Rainbow Electronics MAX31723 - Manual (Page 9)

MAX31722/MAX31723
Digital Thermometers and Thermostats
with SPI/3-Wire Interface
9
Thermostat
The devices’ thermostat can be programmed to power
up in either comparator mode or interrupt mode, which
activate and deactivate the open-drain thermostat output
(TOUT) based on user-programmable trip points (T
HIGH
and T LOW ). The T HIGH and T LOW registers contain
Celsius temperature values in two’s complement format
and are stored in EEPROM memory. As such, the values
are nonvolatile and can be programmed prior to install-
ing the devices for stand-alone operation.
The data format of the T
HIGH
and T
LOW
registers is
identical to that of the temperature register (Figure 5).
After every temperature conversion, the measurement is
compared to the values stored in the T
HIGH and T LOW
registers. The T HIGH register is assigned to address
locations 03h (LSB) and 04h (MSB), and the T LOW reg-
ister is assigned to address locations 05h (LSB) and
06h (MSB). The TOUT output is updated based on the
result of the comparison and the operating mode of the
devices. The number of T
HIGH and T LOW bits used dur-
ing the thermostat comparison is equal to the conversion
resolution set by the R1 and R0 bits in the configuration/
status register. For example, if the resolution is 9 bits,
only the nine MSBs of T
HIGH
and T
LOW
are used by the
thermostat comparator.
If the user does not wish to use the thermostat capa-
bilities of the devices, the TOUT output should be left
unconnected. Note that if the thermostat is not used, the
T
HIGH
and T
LOW
registers can be used for general stor-
age of system data.
Comparator Mode
When the thermostat is in comparator mode, TOUT
can be programmed to operate with any amount of
hysteresis. The TOUT output becomes active when the
measured temperature exceeds the T
HIGH value. TOUT
then stays active until the first time the temperature falls
below the value stored in T
LOW . Putting the devices into
shutdown mode does not clear TOUT in comparator
mode. Figure 6 illustrates thermostat comparator mode
operation.
Interrupt Mode
In interrupt mode, the TOUT output first becomes active
when the measured temperature exceeds the T
HIGH
value. Once activated, in continuous conversion mode
TOUT can only be cleared by either putting the devices
into shutdown mode or by reading from any register
(configuration/status, temperature, T
HIGH , or T LOW )
on the devices. In one-shot mode, TOUT can only be
cleared by reading from any register (configuration/
status, temperature, T
HIGH , or T LOW ) on the devices.
Figure 6. TOUT Operation Example
T HIGH
T LOW
TEMPERATURE
TOUT OUTPUT—COMPARATOR MODE
TOUT OUTPUT—INTERRUPT MODE
INACTIVE
INACTIVE
ACTIVE
ACTIVE
CONVERSIONS
ASSUMES A READ
HAS OCCURED