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DS1721S-DS1721S/T&R-DS1721S+-DS1721U-DS1721U+
Digital Thermometer and Thermostat
FEATURES Temperature measurements require no
external components with ±1°C accuracy
Measures temperatures from -55°C to
+125°C; Fahrenheit equivalent is -67°F to
+257°F
Temperature resolution is configurable from 9
to 12 (default) bits (0.5°C to 0.0625°C
resolution)
Maximum conversion time (9-bit resolution)
of 93.75 ms
Thermostatic settings are user-definable
Data is read/written via a 2-wire serial
interface (open drain I/O lines); 3-bit
addressability
Wide power supply range (2.7V - 5.5V)
Applications include personal computers,
cellular telephones, office equipment, or any
thermally sensitive system
8-pin, 150-mil SOIC package and 8-pin µSOP
package
PIN ASSIGNMENT
PIN DESCRIPTION SDA - 2-Wire Serial Data Input/Output
SCL - 2-Wire Serial Clock
GND - Ground
TOUT - Thermostat Output Signal
A0 - Chip Address Input
A1 - Chip Address Input
A2 - Chip Address Input
VDD - Power Supply Voltage (+5V)
DESCRIPTION The DS1721 2-Wire Digital Thermometer and Thermostat provides 12-bit temperature readings, which
indicate the temperature of the device. Thermostatic settings and temperature readings are all
communicated to/from the DS1721 over a simple 2-wire serial interface. No additional components are
required; the device is truly a “temperature-to-digital” converter.
The DS1721 has three address bits that allow a user to multidrop up to eight sensors along the 2-wire bus,
greatly simplifying the bussing of distributed temperature sensing networks.
The thermal alarm output, TOUT, is active when the temperature of the device exceeds a user-defined
temperature TH. The output remains active until the temperature is equal to or below the user-defined
temperature TL, allowing for any hysteresis necessary. The active state of TOUT is configurable by the
user.
For applications that require faster conversion times, the user can adjust the readout resolution from 12 to
9 bits, effectively reducing the conversion time from 750ms (MAX) to 93.75 ms (MAX). This is
particularly useful in applications where temperature changes large magnitudes very rapidly.
DS1721
2-Wire Digital Thermometer
and Thermostat
DS1721U
8-Pin µ-SOP
VDD
A0
A1
A2
SDA
SCL
TOUT
GND
1 8
2 7
3 6
4 5
DS1721S
8-Pin SOIC (150-mil)
SDA
SCL
TOUT
GND
VDD
A0
A1
A2
DS1721
ORDERING INFORMATION
ORDERING
INFORMATION
PACKAGE
MARKING
DESCRIPTION DS1721S DS1721 DS1721 in 150 mil 8-pin SO
DS1721S+ DS1721 (See Note) DS1721 in Lead-Free 150 mil 8-pin SO
DS1721S/T&R DS1721 DS1721 in 150 mil 8-pin SO, 2500 Piece Tape-and-Reel
DS1721S+T&R DS1721 (See Note) DS1721 in Lead-Free 150 mil 8-pin SO, 2500 Piece Tape-
and-Reel
DS1721U 1721 DS1721 in 8-pin uSOP
DS1721U+ 1721 (See Note) DS1721 in Lead-Free 8-pin uSOP
DS1721U/T&R 1721 DS1721 in 8-pin uSOP, 3000 Piece Tape-and-Reel
DS1721U+T&R 1721 (See Note) DS1721 in Lead-Free 8-pin uSOP, 3000 Piece Tape-and-Reel
Note: A ”+” symbol will also be marked on the package near the Pin 1 indicator.
DETAILED PIN DESCRIPTION Table 1
PIN SYMBOL DESCRIPTION 1 SDA
Data input/output pin. For 2-wire serial communication port. 2 SCL
Clock input/output pin. For 2-wire serial communication port. 3 TOUT
Thermostat output. Active when temperature exceeds TH; will reset when temperature falls below TL.
4 GND
Ground pin. 5 A2
Address input pin. 6 A1
Address input pin. 7 A0
Address input pin. 8 VDD
Supply Voltage. 2.7V to 5.5V input power pin.
OVERVIEW A block diagram of the DS1721 is shown in Figure 1.
The DS1721 consists of five major components:
1. Precision temperature sensor
2. Analog-to-digital converter
3. 2-wire interface electronics
4. Data registers
5. Thermostat comparator
The factory-calibrated temperature sensor requires no external components. Upon power-up, the DS1721
is in an idle mode. Upon issuance of a Start Convert T command [51h], the DS1721 begins temperature
conversions with the default resolution of 12 bits (0.0625°C resolution). Following an 8-bit command
protocol, temperature data can be read over the 2-wire interface. The host can periodically read the value
in the temperature register, which contains the last completed conversion. As conversions are performed
in the background, reading the temperature register does not affect the conversion in progress.
In power-sensitive applications, the user can put the DS1721 in a “one-shot” mode, under which the
DS1721
Each additional bit of resolution approximately doubles the conversion time, so 9-bit conversions can be
performed in less than a quarter of a second.
This is accomplished by issuing a command protocol to the configuration register. It is recommended that
the user issue the command protocol to program the configuration register before any temperature
conversion commands are issued after power-up. This is due to the fact that the configuration data is
stored in volatile memory and will always power-up in the default state. The configuration register
defines the conversion mode, thermometer resolution/conversion time, and active state of the thermostat
comparator output. It also contains 3 status bits denoting the state of temperature conversions and
thermostat flags.
The user can also program over-temperature (TH) and under-temperature (TL) setpoints for thermostatic
operation. The power-up state of TH is 80°C and that for TL is 75°C. The result of each temperature
conversion is compared with these setpoints. The thermostat output (TOUT) becomes active when the
measured temperature exceeds the programmed TH, and remains latched in the active state until
temperature falls below TL. Thus, any hysteresis can be realized for fan control without external
components.
Digital data is written to/read from the DS1721 via a 2-wire interface, and all communication is MSb
first. Multipoint sensing is possible with the DS1721 by uniquely setting the 3-bit address of up to eight
parts on the 2-wire bus.
DS1721 FUNCTIONAL BLOCK DIAGRAM Figure 1 DS1721
OPERATION-Measuring Temperature The core of DS1721 functionality is its direct-to-digital temperature sensor. The DS1721 measures
temperature through the use of an on-chip temperature measurement technique with an operation range
from -55°C to +125°C. The device can be configured to perform continuous conversions with the most
recent result being stored in the thermometer register. The device can also be configured to perform a
single conversion, store the result, and return to a standby mode. Regardless of the mode used, the digital
temperature is retrieved from the temperature register using the Read Temperature (AAh) command, as
described in detail in the “Command Set” section. The DS1721 power-up default has the sensor set to
automatically perform 12-bit conversions continuously once the Start Convert T (51h) command is
issued. Details on how to change the settings after power-up are contained in the “OPERATION-
Programming” section.
The resolution of the temperature conversion can be configured as 9, 10, 11, or 12 bits, with 12-bit
readings as the default state. This equates to a temperature resolution of 0.5°C, 0.25°C, 0.125°C, or
0.0625°C. Following each conversion thermal data is stored in the thermometer register in two’s
complement format; the information can be retrieved over the 2-wire interface by issuing a Read
Temperature (AAh) command. Table 2 describes the exact relationship of output data to measured
temperature. The table assumes the DS1721 is configured for 12-bit resolution; if the device is configured
in a lower resolution mode, insignificant bits will contain zeros. The data is transmitted serially over the
2-wire serial interface, MSb first. The MSb of the temperature register contains the “sign” (S) bit,
denoting whether the temperature is positive or negative. For Fahrenheit usage, a lookup table or
conversion routine must be used.
TEMPERATURE/DATA RELATIONSHIPS Table 2 S 26 25 24 23 22 21 20 MSB
MSb (UNIT = °C) LSb -1 2-2 2-3 2-4 0 0 0 0 LSB
TEMP DIGITAL OUTPUT
(Binary)
DIGITAL OUTPUT
(Hex) +125°C 0111 1101 0000 0000 7D00h
+25.0625°C 0001 1001 0001 0000 1910h
+10.125°C 0000 1010 0010 0000 0A20h
+0.5°C 0000 0000 1000 0000 0080h
+0°C 0000 0000 0000 0000 0000h
-0.5°C 1111 1111 1000 0000 FF80h
-10.125°C 1111 0101 1110 0000 F5E0h
-25.0625°C 1110 0110 1111 0000 E6F0h
-55°C 1100 1001 0000 0000 C900h
OPERATION-Thermostat Control In its operating mode, the DS1721 functions as a thermostat with programmable hysteresis, as shown in
Figure 2. The thermostat output updates as soon as a temperature conversion is complete. When the
DS1721’s temperature meets or exceeds the value stored in the high temperature trip register (TH), the
output becomes active, and will stay active until the temperature is equal to or below the temperature
stored in the low temperature trigger register (TL). In this way, any amount of hysteresis may be
DS1721
The active state for the totem-pole output is programmable by the user. The power-up default of the
DS1721 has TH=80°C, TL=75°C, and the output state active high. Refer to the “OPERATION-
Programming” section for instructions in adjusting the thermostat setpoints and TCOM active state.
THERMOSTAT OUTPUT OPERATION Figure 2
OPERATION-Programming There are two areas of interest in programming the DS1721: the Configuration/Status register and the
thermostat setpoints. All programming is done via the 2-wire interface using the protocols discussed in
the “Command Set” section.
Configuration/Status Register Programming The configuration/status register is accessed via the Access Config (ACh) function command. Writing to
or reading from the register is determined by the R/W bit of the 2-wire control byte (See “2-wire Serial
Data Bus” section). Data is read from or written to the configuration register MSb first. The format of the
register is illustrated below in Figure 3. The effect each bit has on DS1721 functionality is described
below along with the power-up state of the bit and its ability to be read or written to. The entire register is
volatile and will always power-up in the default state. Therefore, it is recommended that the user issue
any configuration programming commands immediately after power is cycled, before any other
commands are issued.
CONFIGURATION/STATUS REGISTER Figure 3 DONE X X U R1 R0 POL 1SHOT
MSb LSb
1SHOT = Temperature Conversion Mode. If 1SHOT is "1", the DS1721 will perform and store one
temperature conversion upon reception of the Start Convert T (51h) command. If 1SHOT is "0", the
DS1721 will continuously perform temperature conversions and store the last completed result in the
Thermometer Register. The user has read/write access to the bit and the power-up default state is "0"
(continuous mode).
POL = TCOM Polarity Bit. If POL is "1", the active state of the TCOM output will be high. A "0" stored in this location sets the thermostat output to an active low state. The user has read/write access to the
POL bit, and the power-up default state is "1" (active high).
U = Undefined. This bit is used internally by the DS1721. It will be a "0" at power-up and will change to a "1" once the Start Convert T [51h] command is issued. This is a “Don’t Care” on a write; i.e. The
DS1721 will ignore writes to this location.
DS1721
R0, R1 = Thermometer Resolution Bits. Table 3 below defines the resolution of the digital thermometer, based on the settings of these two bits. There is a direct tradeoff between resolution and conversion time,
as depicted in the DC Electrical Characteristics: Digital Thermometer table. The designer has read/write
access to R0 and R1, and the default state is R0="1" and R1="1" (12-bit conversions).
THERMOMETER RESOLUTION CONFIGURATION Table 3
R1
R0
THERMOMETER
RESOLUTION
MAX
CONVERSION
TIME 0 0 9-BIT 93.75ms
0 1 10-BIT 187.5ms
1 0 11-BIT 375ms
1 1 12-BIT 750ms
X = Undefined. These bits are used internally by the DS1721.
DONE = Temperature Conversion Status Bit. "1" = conversion complete and "0" = conversion in
progress. The DONE bit is read-only, and the power-up state is "1". In the continuous conversion mode,
DONE = "0".
Thermostat Setpoints Programming The thermostat registers (TH and TL) define the setpoints for operation of the TCOM output. The
respective register can be accessed over the 2-wire bus via the Access TH (A1h) or Access TL (A2h)
commands. Reading from or writing to the respective register is controlled by the state of the R/W bit in
the 2-wire control byte (See “2-Wire Serial Data Bus” section).
The format of the TH and TL registers is a 12-bit 2’s complement representation of the temperature in °C.
The user can program the number of bits (9, 10, 11, or 12) for each TH and TL that correspond to the
thermometer resolution configuration chosen. If the 9-bit mode is chosen, for example, the 3 least
significant bits of TH and TL will be ignored by the thermostat comparator. The format for both TH and
TL is shown in Figure 4. The power-up default of TH is 80°C and that for TL is 75°C.
TEMPERATURE/DATA RELATIONSHIPS Figure 4 S 26 25 24 23 22 21 20 MSB
MSb (UNIT = °C) LSb -1 2-2 2-3 2-4 0 0 0 0 LSB
TEMP DIGITAL OUTPUT
(Binary)
DIGITAL OUTPUT
(Hex) +80°C 0101 0000 0000 0000 5000h
+75°C 0100 1011 0000 0000 4B00h
+10.125°C 0000 1010 0010 0000 0A20h
+0.5°C 0000 0000 1000 0000 0080h
+0°C 0000 0000 0000 0000 0000h
-0.5°C 1111 1111 1000 0000 FF80h
DS1721
If the user does not wish to take advantage of the thermostat capabilities of the DS1721, the 24 bits can be
used for general storage of system data that need not be maintained following a power loss. However, the
TOUT pin should be left floating if general data is stored in TH/TL.
2-WIRE SERIAL DATA BUS The DS1721 supports a bi-directional 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 which generates the serial clock (SCL), controls the bus access, and
generates the START and STOP conditions. The DS1721 operates as a slave on the 2-wire bus.
Connections to the bus are made via the open-drain I/O lines SDA and SCL.
The following bus protocol has been defined (See Figure 5): 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 will be 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 9th
bit.
Within the bus specifications a regular mode (100 kHz clock rate) and a fast mode (400 kHz clock rate)
are defined. The DS1721 works in both modes.
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. Of
course, setup and hold times must be taken into account. A master must signal an end of data to the slave
DS1721
DATA TRANSFER ON 2-WIRE SERIAL BUS Figure 5 Figure 6 details how data transfer is accomplished on the two-wire bus. Depending upon the state of the W bit, two types of data transfer are possible:
Data transfer from a master transmitter to a slave receiver. The 1st 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 1st 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 will not be released.
The DS1721 may operate in the following two modes:
Slave receiver 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.
Slave transmitter mode: The first byte is received and handled as in the slave receiver mode. However, in this mode, the direction bit will indicate that the transfer direction is reversed. Serial data is transmitted
on SDA by the DS1721 while the serial clock is input on SCL. START and STOP conditions are
recognized as the beginning and end of a serial transfer.
DS1721
2-WIRE SERIAL COMMUNICATION WITH DS1721 Figure 6
