DS1821
7 of 18
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 T
H
register, and will remain active until
the temperature drops below the user-defined limit in the T
L
register as illustrated in Figure 4. Thus, the
user can select T
H
and T
L
to provide the desired amount of thermostat output hysteresis.
The user-defined 8-bit centigrade trip-point values (T
H
and T
L
) 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
H
and T
L
registers must be
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
H
and T
L
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 T
H
register and will remain a 1 until the user rewrites the
bit with a 0. The TLF (temperature low flag) bit will be set to 1 if the temperature is ever lower than the
value in the T
L
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 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
T
L
T
H
Temp (°C)
DQ
Operating Mode = Thermostat
POL=1 (DQ is active high)
