DS1621

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102299

THF

= Temperature High Flag. This bit will be set to “1” when the temperature is greater than or

equal to the value of TH. It will remain “1” until reset by writing “0” into this location or removing power

from the device. This feature provides a method of determining if the DS1621 has ever been subjected to

temperatures above TH while power has been applied.

TLF

= Temperature Low Flag. This bit will be set to “1” when the temperature is less than or equal

to the value of TL. It will remain “1” until reset by writing “0” into this location or removing power from

the device. This feature provides a method of determining if the DS1621 has ever been subjected to

temperatures below TL while power has been applied.

NVB

= Nonvolatile Memory Busy flag. “1” = Write to an E

nonvolatile memory is not busy. A copy to E

POL

= Output Polarity Bit. “1” = active high, “0” = active low. This bit is nonvolatile.

1SHOT = One Shot Mode. If 1SHOT is “1”, the DS1621 will perform one temperature conversion upon

receipt of the Start Convert T protocol.

If 1SHOT is “0”, the DS1621 will continuously perform

temperature conversions. This bit is nonvolatile.

For typical thermostat operation the DS1621 will operate in continuous mode. However, for applications

where only one reading is needed at certain times or to conserve power, the one–shot mode may be used.

conversion cycle when operating in one–shot mode.

2–WIRE SERIAL DATA BUS

The DS1621 supports a bidirectional 2–wire bus and data transmission protocol. A device that sends data

onto the bus is defined as a transmitter, and a device receiving data as a receiver. The device that controls

the message is called a “master." The devices that are controlled by the master are “slaves." The bus must

be controlled by a master device which generates the serial clock (SCL), controls the bus access, and

generates the START and STOP conditions. The DS1621 operates as a slave on the 2–wire bus.

Connections to the bus are made via the open–drain I/O lines SDA and SCL.

The following bus protocol has been defined (See Figure 4):

• Data transfer may be initiated only when the bus is not busy.

• During data transfer, the data line must remain stable whenever the clock line is HIGH. Changes in

the data line while the clock line is high will be interpreted as control signals.

Accordingly, the following bus conditions have been defined:

Bus not busy: Both data and clock lines remain HIGH.

Start data transfer: A change in the state of the data line, from HIGH to LOW, while the clock is HIGH,

defines a START condition.

Stop data transfer: A change in the state of the data line, from LOW to HIGH, while the clock line is

HIGH, defines the STOP condition.