Data from the DS1821 can function as a standalone thermostat with a user programmable trigger point or as an 8-bit temperature sensor with a 1-wire digital interface. The trigger point for the thermostat is stored in non-volatile memory, so the DS1821 unit can be programmed for true stand-alone operation by plugging it into the system in advance. The DS1821 has an operating temperature range of -55°C to + 125 °C, 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, which also serves as the output of the thermostat.

The function of the temperature sensor

The core feature of the DS1821 is its proprietary direct digital temperature sensor, which provides 8-bit (1°C increments) temperature readings in the range of -55°C to +125°C. The block diagram of the temperature measurement circuit is shown. The circuit measures a gate period determined by the high temperature-to-common oscillator by counting the number of clock cycles generated by the oscillator with the temperature coefficient of low temperature (temperature-to-common) during the period. The cryo-CO counter is preset to the number of bases corresponding to -55 °C. If the gate of the counter reaches before the end of period 0, the temperature register, which is preset to -55°C, is incremented by 1 degree, and the counter is preset again to an initial value determined by the slope accumulator circuit. The preset counter value is unique for each temperature rise and compensates for the parabolic behavior of the oscillator over temperature. At this point, the counter is re-timed until it reaches 0, if the gate period has not ended, when the counter reaches 0, the temperature register is incremented again. The counter is preset, the process of 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 high temperature-common oscillator gate period. When this iterative process is complete, the value in the temperature register will indicate the temperature of the device in degrees Celsius.

operating mode

The DS1821 has two modes of operation: 1-wire mode and temperature-controlled mode. Power-up operation if T/R=0: Mode determined by user programmable T/R bits in status/configuration register ˉˉ Device power-up, in 1-wire mode, if T/R=1 device power-up, in constant temperature state . The T/R bits ˉˉ are stored in non-volatile memory (EEPROM) and therefore retain their value when the device is powered down.

1-wire mode

1-wire mode (T/R=0) is reached from the factory DS1821. In this mode, the DQ pins of the DS1821 are configured as 1-wire ports for use with microprocessors that are using the protocol described in the 1-Wire bus system section of this data sheet. These communications include reading and writing the high and low thermostat trip point registers (TH and TL) and configuration registers, and reading the temperature, counter and slope accumulator registers. Alternatively, in this mode, the microprocessor can initiate and stop temperature measurements as described in the operation - measure the temperature segment of the data sheet. After tH and TL some bits in the Status/Configuration register and certain bits (THF, TLF, T/R, POL and 1SHOT) ˉ registers are stored in non-volatile EEPROM memory, so they will retain data when the device is powered down. This allows these registers to be pre-programmed with data from the DS1821 to be used as an independent thermostat. Writing to these nonvolatile registers can take up to 10ms. To avoid data corruption, no writes to the non-volatile memory should be initiated at the same time the write to the non-volatile memory is in progress.

Thermostat Mode

In thermostat mode (T/R=1), the DS1821 can act as a stand-alone thermostat, programmed from trigger operation ˉ to TH and TL travel points, while the device is in 1-wire mode. In constant temperature mode, the DS1821 powers continuous temperature conversion, and the DQ pin is used as the output of the thermostat. Provides detailed operation on operating the thermostat output - the separate temperature section of this data sheet. Communication can be re-established with the DS1821 while it is in thermostat mode by pulling VDDto0V while the DQ line is held high, then switching to the DQ line low 16 times. This will temporarily place the DS1821 in 1-Wire mode, allowing communication with the microprocessor through the DQ pin of the DS1821. The I/O functions at this time can perform such as read/write of TH, TL or configuration register or read temperature register. Returning to the thermostat mode, the same process can be done (pull VDD to 0V while the DQ line is held high, then clock 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 bits in the ˉStatus/Configuration register. Also note that holding both VDD and DQ low for longer than about 10 seconds will cause the DS1821 to power down.