DS1992/DS1993

10of 10

1-WIRE BUS SYSTEM

The 1-Wire bus is a system that has a single bus master and one or more slaves. In all instances the

DS199_ is a slave device. The bus master is typically a microcontroller or PC. For small configurations

the 1-Wire communication signals can be generated under software control using a single port pin. For

multisensor networks, the DS2480B 1-Wire line driver chip or serial port adapters based on this chip

(DS9097U series) are recommended. This simplifies the hardware design and frees the microprocessor

from responding in real-time.

The discussion of this bus system is broken down into three topics: hardware configuration, transaction

sequence, and 1-Wire signaling (signal types and timing). The 1-Wire protocol defines bus transactions in

terms of the bus state during specific time slots that are initiated on the falling edge of sync pulses from

the bus master. For a more detailed protocol description, refer to Chapter 4 of the Book of DS19xx iButton

Standards.

HARDWARE CONFIGURATION

The 1-Wire bus has only a single line by definition; it is important that each device on the bus be able to

drive it at the appropriate time. To facilitate this, each device attached to the 1-wire bus must have open-

drain or three-state outputs. The 1-Wire port of the DS199_ is open drain with an internal circuit

equivalent to that shown in Figure 8. A multidrop bus consists of a 1-Wire bus with multiple slaves

attached. The 1-Wire bus has a maximum data rate of 16.3kbps and requires a pullup resistor of

approximately 5k

W. 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. If this does not occur and

the bus is left low for more than 120

ms, one or more of the devices on the bus may be reset.

Figure 8. HARDWARE CONFIGURATION

Open Drain

Port Pin

RX = RECEIVE

TX = TRANSMIT

100

W

MOSFET

RX

TX

TX

RX

DATA

5 µA

Typ.

BUS MASTER

DS199X 1-Wire PORT

TRANSACTION SEQUENCE

The protocol for accessing the DS199_ through the 1-Wire port is as follows:

§ Initialization

§ ROM Function Command

§ Memory Function Command

§ Transaction/Data

INITIALIZATION

All transactions on the 1-wire bus begin with an initialization sequence. The initialization sequence

consists of a reset pulse transmitted by the bus master followed by presence pulse(s) transmitted by the