Rainbow Electronics DS1821 User Manual

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PRELIMINARY

DS1821

Programmable Digital Thermostat and

Thermomete

FEATURES

 Requires no external components  Unique 1-Wire

port pin for communication

 Operates over a -55°C to +125°C (67°F to

+257°F) temperature range

 Functions as a standalone thermostat with

user-definable trip-points

 Provides 8-bit (1°C resolution) centigrade

temperature measurements

 Accuracy is ±1°C over 0°C to +85°C range  Converts temperature to a digital word in 1

second (max)

 Available in 3-pin PR35 and 8-pin SOIC

packages

 Applications include thermostatic controls,

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

interface requires only one

PIN ASSIGNMENT

DALLAS

DS1821

GND

BOTTOM VIEW

DQ ND

NC NC

DS1821S

8-pin 208-mil SOIC

DS1821S

NC NC

PR35

(DS1821)

PIN DESCRIPTION

GND - Ground DQ - Data In/Out and Thermostat Output V

- Power Supply Voltage

NC - No Connect

DESCRIPTION

The DS1821 can function as a standalone thermostat with user-programmable trip-points or as 8-bit temperature sensor with a 1-wire digital interface. The thermostat trip-points are stored in nonvolatile memory, so DS1821 units can be programmed prior to system insertion for true standalone operation.

The DS1821 has an operating temperature range of –55°C to +125°C and is accurate to ±1°C over a range

of 0°C to +85°C. Communication with the DS1821 is accomplished through the open-drain DQ pin; this pin also serves as the thermostat output.

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DETAILED PIN DESCRIPTIONS Table 1

GIS

PR35 8-PIN

SOIC*

SYMBOL DESCRIPTION

DS1821

1 2 GND

Ground pin.

21 DQOpen drain data input/output pin – 1-wire operation; Open drain

thermostat output pin –thermostat operation.

38 V

Power supply pin.

*All pins not specified in this table are “No Connect” pins.

OVERVIEW

Figure 1 shows a block diagram of the DS1821 and pin descriptions are given in Table 1. The DS1821 can operate as a standalone thermostat with user-programmable trip-points or as 8-bit temperature sensor with a 1-wire digital interface. The open-drain DQ pin functions as the thermostat output for thermostat operation and as the data I/O pin for 1-wire communications. The 1-wire interface provides user access to the nonvolatile (EEPROM) thermostat trip-point registers (TH and TL), the status/configuration register, and the temperature register.

When configured as standalone thermostat, temperature conversions start immediately at power-up. In this mode, the DQ pin becomes active when the temperature of the DS1821 exceeds the limit programmed into the TH register, and remains active until the temperature drops below the limit programmed into the TL register.

The DS1821 uses Dallas’ exclusive 1-wire bus protocol that implements bus communication with one control signal. This system is explained in detail in the 1-WIRE BUS SYSTEM section of this datasheet.

DS1821 BLOCK DIAGRAM Figure 1

CONFIGURATION REGISTER

ND CONTROL LOGIC

TEMPERATURE SENSOR

TH REGISTER

TL RE

TER

DIGITAL

COMPARATOR/

LOGIC

4.7K

GND

DS1821

1-WIRE

INTERFACE

AND

I/O CONTROL

POWER

SUPPLY

SENSE

TEMPERATURE SENSOR FUNCTIONALITY

The core functionality of the DS1821 is its proprietary direct-to-digital temperature sensor, which

provides 8-bit (1°C increment) centigrade temperature readings over the range of -55°C to +125°C.

A block diagram of the temperature measurement circuitry is shown in Figure 2. This circuit measures the temperature by counting the number of clock cycles generated by an oscillator with a low temperature coefficient (temp-co) during a gate period determined by a high temp-co oscillator. The low temp-co

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DS1821

counter is preset with a base count that corresponds to –55°C. If the counter reaches 0 before the gate period is over, the temperature register, which is preset to –55°C, is incremented by one degree, and the

counter is again preset with a starting value determined by the slope accumulator circuitry. The preset counter value is unique for every temperature increment and compensates for the parabolic behavior of the oscillators over temperature.

At this time, the counter is clocked again until it reaches 0. If the gate period is not over when the counter reaches 0, the temperature register is incremented again. This process of presetting the counter, counting down to zero, and incrementing the temperature register is repeated until the counter takes less time to reach zero than the duration of the gate period of the high temp-co oscillator. When this iterative process is complete, the value in the temperature register will indicate the centigrade temperature of the device.

TEMPERATURE MEASURING CIRCUITRY Figure 2

SLOPE ACCUMULATOR

PRESET COMPARE

LOW TEMPERATURE

COEFFICIENT OSCILLATOR

HIGH TEMPERATURE

COEFFICIENT OSCILLATOR

COUNTER PRESET

INC

COUNTER

STOP

TEMPERATURE REGISTER

SET/CLEAR LSB

OPERATING MODES

The DS1821 has two operating modes: 1-wire mode and thermostat mode. The power-up operating mode

is determined by the user-programmable T/R¯ bit in the status/configuration register: if T/R¯ = 0 the device powers-up in 1-wire mode, and if T/R¯ = 1 the device powers-up in thermostat mode. The T/R¯ bit is

stored in nonvolatile memory (EEPROM), so it will retain its value when the device is powered down.

1-WIRE MODE

The DS1821 arrives from the factory in 1-wire mode (T/R¯ = 0). In this mode, the DQ pin of the DS1821

is configured as a 1-wire port for communication with a microprocessor using the protocols described in

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DS1821

the 1-WIRE BUS SYSTEM section of this datasheet. These communications can include reading and writing the high and low thermostat trip-point registers (TH and TL) and the configuration register, and reading the temperature, counter, and slope accumulator registers. Also in this mode, the microprocessor can initiate and stop temperature measurements as described in the OPERATION – MEASURING TEMPERATURE section of this datasheet.

The T

and T

registers and certain bits (THF, TLF, T/R¯, POL and 1SHOT) in the status/configuration

register are stored in nonvolatile EEPROM memory, so they will retain data when the device is powered down. This allows these registers to be pre-programmed when the DS1821 is to be used as a standalone thermostat. Writes to these nonvolatile registers can take up to 10ms. To avoid data corruption, no writes to nonvolatile memory should be initiated while a write to nonvolatile memory is in progress. Nonvolatile write status can be monitored by reading the NVB bit in the status/configuration register: NVB = 0 – a write to EEPROM memory is in progress, NVB = 0 – nonvolatile memory is idle.

THERMOSTAT MODE

In thermostat mode (T/R¯ = 1), the DS1821 can operate as a standalone thermostat that triggers according

to the TH and TL trip-points programmed while the device was in 1-wire mode. In thermostat mode the DS1821 powers-up performing continuous temperature conversions, and the DQ pin acts as the thermostat output. Detailed operation of the thermostat output is provided in the OPERATION – STANDALONE THERMOSTAT section of this datasheet.

Communications can be re-establish with the DS1821 while it is in thermostat mode by pulling VDD to 0V while the DQ line is held high, and then toggling the DQ line low 16 times as shown in Figure 12. This temporarily places the DS1821 in 1-wire mode, allowing microprocessor communication with the DS1821 via the DQ pin. At this time any I/O function can be performed, such as reading/writing the TH, TL or configuration registers or reading the temperature register. To return to thermostat mode, the same procedure can be performed (pulling VDD to 0V while the DQ line is held high, and then clocking the DQ line 16 times) or the power can be cycled. Note that temporarily putting the DS1821 into 1-wire mode

does not change the power-up mode of the device; this can only be changed by rewriting the T/R¯ bit in

the status/configuration register. Also note that holding both V approximately 10 seconds will cause the DS1821 to be powered down.

and DQ low for more than

OPERATION – MEASURING TEMPERATURE

DS1821 output temperature data is calibrated in degrees centigrade and is stored in two’s complement format in the 1-byte (8-bit) temperature register (see Figure 3), which the user can access when the

DS1821 is in 1-wire mode (T/R¯ = 0 in the status/configuration register). The sign bit (S) indicates if the

temperature is positive or negative; for positive numbers S = 0 and for negative numbers S = 1. Table 2 gives examples of digital output data and the corresponding temperature reading. For Fahrenheit measurements, a lookup table or conversion routine must be used.

The DS1821 can be configured by the user to take continuous temperature measurements (continuous conversion mode) or single measurements (one-shot mode). The desired configuration can be achieved by setting the nonvolatile1SHOT bit in the status/configuration register: 1SHOT = 0 – continuous conversion mode, 1SHOT = 1 – one-shot mode. Note that the 1SHOT setting only controls the operation of the device in 1-wire mode; in thermostat mode, continuous temperature conversions are started automatically at power-up.

In continuous conversion mode, the Start Convert T [EEh] command initiates continuous temperature conversions, which can be stopped using the Stop Convert T [22h] command. In one-shot mode the Start Convert T [EEh] command initiates a single temperature conversion after which the DS1821 returns to a low-power standby state. In this mode, the microprocessor can monitor the DONE bit in the

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DS1821

configuration register to determine when the conversion status: DONE = 0 ― conversion in progress, DONE = 1 ― conversion complete. The DONE bit does not provide conversion status in continuous conversion mode since measurements are constantly in progress (i.e., DONE will always be 0).

TEMPERATURE, TH and TL REGISTER FORMAT Figure 3

bit 7 bit 6 bit 5 bit 4 bit 3 bit 2 bit 1 bit 0

S262

TEMPERATURE/DATA RELATIONSHIP Table 2

TEMPERATURE DIGITAL OUTPUT

(Binary)

DIGITAL OUTPUT

(Hex)

+125°C* 0111 1101 7Dh

+85°C 0101 0101 55h +25°C 0001 1001 19h

0°C 0000 0000 00h

-1°C 1111 1111 FFh

-25°C 1110 0111 E7h

-55°C 1100 1001 C9h

HIGH-RESOLUTION TEMPERATURE READINGS

The user can calculate temperature values with higher than 8-bit resolution using the data remaining in the counter and slope accumulator when the temperature conversion is complete. To do this the user must first read the temperature from the 8-bit temperature register. This value is called TEMP_READ in the high-resolution equation (see Eq. 1). The 9-bit counter value must then be obtained by issuing the Read Counter [A0h] command. This value is the count remaining in the counter at the end of the gate period and is called COUNT_REMAIN in Eq. 1. Next the Load Counter [41h] command must be issued, which loads the 9-bit slope accumulator value into the counter register. The slope accumulator value (called COUNT_PER_C in Eq. 1) can then be read from the counter by again issuing the Read Counter [A0h] command. The slope accumulator value is called “COUNT_PER_C” because it represents the number of counts needed for an accurate measurement at a given temperature (i.e., the counts per degree C). The high-resolution temperature can then be calculated using Eq. 1:

Eq. 1) TEMPERATURE = TEMP_READ − 0.5 +

REMAINCOUNTCPERCOUNT

CPERCOUNT

)___( −

Additional information about high-resolution temperature calculations can be found in Application Note 105: “High Resolution Temperature Measurement with Dallas Direct-to-Digital Temperature Sensors”.

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DS1821

(DQ

)

OPERATION – THERMOSTAT

When the DS1821 is in thermostat mode (T/R¯ = 1 in the status/configuration register), temperature

conversions are performed continuously beginning at power-up (regardless of the value of the 1SHOT bit), and the DQ pin serves as the thermostat output. The DQ output will become active when the temperature of the DS1821 exceeds the user-defined limit in the TH register, and will remain active until the temperature drops below the user-defined limit in the TL register as illustrated in Figure 4. Thus, the user can select TH and TL to provide the desired amount of thermostat output hysteresis.

The user-defined 8-bit centigrade trip-point values (TH and TL) must be stored in two’s complement format as shown in Figure 3. The sign bit (S) indicates if the temperature is positive or negative; for positive numbers S = 0 and for negative numbers S = 1. The non-volatile T programmed when the DS1821 is in 1-wire mode as explained in the OPERATING MODES section of this datasheet. The DS1821 can be temporarily switched from thermostat mode to 1-wire mode to change the T

and TL values as also explained in the OPERATING MODES section.

The polarity (i.e., the active state) of the DQ output is user-selectable with the nonvolatile POL bit in the status/configuration register. DQ is active-high when POL = 1, and DQ is active-low when POL = 0.

Two bits in the status/configuration register, THF and TLF, provide additional thermostatic information. The value of these bits is normally 0. The THF (temperature high flag) bit will be set to 1 if the measured temperature is ever greater than the value in the TH register and will remain a 1 until the user rewrites the bit with a 0. The THL (temperature low flag) bit will be set to 1 if the temperature is ever lower than the value in the TL register and will remain a 1 until the user rewrites the bit with a 0. These bits provide a record of the device temperature relative to the thermostat trip-points over a period of time. They are stored in nonvolatile memory, so the data stored in THF and TLF can be analyzed after any number of power cycles. The THF and THL bits function in both 1-wire and thermostat mode.

and TL registers must be

DQ OPERATION IN THERMOSTATE MODE Figure 4

Operating Mode = Thermostat

POL=1

is active high

Temp (°C)

STATUS/CONFIGURATION REGISTER

The status/configuration register provides information to the user about conversion status, EEPROM activity and thermostat activity. It also allows the user to program various DS1821 options such as power-up operating mode, thermostat output polarity and conversion mode. The status/configuration register is arranged as shown in Figure 5 and detailed descriptions of each bit are provided in Table 3.

Note that the THF, THL T/R¯, POL and 1SHOT bits are stored in nonvolatile memory (EEPROM).

CONFIGURATION REGISTER Figure 5

bit 7 bit 6 bit 5 bit 4 bit 3 bit 2 bit 1 bit 0

DONE 1 NVB THF* THL*

*Stored in EEPROM

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T/R¯*

POL* 1SHOT*

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