DS1821

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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

format as shown in Figure 3. The sign bit (S) indicates if the temperature is positive or negative; for

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

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

bit with a 0. The TLF (temperature low flag) bit will be set to 1 if the temperature is ever lower than the

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 TLF bits function in both 1-Wire and thermostat mode.

DQ OPERATION IN THERMOSTATE MODE Figure 4

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, TLF 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*

TLF*

T/R

¯* POL* 1SHOT*

*

Stored in EEPROM

Temp (°C)

DQ

Operating Mode = Thermostat

POL=1 (DQ is active high)