DS1821 datasheet(7/16 Pages) DALLAS

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DS1821

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RETURNING TO 1-WIRE MODE FROM THERMOSTAT MODE

The operating mode of the DS1821 is determined at power-up, depending upon the setting of the T/ R bit.

If the T/ R bit is set to a 1, the DS1821 will power up in thermostat mode. In this mode, the device cannot

be written to or read from over the DQ line. However, it is possible to return to the 1-Wire “read” mode

temporarily, in cases where thermostat limits may need to be changed after insertion and use in a system.

To return to the 1-Wire “read” mode, the VDD pin of the DS1821 is brought to 0V while the DQ line is

held high. The DQ line must then be clocked low 16 times. After this is accomplished, the VDD line may

be brought high again, and the DS1821 will then be in 1-Wire “read” mode.

To toggle between modes, VDD is brought low while DQ is held high and then clocked 16 times. When

VDD is brought high again, the part will then be in thermostat mode again. This technique may be used to

toggle between the two operating modes of the DS1821 as often as required.

When both VDD and DQ are low for more than approximately 10 seconds, the part is powered down.

When powered up again, the part will begin operating in the mode set by T/R* bit (1=thermostat mode,

0=“read” mode).

DS1821 COMMAND SET Table 2

INSTRUCTION

DESCRIPTION

PROTOCOL

1-WIRE BUS DATA

AFTER ISSUING

PROTOCOL

TEMPERATURE CONVERSION COMMANDS

Start Convert T

Initiates temperature conversion.

EEh

idle

Stop Convert T

Halts temperature conversion.

22h

idle

Read Temperature Reads last converted temperature value

from temperature register.

AAh

THERMOSTAT COMMANDS

Write TH

Writes high temperature limit value into

TH register.

01h

Write TL

Writes low temperature limit value into

TL register.

02h

Read TH

Reads stored value of high temperature

limit from TH register.

A1h

Read TL

Reads stored value of low temperature

limit from TL register.

A2h

Write Status

Writes configuration data to

configuration register.

0Ch

Read Status

Reads configuration data from

configuration register.

ACh

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