DS1621

2 of 16

102299

DETAILED PIN DESCRIPTION Table 1

PIN

SYMBOL

DESCRIPTION

1

SDA

Data input/output pin for 2-wire serial communication port.

2

SCL

Clock input/output pin for 2-wire serial communication port.

3

Thermostat output. Active when temperature exceeds TH; will reset when

temperature falls below TL.

4

GND

Ground pin.

5

A2

Address input pin.

6

A1

Address input pin.

7

A0

Address input pin.

8

Supply voltage input power pin. (2.7V – 5.5V)

OPERATION

Measuring Temperature

A block diagram of the DS1621 is shown in Figure 1. The DS1621 measures temperatures through the

use of an onboard proprietary temperature measurement technique. A block diagram of the temperature

measurement circuitry is shown in Figure 2.

The DS1621 measures temperature by counting the number of clock cycles that an oscillator with a low

temperature coefficient goes through during a gate period determined by a high temperature coefficient

oscillator. The 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 also preset to the –55

°C value, is

incremented indicating that the temperature is higher than –55

°C.

At the same time, the counter is preset with a value determined by the slope accumulator circuitry. This

circuitry is needed to compensate for the parabolic behavior of the oscillators over temperature. The

counter is then clocked again until it reaches 0. If the gate period is still not finished, then this process

repeats.

The slope accumulator is used to compensate for the nonlinear behavior of the oscillators over

temperature, yielding a high resolution temperature measurement. This is done by changing the number

of counts necessary for the counter to go through for each incremental degree in temperature. To obtain

the desired resolution, both the value of the counter and the number of counts per

°C (the value of the

slope accumulator) at a given temperature must be known.

This calculation is done inside the DS1621 to provide 0.5

°C resolution. The temperature reading is

provided in a 9–bit, two’s complement reading by issuing the READ TEMPERATURE command. Table

2 describes the exact relationship of output data to measured temperature. The data is transmitted through

the 2–wire serial interface, MSB first. The DS1621 can measure temperature over the range of –55

°C to

+125

°C in 0.5°C increments. For Fahrenheit usage a lookup table or conversion factor must be used.