DS1821

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1-Wire BUS SYSTEM

The 1-Wire bus system uses a single bus master (i.e., a microprocessor) to control slave devices. The

DS1821 functions as a slave device when it is used in 1-Wire mode; however, since the DS1821 is not

addressable or multi-droppable, a single 1-Wire-mode DS1821 must be the only slave device on the bus.

All data and commands are transmitted least significant bit first over the 1-Wire bus.

The following discussion of the 1-Wire bus system is broken down into three topics: hardware

configuration, transaction sequence, and 1-Wire signaling (signal types and timing).

HARDWARE CONFIGURATION

The 1-Wire bus has by definition only a single data line. Each device (in this case, the master and one

DS1821) interfaces to the data line via an open drain or 3–state port. This allows each device to “release”

the data line when the device is not transmitting data so that the bus is available for use by the other

device. The 1-Wire port of the DS1821 (the DQ pin) is open drain with an internal circuit equivalent to

that shown in Figure 6.

The 1-Wire bus requires an external pullup resistor of approximately 5 k

Ω; thus, the idle state for the 1-

Wire bus is high. If for any reason a transaction needs to be suspended, the bus MUST be left in the idle

state if the transaction is to resume. Infinite recovery time can occur between bits so long as the 1-Wire

bus is in the inactive (high) state during the recovery period. If the bus is held low for more than 480

μs,

the DS1821 will be reset.

HARDWARE CONFIGURATION Figure 6

TRANSACTION SEQUENCE

The transaction sequence for accessing the DS1821 via the 1-Wire port is as follows:

• Initialization

• DS1821 Function Command

• Data Transmitted/Received

5 μA

Typ.

DS1821 1-WIRE PORT

100

Ω

MOSFET

4.7K

1-wire bus

DQ

Pin

Micro-

processor