Dallas Semiconductor DS1821S, DS1821 Datasheet

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DS1821

Programmable Digital Thermo stat

FEATURES

 Requires no external components  Measures temperatures from -55°C to +125°C

in 1°C increments. Fahrenheit equivalent is 67°F to +257°F in 1.8°F increments DS1821

 Converts temperature to digital word in 1

second (max.)

 Thermostatic settings are user definable and

nonvolatile

 Available in 3-pin PR35, TO-220, and 8-pin

SOIC packages

 Applications include thermostatic controls,

industrial systems, consumer products, thermometers, or any thermally sensitive system

PIN ASSIGNMENT

DS1821

PR35 PACKAGE

See Mech. Drawing

Sectio

GND

DNC

DS1821S 8-Pin SOIC (208-mil)

See Mech Drawings Section

DS1821T

TO-220 PACKAGE

See Mech. Drawing

Sectio

DNC

PIN DESCRIPTION

GND - Ground DQ - Data In/Out V

- Power Supply Voltage +5V NC - No Connect DNC - Do Not Connect

DESCRIPTION

The DS1821 Programmable Digital Thermostat provides a thermal alarm logic output when the temperature of the device exceeds a user-defined temperature TH. The output remains active until the temperature drops below user defined temperature TL, allowing for any hysteresis necessary.

User-defined temperature settings are stored in nonvolatile memory, so parts can be programmed prior to insertion in a system. Communication to/from the DS1821 is accomplished through the DQ pin in a programming mode; this same pin is used in operation as the thermostat output.

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DS1821

DETAILED PIN DESCRIPTION

PR35

1 2/TAB 2 GND

23 1 DQData input/output pin for 1-Wire

TO-220

8-PIN SOIC SYMBOL DESCRIPTION

Ground.

programming

operation; Thermostat output pin in normal operation.

31 8 V

pin. +5V nominal.

DS1821S (8-pin SOIC): All pins not specified in this table are not to be connected.

OVERVIEW

The block diagram of Figure 1 shows the major components of the DS1821. The DS1821 has two operating modes: 1-Wire

The part arrives from the factory in 1-Wire mode. In this mode, the DQ pin of the DS1821 is configured as a 1-Wire communication port which would be connected to a microprocessor. The microprocessor will write data into the high and low temperature trigger registers, TH and TL, respectively, to set up the temperature limits for thermostat operation. In this mode, the result of the last temperature measurement made by the DS1821 may also be read directly by the microprocessor.

and thermostat.

Once temperature limits have been set and thermometer operation has been verified, the user may convert the DS1821 from a temperature sensor into a thermostat by writing to a bit in the status register. The part will then be configured for thermostat operation; this will also become the default power-up state for the device on the subsequent power up.

In thermostat mode, the DQ line becomes the thermostat output. This open drain output will go to its active state (programmable on/off) when the temperature of the DS1821 goes above the limit set in the TH register, and will remain active until the temperature goes below the limit programmed into the TL register.

If the user wishes to establish communications with the DS1821 once it has been placed in thermostat mode (for example, to change temperature trip point limits), this may be done by dropping V DD while holding the DQ line high, then clocking the DQ line 16 times. The part will then be placed into 1-Wire mode, and will allow the I/O functions of the device to operate, and reads from or writes to the memory are possible. This does not change the power-up state of the device, unless the user writes the configuration bit to do so.

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DS1821

DS1821 BLOCK DIAGRAM Figure 1

OPERATION

Temperature Measurement

The DS1821 measures temperatures through the use of an onboard proprietary temperature measurement technique. The temperature reading is provided in an 8-bit, two’s complement reading. Table 1 describes the exact relationship of output data to measured temperature. The data is transmitted serially over the 1-

Wire interface. The DS1821 can measure temperature over the range of -55°C to +125°C in 1°C

increments. For Fahrenheit usage, a lookup table or conversion factor must be used. Please refer to Application Note 105 for the method to increase the resolution of the DS1821.

TEMPERATURE/DATA RELATIONSHIPS Table 1

DIGITAL

OUTPUT

TEMPERATURE

+125°C 01111101 7Dh

+25°C 00011001 19h

0°C 00000000 00h

-1°C 11111111 FFh

-25°C 11100111 E7h

-55°C 11001001 C9h

(Binary)

DIGITAL

OUTPUT

(Hex)

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DS1821

)

Thermostat Controls

In its thermostat mode, the DS1821 functions as a thermostat with programmable hysteresis, as shown in Figure 2. Temperature conversions begin as soon as V

is applied to the device, and are continually

made, so that the thermostat output updates as soon as a temperature conversion is complete. This is approximately once every second.

When the DS1821’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 falls below the temperature stored in the low temperature trigger register (TL). In this way, any amount of hysteresis may be obtained.

The active state for the output is programmable by the user, so that an active state may either be a logic 1 (+5V, output transistor off) or a logic 0 (0V, output transistor on).

THERMOSTAT OUTPUT OPERATION Figure 2

DQ (Thermostat output, Active=High) V

=5 volts

TL TH T (°C

PROGRAMMING THE DS1821

To program the DS1821, it must be placed in 1-Wire mode. This mode is active when the device arrives from the factory. Once the part has been programmed, and if the user has set the power-up state to thermostat mode, 1-Wire mode may only be achieved by bringing the VDD pin low while holding the DQ line high, then clocking the DQ line 16 times.

The DS1821 has four internal registers that may be accessed through the DQ pin when the device is in 1Wire mode. These registers are the high temperature trigger (TH), low temperature trigger (TL), the actual measured temperature result, and the status register. The TH,TL, and status registers are all nonvolatile.

The DS1821 must have temperature settings resident in the TH and TL registers for thermostatic operation. The temperature result register and the thermostat limit registers (TH and TL) hold an 8-bit number in the two’s complement format described in Table 1.

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DS1821

A status register is also present, indicating the status of the thermostatic control, and allowing configuration of the output polarity as either active high or active low, and establishes the power-up state of the device.

The status register is defined as follows:

where,

DONE 1

VB THF TLF T/R POL 1SHOT

DONE = Conversion Done bit. 1 = Conversion complete, 0 = conversion in progress.

THF = Temperature High Flag. This bit will normally be 0, but will be set to 1 when the temperature exceeds the value of TH. It will remain 1 until reset by writing 0 into this location. This feature provides a method of determining if the DS1821 has ever been subjected to temperatures above TH. This bit is

nonvolatile, and is stored in E

memory.

TLF = Temperature Low Flag. This bit will normally be 0, but will be set to 1 when the temperature is lower than the value of TL. It will remain 1 until reset by writing 0 into this location. This feature provides a method of determining if the DS1821 has ever been subjected to temperatures below TL. This bit is nonvolatile, and is stored in E

NVB = Nonvolatile memory busy flag. 1 = Write to an E memory is not busy. A write to E

memory.

may take up to 10 ms.

memory cell in progress, 0 = nonvolatile

T/R* = Power-up mode bit. If set to a 1, the DS1821 will power up in a thermostat mode. If set to a 0, the device will power up in 1-Wire “read” mode. This bit is nonvolatile.

POL = Output Polarity Bit. 1 = active high, 0 = active low. This bit is nonvolatile.

1SHOT = One-Shot Mode. If 1SHOT is 1, the DS1821 will perform one temperature conversion upon reception of the Start Convert T protocol. If 1SHOT is 0, the DS1821 will continuously perform temperature conversions. Note that the One-Shot mode is available only when the device is in 1-Wire mode. In thermostat mode, the device continuously performs temperature conversions. This bit is nonvolatile.

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