DS1821

2 of 16

DETAILED PIN DESCRIPTION

PIN

PR35

PIN

TO-220

PIN

8-PIN

SOIC

SYMBOL

DESCRIPTION

1

2/TAB

2

GND

Ground.

23

1

DQ

Data input/output pin for 1-Wire

operation; Thermostat output pin in normal operation.

31

8

DS1821S (8-pin SOIC): All pins not specified in this table are not to be connected.

OVERVIEW

The block diagram of Figure 1 shows the major components of the DS1821. The DS1821 has two

The part arrives from the factory in 1-Wire mode. In this mode, the DQ pin of the DS1821 is configured

as a 1-Wire communication port which would be connected to a microprocessor. The microprocessor will

write data into the high and low temperature trigger registers, TH and TL, respectively, to set up the

temperature limits for thermostat operation. In this mode, the result of the last temperature measurement

made by the DS1821 may also be read directly by the microprocessor.

Once temperature limits have been set and thermometer operation has been verified, the user may convert

the DS1821 from a temperature sensor into a thermostat by writing to a bit in the status register. The part

will then be configured for thermostat operation; this will also become the default power-up state for the

device on the subsequent power up.

In thermostat mode, the DQ line becomes the thermostat output. This open drain output will go to its

active state (programmable on/off) when the temperature of the DS1821 goes above the limit set in the

TH register, and will remain active until the temperature goes below the limit programmed into the TL

register.

If the user wishes to establish communications with the DS1821 once it has been placed in thermostat

holding the DQ line high, then clocking the DQ line 16 times. The part will then be placed into 1-Wire

mode, and will allow the I/O functions of the device to operate, and reads from or writes to the memory

are possible. This does not change the power-up state of the device, unless the user writes the

configuration bit to do so.