DS1721

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The active state for the totem-pole output is programmable by the user. The power-up default of the

DS1721 has TH=80°C, TL=75°C, and the output state active high. Refer to the “OPERATION-

THERMOSTAT OUTPUT OPERATION Figure 2

OPERATION-Programming

There are two areas of interest in programming the DS1721: the Configuration/Status register and the

thermostat setpoints. All programming is done via the 2-wire interface using the protocols discussed in

the “Command Set” section.

Configuration/Status Register Programming

The configuration/status register is accessed via the Access Config (ACh) function command. Writing to

Data Bus” section). Data is read from or written to the configuration register MSb first. The format of the

register is illustrated below in Figure 3. The effect each bit has on DS1721 functionality is described

below along with the power-up state of the bit and its ability to be read or written to. The entire register is

volatile and will always power-up in the default state. Therefore, it is recommended that the user issue

any configuration programming commands immediately after power is cycled, before any other

commands are issued.

CONFIGURATION/STATUS REGISTER Figure 3

DONE

X

X

U

R1

R0

POL

1SHOT

MSb

LSb

1SHOT = Temperature Conversion Mode. If 1SHOT is "1", the DS1721 will perform and store one

temperature conversion upon reception of the Start Convert T (51h) command. If 1SHOT is "0", the

DS1721 will continuously perform temperature conversions and store the last completed result in the

Thermometer Register. The user has read/write access to the bit and the power-up default state is "0"

(continuous mode).

POL = TCOM Polarity Bit. If POL is "1", the active state of the TCOM output will be high. A "0" stored

in this location sets the thermostat output to an active low state. The user has read/write access to the

POL bit, and the power-up default state is "1" (active high).

U = Undefined. This bit is used internally by the DS1721. It will be a "0" at power-up and will change to

a "1" once the Start Convert T [51h] command is issued. This is a “Don’t Care” on a write; i.e. The

DS1721 will ignore writes to this location.