DS28EA00U+T&R【PDF 17/29 页】IC THERMOMETER 1-WIRE 8-USOP

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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 DS28EA00

is a slave device. The bus master is typically a micro-

controller. The discussion of this bus system is broken

down into three topics: hardware configuration, trans-

action sequence, and 1-Wire signaling (signal types

and timing). The 1-Wire protocol defines bus transac-

tions in terms of the bus state during specific time slots,

which are initiated on the falling edge of sync pulses

from the bus master.

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 DS28EA00 is

open drain with an internal circuit equivalent to that

shown in Figure 10.

A multidrop bus consists of a 1-Wire bus with multiple

slaves attached. The DS28EA00 supports both a stan-

dard and overdrive communication speed of 15.3kbps

(max) and 125kbps (max), respectively. Note that lega-

cy 1-Wire products support a standard communication

speed of 16.3kbps and overdrive of 142kbps. The

slightly reduced rates for the DS28EA00 are a result of

additional recovery times, which in turn are driven by a

1-Wire physical interface enhancement to improve

noise immunity. The value of the pullup resistor primari-

ly depends on the network size and load conditions.

The DS28EA00 requires a pullup resistor of 2.2k?/DIV>

(max) at any speed.

The idle state for the 1-Wire bus is high. If for any rea-

son 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

16祍 (overdrive speed) or more than 120祍 (standard

speed), one or more devices on the bus could be reset.

Transaction Sequence

The protocol for accessing the DS28EA00 through the

1-Wire port is as follows:

" Initialization

" ROM Function Command

" Control Function Command

" Transaction/Data

Initialization

All transactions on the 1-Wire bus begin with an initial-

ization sequence. The initialization sequence consists

of a reset pulse transmitted by the bus master followed

by presence pulse(s) transmitted by the slave(s). The

presence pulse lets the bus master know that the

DS28EA00 is on the bus and is ready to operate. For

more details, see the 1-Wire Signaling section.

1-Wire Digital Thermometer with

Sequence Detect and PIO

DS28EA00

Maxim Integrated

PUP

BUS MASTER

OPEN-DRAIN

PORT PIN

100?MOSFET

DATA

DS28EA00 1-Wire PORT

Rx = RECEIVE

Tx = TRANSMIT

Figure 10. Hardware Configuration

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