated Companies, conveys a
license under the Philips I2C Patent Rights to use these components in an I2C system, provided that the system conforms to the I2C
Standard Specification as defined by Philips.
DS3231
VCC
SCL
RPU
RPU = tR/CB
RPU
INT/SQW
32kHz
VBAT
PUSHBUTTON
RESET
SDA
RST
N.C.
N.C.
N.C.
N.C.
VCC
VCC
GND
VCC
CPU
N.C.
N.C.
N.C.
N.C.
Typical Operating Circuit
+Denotes lead-free
DS3231
Extremely Accurate I2C-Integrated
RTC/TCXO/Crystal
2_____________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
RECOMMENDED DC OPERATING CONDITIONS
(TA= TMIN to TMAX, unless otherwise noted.) (Notes 1, 2)
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
absolute maximum rating conditions for extended periods may affect device reliability.
Voltage Range on VCC, VBAT, 32kHz, SCL, SDA, RST,
INT/SQW Relative to Ground.............................-0.3V to +6.0V
Operating Temperature Range
(noncondensing) .............................................-40°C to +85°C
Junction Temperature......................................................+125°C
Storage Temperature Range ...............................-40°C to +85°C
Lead Temperature
(Soldering, 10s).....................................................+260°C/10s
Soldering Temperature....................................See the Handling,
PC Board Layout, and Assembly section.
PARAMETER
SYMBOL
CONDITIONS
MIN TYP MAX
VCC 2.3 3.3 5.5 V
Supply Voltage VBAT 2.3 3.0 5.5 V
Logic 1 Input SDA, SCL VIH 0.7 x
VCC
VCC +
0.3 V
Logic 0 Input SDA, SCL VIL
-0.3
+0.3 x
VCC
V
Pullup Voltage
(SDA, SCL, 32kHz, INT/SQW) VPU VCC = 0V
5.5V
V
ELECTRICAL CHARACTERISTICS
(VCC = 2.3V to 5.5V, VCC > VBAT, TA= TMIN to TMAX, unless otherwise noted.) (Typical values are at VCC = 3.3V, VBAT = 3.0V, and
TA= +25°C, unless otherwise noted.) (Notes 1, 2)
PARAMETER
SYMBOL
CONDITIONS
MIN TYP MAX
UNITS
VCC = 3.63V
200
Active Supply Current ICCA (Notes 3, 4) VCC = 5.5V
300
µA
VCC = 3.63V
110
Standby Supply Current ICCS
I2C bus inactive, 32kHz
output on, SQW output off
(Note 4) VCC = 5.5V
170
µA
VCC = 3.63V
575
Temperature Conversion Current
ICCSCONV
I2C bus inactive, 32kHz
output on, SQW output off
VCC = 5.5V
650
µA
Power-Fail Voltage VPF
2.45 2.575 2.70
V
Logic 0 Output, 32kHz, INT/SQW,
SDA
VOL
IOL = 3mA
0.4
V
Logic 0 Output, RST
VOL
IOL = 1mA
0.4
V
Output Leakage Current 32kHz,
INT/SQW, SDA
ILO
Output high impedance
-1
0
+1 µA
Input Leakage SCL
ILI
-1
+1 µA
RST Pin I/O Leakage
IOL
RST high impedance (Note 5)
-200
+10
µA
VBAT Leakage Current
(VCC Active)
IBATLKG
25
100
nA
DS3231
Extremely Accurate I2C-Integrated
RTC/TCXO/Crystal
_____________________________________________________________________ 3
ELECTRICAL CHARACTERISTICS (continued)
(VCC = 2.3V to 5.5V, VCC > VBAT, TA= TMIN to TMAX, unless otherwise noted.) (Typical values are at VCC = 3.3V, VBAT = 3.0V, and
TA= +25°C, unless otherwise noted.) (Notes 1, 2)
PARAMETER
SYMBOL
CONDITIONS
MIN TYP MAX
UNITS
Output Frequency
fOUT
VCC = 3.3V or VBAT = 3.3V
32.768
kHz
0°C to +40°C
±2
Frequency Stability vs.
Temperature (Commercial)
∆f/fOUT
VCC = 3.3V or
VBAT = 3.3V,
aging offset = 00h
>40°C to +70°C
±3.5
ppm
-40°C to <0°C
±3.5
0°C to +40°C
±2
Frequency Stability vs.
Temperature (Industrial)
∆f/fOUT
VCC = 3.3V or
VBAT = 3.3V,
aging offset = 00h
>40°C to +85°C
±3.5
ppm
Frequency Stability vs. Voltage
∆f/V
1
ppm/V
-40°C
0.7
+25°C
0.1
+70°C
0.4
Trim Register Frequency
Sensitivity per LSB
∆f/LSB
Specified at:
+85°C
0.8
ppm
Temperature Accuracy Temp VCC = 3.3V or VBAT = 3.3V -3 +3 °C
First year
±1.0
Crystal Aging ∆f/f0After reflow,
not production tested 0–10 years
±5.0
ppm
ELECTRICAL CHARACTERISTICS
(VCC = 0V, VBAT = 2.3V to 5.5V, TA= TMIN to TMAX, unless otherw.ise noted.) (Note 1)
PARAMETER
SYMBOL
CONDITIONS
MIN TYP MAX
VBAT = 3.63V 70
Active Battery Current IBATA
EOSC = 0, BBSQW = 0,
SCL = 400kHz (Note 4) VBAT = 5.5V 150 µA
VBAT = 3.63V
0.84
3.0
Timekeeping Battery Current IBATT
EOSC = 0, BBSQW = 0,
EN32kHz = 1,
SCL = SDA = 0V or
S C L = S D A = VBAT ( Note 4)
VBAT = 5.5V 1.0 3.5
µA
VBAT = 3.63V 575
Temperature Conversion Current
IBATTC
EOSC = 0, BBSQW = 0,
SCL = SDA = 0V or
SCL = SDA = VBAT VBAT = 5.5V 650
µA
Data-Retention Current
IBATTDR
EOSC = 1, SCL = SDA = 0V, +25°C 100 nA
DS3231
Extremely Accurate I2C-Integrated
RTC/TCXO/Crystal
4_____________________________________________________________________
AC ELECTRICAL CHARACTERISTICS
(VCC = VCC(MIN) to VCC(MAX) or VBAT = VBAT(MIN) to VBAT(MAX), VBAT > VCC, TA= TMIN to TMAX, unless otherwise noted.) (Note 1)
PARAMETER
SYMBOL
CONDITIONS
MIN TYP MAX
UNITS
Fast mode
100
400
SCL Clock Frequency fSCL Standard mode 0 100 kHz
Fast mode 1.3
Bus Free Time Between STOP
and START Conditions tBUF Standard mode 4.7 µs
Fast mode 0.6
Hold Time (Repeated) START
Condition (Note 6)
tHD:STA
Standard mode 4.0 µs
Fast mode 1.3
Low Period of SCL Clock tLOW Standard mode 4.7 µs
Fast mode 0.6
High Period of SCL Clock tHIGH Standard mode 4.0 µs
Fast mode 0 0.9
Data Hold Time (Notes 7, 8)
tHD:DAT
Standard mode 0 0.9 µs
Fast mode
100
Data Setup Time (Note 9)
tSU:DAT
Standard mode
250
ns
Fast mode 0.6
Start Setup Time tSU:STA Standard mode 4.7 µs
Fast mode 300
Rise Time of Both SDA and SCL
Signals (Note 10) tRStandard mode
20 +
0.1CB1000
ns
Fast mode 300
Fall Time of Both SDA and SCL
Signals (Note 10) tFStandard mode
20 +
0.1CB
300 ns
Fast mode 0.6
Setup Time for STOP Condition
tSU:STO
Standard mode 4.7 µs
Capacitive Load for Each Bus
Line (Note 10) CB400 pF
Capacitance for SDA, SCL CI/O 10 pF
Pulse Width of Spikes That Must
Be Suppressed by the Input Filter
tSP 30 ns
Pushbutton Debounce PBDB
250
ms
Reset Active Time tRST
250
ms
Oscillator Stop Flag (OSF) Delay
tOSF (Note 11)
100
ms
Temperature Conversion Time tCONV
125
200 ms
POWER-SWITCH CHARACTERISTICS
(TA= TMIN to TMAX)
PARAMETER
SYMBOL
CONDITIONS
MIN TYP MAX
UNITS
VCC Fall Time; VPF(MAX) to
VPF(MIN) tVCCF
300
µs
VCC Rise Time; VPF(MIN) to
VPF(MAX) tVCCR 0µs
Recovery at Power-Up tREC (Note 12)
250
300 ms
DS3231
Extremely Accurate I2C-Integrated
RTC/TCXO/Crystal
_____________________________________________________________________ 5
Pushbutton Reset Timing
tRST
PBDB
RST
Power-Switch Timing
VCC
tVCCF tVCCR
tREC
VPF(MAX)
VPF VPF
VPF(MIN)
RST
DS3231
Extremely Accurate I2C-Integrated
RTC/TCXO/Crystal
6_____________________________________________________________________
Data Transfer on I2C Serial Bus
SDA
SCL
tHD:STA
tLOW
tHIGH
tRtF
tBUF
tHD:DAT
tSU:DAT REPEATED
START
tSU:STA
tHD:STA
tSU:STO
tSP
STOP START
Note 1: Limits at -40°C are guaranteed by design and not production tested.
Note 2: All voltages are referenced to ground.
Note 3: ICCA—SCL clocking at max frequency = 400kHz.
Note 4: Current is the averaged input current, which includes the temperature conversion current.
Note 5: The RST pin has an internal 50kΩ(nominal) pullup resistor to VCC.
Note 6: After this period, the first clock pulse is generated.
Note 7: A device must internally provide a hold time of at least 300ns for the SDA signal (referred to the VIH(MIN) of the SCL signal)
to bridge the undefined region of the falling edge of SCL.
Note 8: The maximum tHD:DAT needs only to be met if the device does not stretch the low period (tLOW) of the SCL signal.
Note 9: A fast-mode device can be used in a standard-mode system, but the requirement tSU:DAT ≥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) + tSU:DAT = 1000 + 250 = 1250ns
before the SCL line is released.
Note 10: CB—total capacitance of one bus line in pF.
Note 11: The parameter tOSF is the period of time the oscillator must be stopped for the OSF flag to be set over the voltage range of
0.0V ≤VCC ≤VCC(MAX) and 2.3V ≤VBAT ≤3.4V.
Note 12: This delay applies only if the oscillator is enabled and running. If the EOSC bit is a 1, the startup time of the oscillator is
added to this delay.
DS3231
Extremely Accurate I2C-Integrated
RTC/TCXO/Crystal
_____________________________________________________________________ 7
STANDBY SUPPLY CURRENT
vs. SUPPLY VOLTAGE
DS3231 toc01
VCC (V)
ICCS (µA)
5.04.03.0
50
100
150
0
2.0
RST ACTIVE
SUPPLY CURRENT
vs. SUPPLY VOLTAGE
DS3231 toc02
VBAT (V)
IBAT (µA)
5.04.03.0
0.800
0.900
1.000
1.100
1.200
0.700
2.0
VCC = 0V
SUPPLY CURRENT
vs. TEMPERATURE
DS3231 toc03
TEMPERATURE (°C)
IBAT (µA)
80.060.040.020.00.0-20.0
0.700
0.800
0.900
1.000
0.600
-40.0
VBAT = 3.0V
FREQUENCY DEVIATION vs.
TEMPERATURE vs. AGING VALUE
DS3231 toc04
CRYSTAL AGING REGISTER VALUE
FREQUENCY DEVIATION (ppm)
96640 32-64 -32-96
-40
-30
-20
-10
0
10
20
30
40
50
60
-128 128
+85°C
-40°C
+70°C
0°C
+40°C
-40°C
+85°C
+25°C
+40°C
+25°C
+70°C0°C
Typical Operating Characteristics
(VCC = +3.3V, TA = +25°C, unless otherwise noted.)
DS3231
Extremely Accurate I2C-Integrated
RTC/TCXO/Crystal
8_____________________________________________________________________
Block Diagram
N
N
N
RST
VCC
32kHz
INT/SQW
CLOCK AND CALENDAR
REGISTERS
USER BUFFER
(7 BYTES)
I2C INTERFACE AND
ADDRESS REGISTER
DECODE
POWER CONTROL
VCC
VBAT
GND
SCL
SDA
TEMPERATURE
SENSOR
CONTROL LOGIC/
DIVIDER
PUSHBUTTON RESET;
SQUARE-WAVE BUFFER;
INT/SQW CONTROL
CONTROL AND STATUS
REGISTERS
OSCILLATOR AND
CAPACITOR ARRAY
X1
X2
DS3231
DS3231
Extremely Accurate I2C-Integrated
RTC/TCXO/Crystal
_____________________________________________________________________ 9
Pin Description
PIN NAME
FUNCTION
1
32kHz
32kHz Output. This open-drain pin requires an external pullup resistor. It may be left open if not used.
2V
CC DC Power Pin for Primary Power Supply. This pin should be decoupled using a 0.1µF to 1.0µF capacitor.
If not used, connect to ground.
3
INT/SQW
Active-Low Interrupt or Square-Wave Output. This open-drain pin requires an external pullup resistor. It may be
left open if not used. This multifunction pin is determined by the state of the INTCN bit in the Control Register
(0Eh). When INTCN is set to logic 0, this pin outputs a square wave and its frequency is determined by RS2
and RS1 bits. When INTCN is set to logic 1, then a match between the timekeeping registers and either of the
alarm registers activates the INT/SQW pin (if the alarm is enabled). Because the INTCN bit is set to logic 1
when power is first applied, the pin defaults to an interrupt output with alarms disabled.
4RST
Acti ve- Low Reset. Thi s p i n i s an op en- d r ai n i np ut/outp ut. It i nd i cates the status of V
C C
r el ati ve to the
V
P F
sp eci fi cati on. As V
C C
fal l s b el ow V
P F
, the RS T p i n i s d r i ven l ow . W hen V
C C
exceed s V
P F
, for tR S T, the RS T
p i n i s d r i ven hi g h i m p ed ance. The acti ve- l ow , op en- d r ai n outp ut i s com b i ned w i th a d eb ounced p ushb utton
i np ut functi on. Thi s p i n can b e acti vated b y a p ushb utton r eset r eq uest. It has an i nter nal 50kΩ nom i nal val ue
p ul l up r esi stor to V
C C
. N o exter nal p ul l up r esi stor s shoul d b e connected . If the cr ystal osci l l ator i s d i sab l ed , the
star tup ti m e of the osci l l ator i s ad d ed to the tR S T d el ay.
5–12
N.C. No Connection. Must be connected to ground.
13 GND Ground
14 VBAT
Backup Power-Supply Input. This pin should be decoupled using a 0.1µF to 1.0µF low-leakage capacitor.
If the I2C interface is inactive whenever the device is powered by the VB A T input, the decoupling capacitor is
not required. If VB A T is not used, connect to ground. UL recognized to ensure against reverse charging when
used with a lithium battery. Go to www.maxim-ic.com/qa/info/ul.
15 SDA Serial Data Input/Output. This pin is the data input/output for the I2C serial interface. This open-drain pin
requires an external pullup resistor.
16 SCL Serial Clock Input. This pin is the clock input for the I2C serial interface and is used to synchronize data
movement on the serial interface.
Detailed Description
The DS3231 is a serial RTC driven by a temperature-
compensated 32kHz crystal oscillator. The TCXO pro-
vides a stable and accurate reference clock, and
maintains the RTC to within ±2 minutes per year accu-
racy from -40°C to +85°C. The TCXO frequency output
is available at the 32kHz pin. The RTC is a low-power
clock/calendar with two programmable time-of-day
alarms and a programmable square-wave output. The
INT/SQW provides either an interrupt signal due to
alarm conditions or a square-wave output. The clock/cal-
endar provides seconds, minutes, hours, day, date,
month, and year information. The date at the end of the
month 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 internal registers are
accessible though an I2C bus interface.
A temperature-compensated voltage reference and
comparator circuit monitors the level of VCC to detect
power failures and to automatically switch to the back-
up supply when necessary. The RST pin provides an
external pushbutton function and acts as an indicator
of a power-fail event.
DS3231
Operation
The block diagram shows the main elements of the
DS3231. The eight blocks can be grouped into four
functional groups: TCXO, power control, pushbutton
function, and RTC. Their operations are described sep-
arately in the following sections.
32kHz TCXO
The temperature sensor, oscillator, and control logic
form the TCXO. The controller reads the output of the
on-chip temperature sensor and uses a lookup table to
determine the capacitance required, adds the aging
correction in AGE register, and then sets the capaci-
tance selection registers. New values, including
changes to the AGE register, are loaded only when a
change in the temperature value occurs, or when a
user-initiated temperature conversion is completed.
The temperature is read on initial application of VCC
and once every 64 seconds afterwards.
Power Control
This function is provided by a temperature-compensat-
ed voltage reference and a comparator circuit that
monitors the VCC level. When VCC is greater than VPF,
the part is powered by VCC. When VCC is less than VPF
but greater than VBAT, the DS3231 is powered by VCC.
If VCC is less than VPF and is less than VBAT, the
device is powered by VBAT. See Table 1.
To preserve the battery, the first time VBAT is applied to
the device, the oscillator will not start up until VCC is
applied, or until a valid I2C address is written to the
part. Typical oscillator startup time is less than one sec-
ond. Approximately 2 seconds after VCC is applied, or
a valid I2C address is written, the device makes a tem-
perature measurement and applies the calculated cor-
rection to the oscillator. Once the oscillator is running, it
continues to run as long as a valid power source is
available (VCC or VBAT), and the device continues to
measure the temperature and correct the oscillator fre-
quency every 64 seconds.
Pushbutton Reset Function
The DS3231 provides for a pushbutton switch to be con-
nected to the RST output pin. When the DS3231 is not in
a reset cycle, it continuously monitors the RST signal for a
low going edge. If an edge transition is detected, the
DS3231 debounces the switch by pulling the RST low.
After the internal timer has expired (PBDB), the DS3231
continues to monitor the RST line. If the line is still low, the
DS3231 continuously monitors the line looking for a rising
edge. Upon detecting release, the DS3231 forces the
RST pin low and holds it low for tRST.
The same pin, RST, is used to indicate a power-fail con-
dition. When VCC is lower than VPF, an internal power-fail
signal is generated, which forces the RST pin low. When
VCC returns to a level above VPF, the RST pin is held low
for approximately 250ms (tREC) to allow the power sup-
ply to stabilize. If the oscillator is not running (see the
Power Control section) when VCC is applied, tREC is
bypassed and RST immediately goes high.
Real-Time Clock
With the clock source from the TCXO, the RTC provides
seconds, minutes, hours, day, date, month, and year
information. The date at the end of the month is automati-
cally adjusted for months with fewer than 31 days, includ-
ing corrections for leap year. The clock operates in either
the 24-hour or 12-hour format with an AM/PM indicator.
The clock provides two programmable time-of-day
alarms and a programmable square-wave output. The
INT/SQW pin either generates an interrupt due to alarm
condition or outputs a square-wave signal and the
selection is controlled by the bit INTCN.
Address Map
Figure 1 shows the address map for the DS3231 time-
keeping registers. During a multibyte access, when the
address pointer reaches the end of the register space
(12h), it wraps around to location 00h. On an I2C
START or address pointer incrementing to location 00h,
the current time is transferred to a second set of regis-
ters. The time information is read from these secondary
registers, while the clock may continue to run. This
eliminates the need to reread the registers in case the
main registers update during a read.
I2C Interface
The I2C interface is accessible whenever either VCC or
VBAT is at a valid level. If a microcontroller connected to
the DS3231 resets because of a loss of VCC or other
event, it is possible that the microcontroller and DS3231
I2C communications could become unsynchronized,
e.g., the microcontroller resets while reading data from
the DS3231. When the microcontroller resets, the
Extremely Accurate I2C-Integrated
RTC/TCXO/Crystal
10 ____________________________________________________________________
SUPPLY CONDITION POWERED BY
VCC < VPF, VCC < VBAT VBAT
VCC < VPF, VCC > VBAT VCC
VCC > VPF, VCC < VBAT VCC
VCC > VPF, VCC > VBAT VCC
Table 1. Power Control
DS3231 I2C interface may be placed into a known state
by toggling SCL until SDA is observed to be at a high
level. At that point the microcontroller should pull SDA
low while SCL is high, generating a START condition.
Clock and Calendar
The time and calendar information is obtained by read-
ing the appropriate register bytes. Figure 1 illustrates the
RTC registers. The time and calendar data are set or ini-
tialized by writing the appropriate register bytes. The con-
tents of the time and calendar registers are in the
binary-coded decimal (BCD) format. The DS3231 can be
run in either 12-hour or 24-hour mode. Bit 6 of the hours
register is defined as the 12- 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 second 10-hour bit (20–23
hours). The century bit (bit 7 of the month register) is tog-
gled when the years register overflows from 99 to 00.
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, sec-
ondary (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 and when the register
pointer rolls over to zero. The time information is read
from these secondary registers, while the clock contin-
ues to run. This eliminates the need to reread the regis-
ters in case the main registers update during a read.
DS3231
Extremely Accurate I2C-Integrated
RTC/TCXO/Crystal
____________________________________________________________________ 11
Figure 1. Timekeeing Registers
Note: Unless otherwise specified, the registers’ state is not defined when power is first applied.
ADDRESS
BIT 7
MSB
BIT 6
BIT 5
BIT 4 BIT 3
BIT 2
BIT 1
BIT 0
LSB
FUNCTION RANGE
00H 0 10 Seconds Seconds Seconds 00–59
01H 0 10 Minutes Minutes Minutes 00–59
AM/PM
02H 0
12/24
10 Hour 10 Hour
Hour Hours
1–12 + AM/PM
00–23
03H 0 0 0 0 0 Day Day 1–7
04H 0 0 10 Date Date Date 00–31
05H
Century
00
10 Month
Month Month/
Century
01–12 +
Century
06H 10 Year Year Year 00–99
07H
A1M1
10 Seconds Seconds
Alarm 1 Seconds
00–59
08H
A1M2
10 Minutes Minutes
Alarm 1 Minutes
00–59
AM/PM
09H
A1M3 12/24
10 Hour 10 Hour
Hour
Alarm 1 Hours
1–12 + AM/PM
00–23
Day Alarm 1 Day 1–7
0AH
A1M4 DY/DT
10 Date Date Alarm 1 Date 1–31
0BH
A2M2
10 Minutes Minutes
Alarm 2 Minutes
00–59
AM/PM
0CH
A2M3 12/24
10 Hour 10 Hour
Hour
Alarm 2 Hours
1–12 + AM/PM
00–23
Day Alarm 2 Day 1–7
0DH
A2M4 DY/DT
10 Date Date Alarm 2 Date 1–31
0EH
EOSC BBSQW CONV
RS2 RS1
INTCN A2IE A1IE
Control —
0FH
OSF
00 0
EN32kHz BSY A2F A1F
Control/Status —
10H
SIGN DATA DATA DATA
DATA
DATA DATA DATA
Aging Offset —
11H
SIGN DATA DATA DATA
DATA
DATA DATA DATA
MSB of Temp —
12H
DATA
DATA
00 0000LSB of Temp —
DS3231
The countdown chain is reset whenever the seconds
register is written. Write transfers occur on the acknowl-
edge from the DS3231. 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.
Alarms
The DS3231 contains two time-of-day/date alarms.
Alarm 1 can be set by writing to registers 07h to 0Ah.
Alarm 2 can be set by writing to registers 0Bh to 0Dh.
The alarms can be programmed (by the alarm enable
and INTCN bits of the control register) to activate the
INT/SQW output on an alarm match condition. Bit 7 of
each of the time-of-day/date alarm registers are mask
bits (Table 2). When all the mask bits for each alarm
are logic 0, an alarm only occurs when the values in the
timekeeping registers match the corresponding values
stored in the time-of-day/date alarm registers. The
alarms can also be programmed to repeat every sec-
ond, minute, hour, day, or date. Table 2 shows the pos-
sible settings. Configurations not listed in the table will
result in illogical operation.
The DY/DT bits (bit 6 of the alarm day/date registers)
control whether the alarm value stored in bits 0 to 5 of
that register reflects the day of the week or the date of
the month. If DY/DT is written to logic 0, the alarm will
be the result of a match with date of the month. If
DY/DT is written to logic 1, the alarm will be the result of
a match with day of the week.
When the RTC register values match alarm register set-
tings, the corresponding Alarm Flag ‘A1F’ or ‘A2F’ bit is
set to logic 1. If the corresponding Alarm Interrupt
Enable ‘A1IE’ or ‘A2IE’ is also set to logic 1 and the
INTCN bit is set to logic 1, the alarm condition will acti-
vate the INT/SQW signal. The match is tested on the
once-per-second update of the time and date registers.
Extremely Accurate I2C-Integrated
RTC/TCXO/Crystal
12 ____________________________________________________________________
Table 2. Alarm Mask Bits
ALARM 1 REGISTER MASK BITS (BIT 7)
DY/DT
A1M4 A1M3 A1M2 A1M1 ALARM RATE
X1111Alarm once per second
X1110Alarm when seconds match
X1100Alarm when minutes and seconds match
X1000Alarm when hours, minutes, and seconds match
00000Alarm when date, hours, minutes, and seconds match
10000Alarm when day, hours, minutes, and seconds match
ALARM 2 REGISTER MASK BITS (BIT 7)
DY/DT
A2M4 A2M3 A2M2 ALARM RATE
X1 11Alarm once per minute (00 seconds of every minute)
X1 10Alarm when minutes match
X1 00Alarm when hours and minutes match
00 00Alarm when date, hours, and minutes match
10 00Alarm when day, hours, and minutes match
Special-Purpose Registers
The DS3231 has two additional registers (control and
status) that control the real-time clock, alarms, and
square-wave output.
Control Register (0Eh)
Bit 7: Enable Oscillator (EOSC). When set to logic 0,
the oscillator is started. When set to logic 1, the oscilla-
tor is stopped when the DS3231 switches to VBAT. This
bit is clear (logic 0) when power is first applied. When
the DS3231 is powered by VCC, the oscillator is always
on regardless of the status of the EOSC bit.
Bit 6: Battery-Backed Square-Wave Enable
(BBSQW). When set to logic 1 and the DS3231 is
being powered by the VBAT pin, this bit enables the
square-wave output when VCC is absent. When
BBSQW is logic 0, the INT/SQW pin goes high imped-
ance when VCC falls below the power-fail trip point.
This bit is disabled (logic 0) when power is first applied.
Bit 5: Convert Temperature (CONV). Setting this bit to
1 forces the temperature sensor to convert the temper-
ature into digital code and execute the TCXO algorithm
to update the capacitance array to the oscillator. This
can only happen during the idle period. The status bit,
BSY, prevents the bit from being set when BSY = 1.
The user should check the status bit BSY before forcing
the controller to start a new TCXO execution. A user-ini-
tiated temperature conversion does not affect the inter-
nal 64-second update cycle.
A user-initiated temperature conversion does not affect
the BSY bit for approximately 2ms. The CONV bit
remains at a 1 from the time it is written until the conver-
sion is finished, at which time both CONV and BSY go
to 0. The CONV bit should be used when monitoring
the status of a user-initiated conversion.
Bits 4 and 3: Rate Select (RS2 and RS1). These bits
control the frequency of the square-wave output when
the square wave has been enabled. The following table
shows the square-wave frequencies that can be select-
ed with the RS bits. These bits are both set to logic 1
(8.192kHz) when power is first applied.
Bit 2: Interrupt Control (INTCN). This bit controls the
INT/SQW signal. When the INTCN bit is set to logic 0, a
square wave is output on the INT/SQW pin. When the
INTCN bit is set to logic 1, then a match between the
timekeeping registers and either of the alarm registers
activates the INT/SQW (if the alarm is also enabled).
The corresponding alarm flag is always set regardless
of the state of the INTCN bit. The INTCN bit is set to
logic 1 when power is first applied.
Bit 1: Alarm 2 Interrupt Enable (A2IE). When set to
logic 1, this bit permits the alarm 2 flag (A2F) bit in the
status register to assert INT/SQW (when INTCN = 1).
When the A2IE bit is set to logic 0 or INTCN is set to
logic 0, the A2F bit does not initiate an interrupt signal.
The A2IE bit is disabled (logic 0) when power is first
applied.
Bit 0: Alarm 1 Interrupt Enable (A1IE). When set to
logic 1, this bit permits the alarm 1 flag (A1F) bit in the
status register to assert INT/SQW (when INTCN = 1).
When the A1IE bit is set to logic 0 or INTCN is set to
logic 0, the A1F bit does not initiate the INT/SQW sig-
nal. The A1IE bit is disabled (logic 0) when power is
first applied.
DS3231
Extremely Accurate I2C-Integrated
RTC/TCXO/Crystal
____________________________________________________________________ 13
BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0
EOSC BBSQW CONV RS2 RS1 INTCN A2IE A1IE
RS2 RS1 SQUARE-WAVE OUTPUT
FREQUENCY
00 1Hz
01 1.024kHz
10 4.096kHz
11 8.192kHz
SQUARE-WAVE OUTPUT FREQUENCY
Control Register (0Eh)
DS3231
Status Register (0Fh)
Bit 7: Oscillator Stop Flag (OSF). A logic 1 in this bit
indicates that the oscillator either is stopped or was
stopped for some period and may be used to judge the
validity of the timekeeping data. This bit is set to logic 1
any time that the oscillator stops. The following are
examples of conditions that can cause the OSF bit to
be set:
1) The first time power is applied.
2) The voltages present on both VCC and VBAT are
insufficient to support oscillation.
3) The EOSC bit is turned off in battery-backed mode.
4) External influences on the crystal (i.e., noise, leak-
age, etc.).
This bit remains at logic 1 until written to logic 0.
Bit 3: Enable 32kHz Output (EN32kHz). This bit indi-
cates the status of the 32kHz pin. When set to logic 1,
the 32kHz pin is enabled and outputs a 32.768kHz
square-wave signal. When set to logic 0, the 32kHz pin
goes to a high-impedance state. The initial power-up
state of this bit is logic 1, and a 32.768kHz square-wave
signal appears at the 32kHz pin after a power source is
applied to the DS3231 (if the oscillator is running).
Bit 2: Busy (BSY). This bit indicates the device is busy
executing TCXO functions. It goes to logic 1 when the
conversion signal to the temperature sensor is asserted
and then is cleared when the device is in the 1-minute
idle state.
Bit 1: Alarm 2 Flag (A2F). A logic 1 in the alarm 2 flag
bit indicates that the time matched the alarm 2 regis-
ters. If the A2IE bit is logic 1 and the INTCN bit is set to
logic 1, the INT/SQW pin is also asserted. A2F is
cleared when written to logic 0. This bit can only be
written to logic 0. Attempting to write to logic 1 leaves
the value unchanged.
Bit 0: Alarm 1 Flag (A1F). A logic 1 in the alarm 1 flag
bit indicates that the time matched the alarm 1 regis-
ters. If the A1IE bit is logic 1 and the INTCN bit is set to
logic 1, the INT/SQW pin is also asserted. A1F is
cleared when written to logic 0. This bit can only be
written to logic 0. Attempting to write to logic 1 leaves
the value unchanged.
Crystal Aging
The crystal aging offset register provides an 8-bit code
to add to the codes in the capacitance array registers.
The code is encoded in two’s complement. One LSB
represents one small capacitor to be switched in or out
of the capacitance array at the crystal pins. The offset
register is added to the capacitance array register
under the following conditions: during a normal temper-
ature conversion, if the temperature changes from the
previous conversion, or during a manual user conver-
sion (setting the CONV bit). To see the effects of the
aging register on the 32kHz output frequency immedi-
ately, a manual conversion should be started after each
aging register change.
Positive aging values add capacitance to the array,
slowing the oscillator frequency. Negative values
remove capacitance from the array, increasing the
oscillator frequency.
The change in ppm per LSB is different at different tem-
peratures. The frequency vs. temperature curve is shift-
ed by the values used in this register. At +25°C, one LSB
typically provides about 0.1ppm change in frequency.
Extremely Accurate I2C-Integrated
RTC/TCXO/Crystal
14 ____________________________________________________________________
BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0
Sign Data Data Data Data Data Data Data
Crystal Aging Offset (10h)
BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0
OSF 0 0 0 EN32kHz BSY A2F A1F
Status Register (0Fh)
Temperature Registers (11h–12h)
Temperature is represented as a 10-bit code with a res-
olution of +0.25°C and is accessible at location 11h
and 12h. The temperature is encoded in two’s comple-
ment format. The upper 8 bits are at location 11h and
the lower 2 bits are in the upper nibble at location 12h.
Upon power reset, the registers are set to a default
temperature of 0°C and the controller starts a tempera-
ture conversion. New temperature readings are stored
in this register.
I2C Serial Data Bus
The DS3231 supports a bidirectional I2C bus and data
transmission protocol. A device that sends data onto
the bus is defined as a transmitter and a device receiv-
ing data is defined as a receiver. The device that con-
trols the message is called a master. The devices that
are controlled by the master are slaves. The bus must
be controlled by a master device that generates the
serial clock (SCL), controls the bus access, and gener-
ates the START and STOP conditions. The DS3231
operates as a slave on the I2C bus. Connections to the
bus are made through the SCL input and open-drain
SDA I/O lines. Within the bus specifications, a standard
mode (100kHz maximum clock rate) and a fast mode
(400kHz maximum clock rate) are defined. The DS3231
works in both modes.
The following bus protocol has been defined (Figure 2):
•Data transfer may 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 line 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 a 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 the START and
the 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, which is associ-
ated 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.
DS3231
Extremely Accurate I2C-Integrated
RTC/TCXO/Crystal
____________________________________________________________________ 15
BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0
Sign Data Data Data Data Data Data Data
Temperature Register (Upper Byte) (11h)
BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0
Data Data 0 00000
Temperature Register (Lower Byte) (12h)
DS3231
Figures 3 and 4 detail how data transfer is accom-
plished on the I2C bus. Depending upon the state of
the R/Wbit, two types of data transfer are possible:
Data transfer from a master transmitter to a slave
receiver. The first byte transmitted by the master is
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.
Data transfer from a slave transmitter to a master
receiver. The first byte (the slave address) is trans-
mitted by the master. The slave then returns an
acknowledge bit. Next follows a number of data
bytes transmitted by the slave to the master. 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 the serial clock puls-
es 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 will not be released. Data is transferred with the
most significant bit (MSB) first.
The DS3231 can operate in the following two modes:
Slave receiver mode (DS3231 write mode): Serial
data and clock are received through SDA and SCL.
After each byte is received, an acknowledge bit is
transmitted. START and STOP conditions are recog-
nized as the beginning and end of a serial transfer.
Address recognition is performed by hardware after
reception of the slave address and direction bit. The
slave address byte is the first byte received after the
master generates the START condition. The slave
address byte contains the 7-bit DS3231 address,
which is 1101000, followed by the direction bit (R/W),
which is 0 for a write. After receiving and decoding
the slave address byte, the DS3231 outputs an
Extremely Accurate I2C-Integrated
RTC/TCXO/Crystal
16 ____________________________________________________________________
STOP
CONDITION
OR REPEATED
START
CONDITION
REPEATED IF MORE BYTES
ARE TRANSFERED
ACK
START
CONDITION
ACK
ACKNOWLEDGEMENT
SIGNAL FROM RECEIVER
ACKNOWLEDGEMENT
SIGNAL FROM RECEIVER
SLAVE ADDRESS
MSB
SCL
SDA
R/W
DIRECTION
BIT
12 678 9 12 893–7
Figure 2. I2C Data Transfer Overview
AXXXXXXXXA1101000S 0 XXXXXXXX A XXXXXXXX A XXXXXXXX A P
<SLAVE
ADDRESS>
S = START
A = ACKNOWLEDGE
P = STOP
R/W = READ/WRITE OR DIRECTION BIT ADDRESS = D0H
<RW>
DATA TRANSFERRED
(X + 1 BYTES + ACKNOWLEDGE)
<DATA (n + X)><DATA (n + 1)><DATA (n)>
<WORD
ADDRESS (n)>
Figure 3. Slave Receiver Mode (Write Mode)
AXXXXXXXXA1101000S 1 XXXXXXXX A XXXXXXXX A XXXXXXXX A P
<SLAVE
ADDRESS>
S = START
A = ACKNOWLEDGE
P = STOP
A = NOT ACKNOWLEDGE
R/W = READ/WRITE OR DIRECTION BIT ADDRESS = D1H
<RW>
DATA TRANSFERRED
(X + 1 BYTES + ACKNOWLEDGE)
NOTE: LAST DATA BYTE IS FOLLOWED BY
A NOT ACKNOWLEDGE (A) SIGNAL
<DATA (n + X)><DATA (n + 2)><DATA (n + 1)>
<DATA (n)>
Figure 4. Slave Transmitter Mode (Read Mode)
acknowledge on SDA. After the DS3231 acknowl-
edges the slave address + write bit, the master
transmits a word address to the DS3231. This sets
the register pointer on the DS3231, with the DS3231
acknowledging the transfer. The master may then
transmit zero or more bytes of data, with the DS3231
acknowledging each byte received. The register
pointer increments after each data byte is trans-
ferred. The master generates a STOP condition to
terminate the data write.
Slave transmitter mode (DS3231 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.
Serial data is transmitted on SDA by the DS3231
while the serial clock is input on SCL. START and
STOP conditions are recognized as the beginning
and end of a serial transfer. Address recognition is
performed by hardware after reception of the slave
address and direction bit. The slave address byte is
the first byte received after the master generates a
START condition. The slave address byte contains
the 7-bit DS3231 address, which is 1101000, fol-
lowed by the direction bit (R/W), which is 1 for a
read. After receiving and decoding the slave
address byte, the DS3231 outputs an acknowledge
on SDA. The DS3231 then begins to transmit data
starting with the register address pointed to by the
register pointer. If the register pointer is not written to
before the initiation of a read mode, the first address
that is read is the last one stored in the register point-
er. The DS3231 must receive a not acknowledge to
end a read.
Handling, PC Board Layout,
and Assembly
The DS3231 package contains a quartz tuning-fork
crystal. Pick-and-place equipment can 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.
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 connect-
ed to ground.
Moisture-sensitive packages are shipped from the facto-
ry dry packed. Handling instructions listed on the pack-
age label must be followed to prevent damage during
reflow. See IPC/JEDEC J-STD-020 standard for moisture-
sensitive device (MSD) classifications and reflow pro-
files. Exposure to reflow is limited to 2 times maximum.
DS3231
Extremely Accurate I2C-Integrated
RTC/TCXO/Crystal
____________________________________________________________________ 17
DS3231
Extremely Accurate I2C-Integrated
RTC/TCXO/Crystal
18 ____________________________________________________________________
Chip Information
TRANSISTOR COUNT: 33,000
SUBSTRATE CONNECTED TO GROUND
PROCESS: CMOS
Thermal Information
Theta-JA: +73°C/W
Theta-JC: +23°C/W
16
15
14
13
12
11
10
9
1
2
3
4
5
6
7
8
32kHz SCL
SDA
VBAT
GND
N.C.
N.C.
N.C.
N.C.
TOP VIEW
SO
VCC
INT/SQW
N.C.
RST
N.C.
N.C.
N.C.
DS3231S
Pin Configuration
DS3231
Extremely Accurate I2C-Integrated
RTC/TCXO/Crystal
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 ____________________ 19
©2005 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products, Inc.
is a registered trademark of Dallas Semiconductor Corporation.
Package Information
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information,
go to www.maxim-ic.com/DallasPackInfo).
56-G4009-001.EPS