Computer Circuit Board Chips Programmed Integrated Digital DS1821S

Computer Circuit Board Chips Programmed Integrated Digital DS1821S

DS1821S Programmable Digital Thermostat and Thermometer

FEATURES ƒ

Requires no external components ƒ

Unique 1-Wire® interface requires only one port pin for communication ƒ

Operates over a -55°C to +125°C (-67°F to +257°F) temperature range

ƒ Functions as a standalone thermostat with user-definable trip-points ƒ Provides 8-bit (1°C resolution) centigrade temperature measurements

ƒ Accuracy is ±1°C over 0°C to +85°C range ƒ Converts temperature to a digital word in 1 second (max)

Available in 3-pin PR35 and 8-pin SO packages ƒ

Applications include thermostatic controls, industrial systems, consumer products, thermometers, or any thermally sensitive system

PIN DESCRIPTION

GND - Ground

DQ - Data In/Out and Thermostat Output

VDD - Power Supply

Voltage NC - No Connect

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 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/DATA RELATIONSHIP Table 2

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 + COUNT PER C COUNT PER C COUNT REMAIN _ _ ( _ _ − _ ) High-resolution temperature readings cannot be used while in continuous conversion mode. Also, the Read Counter [A0h] and Load Counter [41h] commands must not be used while in continuous conversion mode.

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