DS1821

3 of 18

DS1821 BLOCK DIAGRAM Figure 1

TEMPERATURE SENSOR FUNCTIONALITY

The core functionality of the DS1821 is its proprietary direct-to-digital temperature sensor, which

provides 8-bit (1°C increment) centigrade temperature readings over the range of -55°C to +125°C.

A block diagram of the temperature measurement circuitry is shown in Figure 2. This circuit measures

the temperature by counting the number of clock cycles generated by an oscillator with a low temperature

coefficient (temp-co) during a gate period determined by a high temp-co oscillator. The low temp-co

counter is preset with a base count that corresponds to –55

°C. If the counter reaches 0 before the gate

period is over, the temperature register, which is preset to –55

°C, is incremented by one degree, and the

counter is again preset with a starting value determined by the slope accumulator circuitry. The preset

counter value is unique for every temperature increment and compensates for the parabolic behavior of

the oscillators over temperature.

At this time, the counter is clocked again until it reaches 0. If the gate period is not over when the counter

reaches 0, the temperature register is incremented again. This process of presetting the counter, 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 gate period of the high temp-co oscillator. When this iterative process

is complete, the value in the temperature register will indicate the centigrade temperature of the device.

4.7K

DQ

GND

TH REGISTER

TL REGISTER

CONFIGURATION REGISTER

AND CONTROL LOGIC

TEMPERATURE SENSOR

1-WIRE

INTERFACE

AND

I/O CONTROL

POWER

SUPPLY

SENSE

DIGITAL

COMPARATOR/

LOGIC

DS1821