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General Description
The DS1371 is a 32-bit binary counter that is
designed to continuously count time in seconds.
An additional counter that can generate a
periodic alarm or serve as a watchdog timer is
also provided. If enabled as a watchdog timer,
the watchdog strobe input pin provides a
hardware reset of the counter. If disabled, this
counter can be used as 3 Bytes of general-
purpose RAM. A configurable output can be
used as an interrupt or provide a square wave at
one of four selectable frequencies. The device is
programmed serially through a 2-wire
bidirectional bus.
Applications
Servers
Point-of-Sale Equipment
Portable Instruments
Elapsed Time Measurements
Typical Configuration
Features
§ 32-Bit Binary Counter
§ 24-Bit Binary Counter Provides Periodic
Alarm, Watchdog Timer, or RAM
§ Strobe Input to Reset Watchdog Timer
§ Single Output Configurable as Interrupt or
Square Wave
§ 2-Wire Serial Interface
§ Low-Voltage Operation
§ Operating Temperature Range:
-40°C to +85°C
§ Available in 8-Pin mSOP
Ordering Information
PART TEMP RANGE
PIN-
PACKAGE
TOP
MARK
DS1371U -40°C to +85°C 8 µSOP DS1371
Pin Configuration
Package Dimension Information is available at:
www.maxim-ic.com/DallasPackInfo.
DS1371
2-Wire, 32-Bit Binary Counte
r
Watchdo
g
Clock
www.maxim-ic.com
TOP VIEW
µSOP
X1
X2
WDS
GND
1
2
3
4
8
7
6
5
VCC
SQW/INT
SCL
SDA
DS1371
DS1371
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ABSOLUTE MAXIMUM RATINGS
Voltage Range on VCC Pin Relative to Ground -0.3V to +6.0V
Voltage Range on SDA, SCL, and WDS Relative to Ground -0.3V to VCC +0.5V
Operating Temperature Range -40°C to +85°C
Storage Temperature Range -55°C to +125°C
Soldering Temperature See IPC/JEDEC J-STD-020A
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 can affect device reliability.
RECOMMENDED DC OPERATING CONDITIONS
(TA = -40°C to +85°C)
PARAMETER SYMBOL MIN TYP MAX UNITS
Supply Voltage (Note 1) VCC 1.7 3.3 5.5 V
Input Logic 1 (Notes 2, 3) VIH 0.7 VCC V
CC + 0.3 V
Input Logic 0 (Notes 2, 3) VIL -0.3 0.3VCC V
DC ELECTRICAL CHARACTERISTICS
(VCC = 1.7V to 5.5V, TA = -40°C to +85°C, unless otherwise noted.) (Note 4)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
Supply Voltage (Note 1) VCC 1.7 5.5 V
Oscillator Operating Voltage
Range (Note 1) VOSC 1.3 5.5 V
Input Leakage ILI (Note 2) 1 mA
I/O Leakage ILO (Note 3) 1 mA
VCC > 2V; VOL = 0.4V
SDA Logic 0 Output (Note 1) IOLSDA VCC < 2V; VOL = 0.2VCC 3 mA
VCC > 2V; VOL = 0.4V 3.0
1.7V < VCC < 2V;
VOL = 0.2VCC 3.0
mA
SQW/INT Logic 0 Output
(Note 1) IOLSQW
1.3V < VCC < 1.7V;
VOL = 0.2VCC 250
mA
Active Supply Current ICCA (Note 5) 100 150 mA
Timekeeping Current
(Oscillator Enabled, INTCN = 1) IOSC0 (Notes 6, 7) 800 nA
Timekeeping Current
(Oscillator Enabled, INTCN = 0) IOSC1 (Notes 6, 7) 1300 nA
Data Retention Current
(Oscillator Disabled) IDDR (Note 6) 50 nA
DS1371
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AC ELECTRICAL CHARACTERISTICS
(VCC = 1.7V to 5.5V, TA = -40°C to +85°C, unless otherwise noted.) (Note 8)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
Fast mode 100 400
SCL Clock Frequency (Note 9) fSCL Standard mode 0 100 kHz
Fast mode 1.3 Bus Free Time Between STOP and
START Conditions tBUF Standard mode 4.7 ms
Fast mode 0.6 Hold Time (repeated) START Condition
(Note 10) tHD:STA Standard mode 4.0 ms
Fast mode 1.3
Low Period of SCL Clock tLOW Standard mode 4.7 ms
Fast mode 0.6
High Period of SCL Clock tHIGH Standard mode 4.0 ms
Fast mode 0 0.9
Data Hold Time (Notes 11, 12) tHD:DAT Standard mode 0 0.9 ms
Fast mode 100
Data Setup Time (Note 13) tSU:DAT Standard mode 250 ns
Fast mode 0.6
Start Setup Time tSU:STA Standard mode 4.7 ms
Fast mode 20 +
0.1CB 300
Rise Time of Both SDA and SCL
Signals (Note 9) tR
Standard mode 20 +
0.1CB 1000
ns
Fast mode 20 +
0.1CB 300
Fall Time of Both SDA and SCL Signals
(Note 9) tF
Standard mode 20 +
0.1CB 300
ns
Fast mode 0.6
Setup Time for STOP Condition tSU:STO Standard mode 4.7 ms
Capacitive Load for Each Bus Line
(Note 7) CB 400 pF
Pulse Width of Spikes that Must be
Suppressed by the Input Filter (Note 14) TSP Fast mode 30 ns
Watchdog Strobe (WDS) Pulse Width tWDS 100 ns
Oscillator Stop Flag (OSF) Delay
(Note 8) tOSF 100 ms
Note 1: All voltages are referenced to ground.
Note 2: SCL and WDS only.
Note 3: SDA and SQW/INT.
Note 4: Limits at -40°C are guaranteed by design and not production tested.
Note 5: ICCA—SCL clocking at max frequency = 400kHz. WDS inactive.
Note 6: Specified with WDS input and 2-wire bus inactive, SCL = SDA = VCC.
Note 7: Measured with a 32.768kHz crystal attached to the X1 and X2 pins.
Note 8: The parameter tOSF is the period of time the oscillator must be stopped in order for the OSF flag to be set over the voltage range of 1.3V
≤ VCC ≤ VCCMAX.
Note 9: A fast mode device can be used in a standard mode system, but the requirement tSU: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 + tSU:DAT = 1000 + 250 = 1250ns before the SCL line is released.
Note 10: After this period, the first clock pulse is generated.
Note 11: A device must internally provide a hold time of at least 300ns for the SDA signal (referred to the VIHMIN of the SCL signal) in order to
bridge the undefined region of the falling edge of SCL.
Note 12: The maximum tHD:DAT has only to be met if the device does not stretch the LOW period (tLOW) of the SCL signal.
Note 13: CB—total capacitance of one bus line in pF.
Note 14: This parameter is not production tested.
OSCILLATOR FREQUENCY vs. VCC
DS1371 toc06
VCC (V)
FREQUENCY (Hz)
5.34.84.33.83.32.82.31.8
32767.61
32767.66
32767.71
32767.76
3.767.56
1.3
IOSC0 vs. WDS FREQUENCY
DS1371 toc05
WDS FREQUENCY (kHz)
SUPPLY CURRENT (µA)
800600400200
5
10
15
20
25
30
35
0
0 1000
ICCA vs. VCC
SQUARE-WAVE ON
10
15
20
25
30
35
40
45
50
55
60
65
70
75
5
0
DS1371 toc04
TEMPERATURE ( C)
SUPPLY CURRENT (µA)
806040200-20-40
IOSC0 vs. TEMPERATURE
VCC = 3.3V
DS1371 toc03
TEMPERATURE ( C)
SUPPLY CURRENT (nA)
806040200-20
550
600
650
700
750
500
-40
IOSC1 vs. VCC
SQUARE-WAVE ON
DS1371 toc02
VCC (V)
SUPPLY CURRENT (nA)
5.04.53.5 4.02.5 3.02.0
450
500
550
600
650
700
750
800
850
900
950
400
1.5 5.5
IOSCO vs. VCC
SQUARE-WAVE OFF
DS1371 toc01
VCC (V)
SUPPLY CURRENT (nA)
5.04.54.03.53.02.52.0
350
400
450
500
550
600
300
1.5 5.5
DS1371
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Typical Operating Characteristics
(VCC = 3.3V, TA = +25°C, unless otherwise noted)
DS1371
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Figure 1. Timing Diagram
Figure 2. Functional Diagram
DS1371
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PIN DESCRIPTION
PIN NAME FUNCTION
1, 2 X1, X2
These signals are connections for a standard 32.768kHz quartz crystal. The internal
oscillator circuitry is designed for operation with a crystal having a specified load
capacitance (CL) of 6pF.
For more information about crystal selection and crystal layout considerations, refer to
Application Note 58: Crystal Considerations with Dallas Real-Time Clocks. The
DS1371 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.
3 WDS
Watchdog Input. A positive edge-triggered hardware interrupt input that restarts the
watchdog counter when this signal transitions from a low to a high. While WDS remains
at a static low or high, the watchdog counter continues to decrement.
4 GND Ground
5 SDA 2-Wire Serial Data. Input/output for 2-wire data.
6 SCL 2-Wire Serial Clock. Input for 2-wire clock.
7 SQW/INT Square-Wave/Interrupt Output. This pin is used to output the programmable square-
wave or alarm interrupt signal. It is open drain and requires an external pullup resistor.
8 VCC Supply Voltage Terminal
Detailed Description
The DS1371 is a real-time clock (RTC) with a 2-wire serial interface that provides elapsed seconds from
a user-defined starting point in a 32-bit counter (Figure 2). A 24-bit counter can be configured as either a
watchdog counter or as an alarm counter. An on-chip oscillator circuit uses a customer-supplied
32.768kHz crystal to keep time.
Oscillator Circuit
The DS1371 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 3 shows a functional schematic of the oscillator circuit. 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 is added by crystal frequency drift caused by temperature shifts. External
circuit noise coupled into the oscillator circuit can result in the clock running fast. Figure 4 shows a
typical PC board layout for isolation of the crystal and oscillator from noise. Refer to Application Note
58: Crystal Considerations with Dallas Real-Time Clocks for detailed information.
DS1371
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Table 1. Crystal Specifications*
PARAMETER SYMBOL
MIN TYP MAX UNITS
Nominal Frequency FO 32.768 kHz
Series Resistance ESR 45 kΩ
Load Capacitance CL 6 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 3. Oscillator Circuit Showing Internal Bias Network
Figure 4. Layout Example
DS1371
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Address Map
Table 2 shows the address map for the registers of the DS1371. During a multibyte access, when the
address pointer reaches the end of the register space (08h), it wraps around to location 00h. On a 2-wire
START, STOP, or address pointer incrementing to location 00h, the current time is transferred to a
second set of registers. 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 of an update of the main registers
during a read.
Table 2. Address Map
ADDRESS BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0 FUNCTION
00H TOD COUNTER BYTE 0 Time-of-Day
Counter
01H TOD COUNTER BYTE 1 Time-of-Day
Counter
02H TOD COUNTER BYTE 2 Time-of-Day
Counter
03H TOD COUNTER BYTE 3 Time-of-Day
Counter
04H WD/ALM COUNTER BYTE 0 Watchdog/Alarm
Counter
05H WD/ALM COUNTER BYTE 1 Watchdog/Alarm
Counter
06H WD/ALM COUNTER BYTE 2 Watchdog/Alarm
Counter
07H EOSC WACE WD/
ALM 0 INTCN RS2 RS1 AIE Control
08H OSF 0 0 0 0 0 0 AF Status
Note: Unless otherwise specified, the state of the registers are not defined when power is first applied.
DS1371
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Time-of-Day Counter
The time-of-day counter is a 32-bit up counter. The contents can be read or written by accessing the
address range 00h–03h. When the counter is read, the current time of day is latched into a register, which
is output on the serial data line while the counter continues to increment. Writing to the counter resets the
countdown chain for the time-of-day counter (Figure 2). The watchdog countdown chain is unaffected. If
the square-wave output is enabled and is set to 1Hz, the output resets when the countdown chain is reset.
Because the other square-wave frequencies are derived before the section of the countdown chain that is
reset, the other frequencies are unaffected by a write to the time-of-day counter.
Watchdog/Alarm Counter
The watchdog/alarm counter is a 24-bit down counter. The contents can be read or written by accessing
the address range 04h–06h. When this counter is written, both the counter and a seed register are loaded
with the written value. The countdown chain for the watchdog counter is reset when the counter is
written. The time-of-day countdown chain is unaffected. When the counter is to be reloaded, it uses the
value in the seed register. In alarm mode, when the counter is read, the current counter value is latched
into a register, which is output on the serial data line while the counter continues to decrement. In
watchdog mode, reading any of the watchdog registers reloads the seed value.
If the counter is not needed, it can be disabled and used as a 24-bit cache of general-purpose RAM by
setting the WACE bit in the control register to logic 0. If all 24 bits of the watchdog/alarm counter are
written to a 0 when WACE = 1, the counter is disabled and the AF bit is not set.
When the WD/ALM bit in the control register is set to a logic 0, the WD/ALM counter decrements every
second, until it reaches 0. At this point, the AF bit in the status register is set, the counter is reloaded, and
restarted.
When the WD/ALM bit is set to a logic 1, the WD/ALM counter decrements every 1/4096 of a second
(approximately every 244µs) until it reaches 0, sets the AF bit in the status register, and stops. If AIE = 1
and INTCN = 1, the SQW/INT pin pulses low for 250ms. The pulse cannot be truncated by writing either
AF or AIE to a 0 during the low time of the SQW/INT pin. At the end of the 250ms pulse, the AF bit is
cleared to a 0 and the SQW/INT pin becomes high impedance. The WD/ALM counter can be reloaded and
restarted before the counter reaches 0 by:
1) Reading or writing any of the WD/ALM counter registers (WDS must be low).
2) A low-to-high transition on the WDS pin.
Note: WDS must be low when configuring the watchdog.
DS1371
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Special Purpose Registers
The DS1371 has two additional registers (07h–08h) that control the WD/ALM counter, square-wave
output, and interrupts.
Control Register (07h)
Bit # 7 6 5 4 3 2 1 0
Name EOSC WACE WD/ALM 0 INTCN RS2 RS1 AIE
Default 0 0 0 0 0 1 1 0
Bit 7/Enable Oscillator (EOSC). When set to logic 0, the oscillator is started. When set to logic 1, the
oscillator is stopped. When this bit is set to a logic 1, the oscillator is stopped and the DS1371 is placed
into a low-power standby mode (IDDR). This bit is clear (logic 0) when power is first applied.
Bit 6/WD/ALM Counter Enable (WACE). When set to logic 1, the WD/ALM counter is enabled. When
set to logic 0, the WD/ALM counter is disabled, and the 24-bits can be used as general-purpose RAM.
This bit is clear (logic 0) when power is first applied.
Bit 5/WD/ALM Counter Select (WD/ALM). When set to logic 0, the counter decrements every second
until it reaches 0 and is then reloaded and restarted. When set to logic 1, the WD/ALM counter
decrements every 1/4096 of a second (approximately every 244µs) until it reaches 0, sets the AF bit in the
status register, and stops. If any of the WD/ALM counter registers are read or written, or a rising edge on
the WDS pin occurs before the counter reaches 0, the counter is reloaded and restarted. This bit is clear
(logic 0) when power is first applied.
Bit 3/Interrupt Control (INTCN). This bit controls the SQW/INT signal. When the INTCN bit is set to
logic 0, a square wave is output on the SQW/INT pin whose frequency is defined by bits RS2 and RS1.
The oscillator must also be enabled for the square wave to be output. When the INTCN bit is set to logic
1, then this permits the alarm flag bit in the status register to assert SQW/INT (provided that the alarm is
also enabled). The alarm flag is always set, regardless of the state of the INTCN bit. The INTCN bit is set
to logic 0 when power is first applied.
Bits 2, 1/Rate Select 1 and 2 (RS2 and RS1). These bits control the frequency of the square-wave output
when the square wave has been enabled. Table 3 shows the square-wave frequencies that can be selected
with the RS bits. These bits are both set (logic 1) when power is first applied.
Bit 0/Alarm Interrupt Enable (AIE). When set to logic 1, this bit permits the AF bit in the status
register to assert SQW/INT (when INTCN = 1). When set to logic 0 or INTCN is set to logic 0, the AF bit
does not initiate the SQW/INT signal. If the WD/ALM bit is set to a logic 1 and the AF flag is set, writing
AIE to a 0 does not truncate the 250ms pulse on the SQW/INT pin. The AIE bit is at logic 0 when power
is first applied.
Table 3. Square-Wave Output Frequency
RS2 RS1
SQUARE-WAVE OUTPUT
FREQUENCY
0 0 1Hz
0 1 4.096kHz
1 0 8.192kHz
1 1 32.768kHz
DS1371
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Status Register (08h)
Bit # 7 6 5 4 3 2 1 0
Name OSF 0 0 0 0 0 0 AF
Default 1 0 0 0 0 0 0 —
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 can 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:
· The first time power is applied.
· The voltage present on VCC is insufficient to support oscillation.
· The EOSC bit is turned off.
· External influences on the crystal (e.g., noise, leakage, etc.)
· This bit remains at logic 1 until written to logic 0.
Bit 0/Alarm Flag (AF). A logic 1 in the AF bit indicates that the WD/ALM counter reached 0. If INTCN
is set to a 1, and WD/ALM is set to 0 and AIE is set to 1, the SQW/INT pin goes low and stays low until
AF is cleared. AF is cleared when written to logic 0. This bit can only be written to logic 0. Attempting to
write logic 1 leaves the value unchanged. If INTCN and WD/ALM are set to 1 and the AIE is set to 1, the
SQW/INT pin pulses low for 250ms when the WD/ALM counter reaches 0 and sets AF = 1. At the
completion of the 250ms pulse, the DS1371 clears the AF bit to a 0. If the 250ms pulse is active, writing
AF to a 0 does not truncate the pulse.
2-Wire Serial Interface
The DS1371 supports a bidirectional 2-wire bus and data transmission protocol. A device that sends data
onto the bus is defined as a transmitter and a device receiving data as a receiver. The device that controls
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
generates the START and STOP conditions. The DS1371 operates as a slave on the 2-wire bus.
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 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.
DS1371
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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. Within the 2-wire bus specifications a standard mode (100kHz clock rate)
and a fast mode (400kHz clock rate) are defined.
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 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.
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. 2-Wire Data Transfer Overview
DS1371
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Figures 6 and 7 detail how data transfer is accomplished on the 2-wire bus. Depending upon the state of
the R/W bit, 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 transfer from a slave transmitter to a master receiver. The first byte (the slave address) is
transmitted 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 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.
The DS1371 can operate in the following two modes:
Slave receiver mode (DS1371 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
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 START condition is generated by the master. The slave address byte contains the 7-bit DS1371
address, which is 1101000, followed by the direction bit (R/W), which for a write is a 0. After receiving
and decoding the slave address byte the DS1371 outputs an acknowledge on SDA. After the DS1371
acknowledges the slave address + write bit, the master transmits a word address to the DS1371. This sets
the register pointer on the DS1371, with the DS1371 acknowledging the transfer. The master may then
transmit zero or more bytes of data, with the DS1371 acknowledging each byte received. The register
pointer increments after each byte is transferred. The master generates a STOP condition to terminate the
data write.
Slave transmitter mode (DS1371 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 DS1371 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 START condition is generated by the master. The slave address byte contains
the 7-bit DS1371 address, which is 1101000, followed by the direction bit (R/W), which for a read is a 1.
After receiving and decoding the slave address byte, the DS1371 outputs an acknowledge on SDA. The
DS1371 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 pointer. The DS1371 must receive a “not acknowledge” to end a read.
DS1371
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Figure 6. 2-Wire Write Protocol
Figure 7. 2-Wire Read Protocol
Chip Information
TRANSISTOR COUNT: 11,036
SUBSTRATE CONNECTED TO GROUND
PROCESS: CMOS
Package Information
Theta JA: 221 °C/W
Theta JC: 39 °C/W
