HIP7010B datasheet(7/20 Pages) INTERSIL

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J1850 VPW Messaging

This section provides an introduction to J1850 multiplexed

communications. It is assumed that the user is or will

become familiar with the appropriate documents published

by the Society of Automotive Engineering (SAE). The follow-

ing discussion is not comprehensive.

Overview

The SAE J1850 Standard (Note 1) (J1850) establishes the

requirements for communications on a Class B multiplexed

wiring network for automotive applications. The J1850 docu-

ment details the requirements in a three layer description

which separately speciﬁes the characteristics of the

physical

layer, the data link layer, and the application layer. There are

several options within each layer which allows vehicle manu-

facturers to customize the network while still maintaining a

level of universality.

NOTE:

1. SAE J1850 Standard, Class B Data Communication Network

Interface, May 1994, Society of Automotive Engineers Inc.

The hardware of the Intersil HIP7010 provides features

which facilitate implementation of the 10.4Kbps Variable

Pulse Width Modulated (VPW) physical layer option of

J1850. In combination with a bus transceiver, such as the

Intersil J1850 Bus Transceiver HIP7020, and appropriate

software algorithms, the HIP7010 circuitry enables the

designer to completely implement a 10.4Kbps VPW Class B

Communications Network Interface per J1850. Features of

such an implementation include:

• Single Wire 10.4Kbps Communications

• Bit-by-Bit Bus Arbitration

• Industry Standard Protocol

• Message Acknowledgment (“In-Frame Response”) Capa-

bilities

• Exceptionally Tolerant of Clock Skew, System Noise, and

Ground Offsets

• Meets CARB and EPA Diagnostic Requirements

• Supports up to 32 Nodes

• Low Error Rates

• Excellent EMC Levels (when interfaced via Intersil J1850

Bus Transceiver HIP7020)

In addition to the standard J1850 features, the HIP7010 hard-

ware provides a high speed mode, (intended for receive only

use) which can signiﬁcantly enhance vehicle maintenance

capabilities. The high speed mode provides a 41.6Kbps com-

munications path to any node built with the HIP7010.

Anatomy of a J1850 VPW Message

All messages in a J1850 VPW system are sent along a single

wire, shared bus. At any given moment the bus can be in

either of two states:

active (high) or passive (low). Multiple

nodes are connected to the bus as a “wired-OR” network in

which the bus is high if

any one (or more) node is generating

an active output. The bus is only low when

no nodes are gen-

erating active outputs. It follows that, when no communica-

tions are taking place the bus will rest in the passive state. A

message begins when the bus is ﬁrst driven to the high state.

Each succeeding state transition (i.e., a change from active to

passive or passive to active) transfers one bit of information

(

symbol) until the message is complete and the bus once

again rests at the passive state. The interpretation of each

symbol in the message is dependent on its duration (and

state), hence, the descriptor Variable Pulse Width (VPW).

Each message has a beginning and an end, the span of

which encompasses the entire

message or frame (refer to

Figure 3). A frame consists of an active

start of frame (SOF)

symbol and a passive

end of frame (EOF) symbol sandwiched

around a series of byte sized (8-bit) groups of symbols. The

ﬁrst byte of the frame contents is always a

header byte, fol-

lowed by possibly additional header bytes, followed by one or

data bytes, followed by an integrity check byte (CRC

byte), followed by a passive

end of data (EOD) symbol, fol-

lowed by possibly one or more

in-frame-response (IFR) bytes.

To keep waiting times low, messages are limited to 12 bytes

total (including header, data, check, and IFR bytes). All mes-

sage bytes are transmitted most signiﬁcant bit (MSB) ﬁrst.

VPW Symbol Deﬁnitions

Within the J1850 scheme, symbols are deﬁned in terms of both

duration and state (passive or active). The duration is mea-

sured as the time between successive transitions. There is one

transition per symbol and one symbol per transition. The end of

one symbol marks the beginning of the next. Since the bus is

passive when a message begins and must return to that same

state when the message completes, all frames have an even

number of transitions and hence an even number of symbols.

There are unique deﬁnitions for data bit symbols (all the sym-

bols which occur within the header, data, and check bytes) and

protocol symbols (including SOF, EOD, and EOF). The duration

of each symbol is expressed in terms of VPW Timing Pulses

(TV values). Table 1 summarizes the TV deﬁnitions. Each TV is

speciﬁed in terms of a

nominal (or ideal) duration and a mini-

mum and maximum duration. The span between the minimum

and maximum limits accommodates system noise sources

such as node to node clock skew, ground offsets, clock jitter,

and electromechanical noise. There are no dead zones

between the maximum of one TV and the minimum of the next.

SOF

HEADER

DATA 1

DATA 2

CRC

EOD

EOF

FIGURE 3. TYPICAL J1850 VPW MESSAGE FRAME

HIP7010

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