Dallas Semiconductor DS1721U, DS1721S Datasheet

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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 150 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
T

OUT

- Thermostat Output Signal

A

0

- Chip Address Input

A

1

- Chip Address Input

A

2

- Chip Address Input

V

DD

- 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, T

OUT

, 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 T

OUT

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 1.2 s (MAX) to 150 ms (MAX). This is particularl y
useful in applications where temperature changes large magnitudes very rapidly.

Applications for DS1721 include personal computers/services, cellular telephones, office equipment, or
any microprocessor-based, thermally sensitive system.

DS1721

2-Wire Digital Thermomete

r

and Thermostat

www.dalsemi.com

*

DS1721U

8-PIN µ-SOP

V

DD

A

0

A

1

A

2

SDA

SCL

T

OUT

GND

1 8
2 7

3

6

4

5

DS1721S

8-Pin SOIC (150-mil)

SDA

SCL

T

OUT

GND

V

DD

A

0

A

1

A

2

DS1721

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DETAILED PIN DESCRIPTION Table 1

PIN SYMBOL DESCRIPTION

1SDAData input/output pin. For 2-wire serial communication port.
2SCLClock input/output pin. For 2-wire serial communication port.
3T

OUT

Thermostat output. Active when temperature exceeds TH; will reset when
temperature falls below TL.

4 GND

Ground pin.

5A2

Address input pin.

6A1

Address input pin.

7A0

Address input pin.

8VDDSupply 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-wir e interfa ce. The host can p eriodical l y 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
sensor will complete and store one temperature conversion and return to a low-power standby state. In
time-sensitive applications, the user can change the conversion resolution from 12 bits to 9, 10, or 11.
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 confi gu ration regi ster. 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 stat e 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-temper ature (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 (T

OUT

) becomes active when the

measured temperature exceeds the programmed TH, and remains latched in the active state until

DS1721

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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

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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 “OPERATION­Programming” 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

S262

5

2

4

2

3

2

2

2

1

2

0

MSB

MSb (UNIT = °C) LSb

2

-1

2

-2

2

-3

2

-4

0000LSB

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 sho wn 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
obtained.

DS1721

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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 “OPERATION­Programming” section for instructions in adjusting the thermostat setpoints and T

COM

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.