DS1821

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TEMPERATURE MEASURING CIRCUITRY Figure 2

OPERATING MODES

The DS1821 has two operating modes: 1-Wire mode and thermostat mode. The power-up operating

mode is determined by the user-programmable T/R

¯ bit in the status/configuration register: if T/R¯ = 0 the

device powers-up in 1-Wire mode, and if T/R

¯ = 1 the device powers-up in thermostat mode. The T/R¯ bit

is stored in nonvolatile memory (EEPROM), so it will retain its value when the device is powered down.

1-Wire MODE

The DS1821 arrives from the factory in 1-Wire mode (T/R

¯ = 0). In this mode, the DQ pin of the DS1821

is configured as a 1-Wire port for communication with a microprocessor using the protocols described in

the 1-Wire BUS SYSTEM section of this datasheet. These communications can include reading and

reading the temperature, counter, and slope accumulator registers. Also in this mode, the microprocessor

can initiate and stop temperature measurements as described in the OPERATION – MEASURING

TEMPERATURE section of this datasheet.

¯, POL and 1SHOT) in the status/configuration

register are stored in nonvolatile EEPROM memory, so they will retain data when the device is powered

down. This allows these registers to be pre-programmed when the DS1821 is to be used as a standalone

thermostat.

Writes to these nonvolatile registers can take up to 10ms. To avoid data corruption, no

writes to nonvolatile memory should be initiated while a write to nonvolatile memory is in progress.

SLOPE ACCUMULATOR

PRESET

COMPARE

LOW TEMPERATURE

COEFFICIENT OSCILLATOR

COUNTER

PRESET

=0

TEMPERATURE REGISTER

HIGH TEMPERATURE

COEFFICIENT OSCILLATOR

COUNTER

=0

INC

STOP

SET/CLEAR

LSB