The DS1721 2-Wire digital thermometer and temperature monitor provides a 12-bit temperature reading which indicates the temperature of the device. Thermostat settings and temperature readings are transmitted to/from the DS1721 via a simple 2-wire serial interface. No additional components are required; the device is a true "temperature-to-digital" converter. The DS1721 has three address bits, which allow the user to multipoint up to eight sensors along a 2-wire bus, greatly simplifying the bussing of distributed temperature sensing networks. Thermal alarm output, TOUT is active when the temperature of the device exceeds the user-defined temperature TH. The output remains active until this temperature is at or below the user-defined temperature TL, allowing for any hysteresis necessary. The active state of T OUT is configurable by the user as described above. For applications requiring faster conversion times, the user can adjust the resolution from 12 readings to 9 bits, effectively reducing the conversion time from 1.2 seconds (max) to 150 milliseconds (max). This is especially useful in applications where the temperature changes to a large extent very rapidly. Applications DS1721 include personal computers/services, cellular phones, office equipment, or any microprocessor-based, heat-sensitive system.

figure 1

At the heart of the DS1721's functionality is its direct digital temperature sensor. The DS1721 measures temperature by using an on-chip temperature measurement technique with an operating range from -55°C to +125°C. The device may be configured to perform successive conversions where the most recent result is stored in the thermometer register. The device can also be configured to perform a single conversion, store the result, and return to standby mode. Regardless of the mode used, the digital temperature is retrieved from the temperature register using the read temperature (AAh) command, as detailed in the "Command Set" section. By default, the DS1721 has been set to automatically perform 12-bit conversion by default when the DS1721 is powered on, and once the conversion starts, the T(51H) command is issued. Details on how to change the settings after power up are in the "Operation - Programming" section. The temperature conversion resolution can be configured to 9, 10, 11, or 12 bits, with 12-bit readings being the default. This is equivalent to 0.5°C, 0.25°C, 0.125°C, or a resolution of 0.0625°C for temperature. The thermal data following each conversion is stored in the thermometer register in two's complement format; this information can be retrieved by issuing a read temperature (AAh) command over the 2-wire interface. Table 2 describes the exact relationship of the output data to the measured temperature. This table assumes that the DS1721 is configured for 12-bit resolution; if the device is configured in a lower resolution mode, the trivial bits will contain zeros. A 2-wire serial interface where data is sent serially, MSB first. The MSB of the temperature register contains the "check-in" (S) bit that indicates whether the temperature is positive or negative. For Fahrenheit use, a look-up table or conversion program must be used.

Table 1

Table 2

To operate the thermostat in its operating mode, the DS1721 acts as a thermostat with programmable hysteresis, as shown in Figure 2. The thermostat output is updated as soon as a temperature conversion is complete. When the temperature of the DS1721 reaches or exceeds the value stored in the high temperature trip register (TH), the output becomes active and will remain active until the temperature equals or falls below the temperature stored in the low temperature trip register (TL). In this way, any amount of hysteresis can be obtained. The active state of the push-pull output is programmable by the user. The power-up default DS1721 has TH = 80°C, TL = 75°C, and the output state is active high. Please refer to the "Operation - Planning" section to adjust the thermostat setpoint and T command COM active status.

figure 2

Operational programming

There are two areas of interest in programming the DS1721: configuration/status registers and thermostat setpoints. All programming is performed through the 2-wire interface using the protocols discussed in the "Command Set" section.