DS1821

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Thermostat Controls

In its thermostat mode, the DS1821 functions as a thermostat with programmable hysteresis, as shown in

made, so that the thermostat output updates as soon as a temperature conversion is complete. This is

approximately once every second.

When the DS1821’s temperature meets or exceeds the value stored in the high temperature trip register

(TH), the output becomes active, and will stay active until the temperature falls below the temperature

stored in the low temperature trigger register (TL). In this way, any amount of hysteresis may be

obtained.

The active state for the output is programmable by the user, so that an active state may either be a logic 1

(+5V, output transistor off) or a logic 0 (0V, output transistor on).

THERMOSTAT OUTPUT OPERATION Figure 2

PROGRAMMING THE DS1821

To program the DS1821, it must be placed in 1-Wire mode. This mode is active when the device arrives

from the factory. Once the part has been programmed, and if the user has set the power-up state to

line high, then clocking the DQ line 16 times.

The DS1821 has four internal registers that may be accessed through the DQ pin when the device is in 1-

Wire mode. These registers are the high temperature trigger (TH), low temperature trigger (TL), the

actual measured temperature result, and the status register. The TH,TL, and status registers are all

nonvolatile.

The DS1821 must have temperature settings resident in the TH and TL registers for thermostatic

operation. The temperature result register and the thermostat limit registers (TH and TL) hold an 8-bit

number in the two’s complement format described in Table 1.

TL

TH

T (°C)