Datasheet

DS1992/DS1993
10 of 17
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
DS199x 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 DS199x 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 5kΩ. 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 120µs, 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 Ω
MOSFET
V
PUP
RX
TX
TX
RX
DATA
R
PU
5 µA
Typ.
BUS MASTER
DS199x 1-Wire PORT
TRANSACTION SEQUENCE
The protocol for accessing the DS199x 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