The Use of CAN Based Networks in Automotive Real Time Systems

8/13/2019 The Use of CAN Based Networks in Automotive Real Time Systems

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The use of CAN based Networks in Automotive Real-

Time SystemsController Area Network a real time bus

Risn Howard

Bachelor of Engineering in Computer EngineeringUniversity of Limerick

Limerick!"!#$%studentmail&ul&ie

AbstractThis paper discusses the importance of the CAN bus

and why it is widely used today in both the automotive segment

where it originate and in industry and automation. This network

is very robust and uses arbitration to organize message delivery.

This minimizes the need for priority based queues at system level.eal time embedded systems and the need for a network like

CAN to implement them is discussed.

Keywords- real-time systems; embedded systems; automotive;

network; controller area network; vehicle; FleRay; Motorol

Interconnect; MI; Local Interconnect Network; LIN; in-vehicle

network; IVN; E!;

'& '()R*+UC)'*(

Real,time systems are of huge importance in all areas of theworld today& - real time system is one which must deliver atask in a timely manner. within a timing constraint& *ne of the

key areas where real,time systems are a necessity is in theautomotive industry& Like all everyday e/uipment. cars arenow full of electronics and em0edded systems& )he need forthese systems to 0e flawless is of huge importance in the safetyof their users&

''& *1ER1'E2

A. What is a Real Time System

- real time system is one which must deliver a task in a timely

manner. within a timing constraint& (ot only does a systemneed to run a control law with time constraints. it must also

schedule communications3 it has to deal with sending and

receiving messages according to deadlines& )iming constraintsfor real,time systems can 0e divided into two types3 hard andsoft& Hard real time systems are safety critical. if they do not

finish within their timing deadline serious conse/uences mayresult& 4oft real time systems on the other hand have deadlines

0ut it is not critical that they meet them& 5any em0edded

systems are hard real,time systems& +eadlines of 6o0s in anem0edded system are typically derived from the re/uired

responsiveness of the sensors and actuators monitored and

controlled 0y it&

B. Real Time Alications

Real-time applications can be divided into thefollowing four types according to their timing

attributes.a! "urely cyclic#

- purely cyclic application e7ecutes every task in the system

periodically& Even '8* operations are polled& 5oreover. itsdemands in resources do not vary significantly from period to

period&

b! $ostly cyclic#

5ost tasks in a mostly cyclic system e7ecute periodically& )hesystem must also respond to some e7ternal events

asynchronously. for e7ample fault recovery and e7ternalcommands&

c! Asynchronous % redictable#

5ost tasks are not periodic in applications such as multimediacommunication. radar signal processing. and tracking& )heduration 0etween consecutive e7ecutions of a task may vary

considera0ly. or the variations in the amounts of resourcesdemanded in different periods may 0e large& However. these

variations have either 0ounded ranges or known statistics&

d! Asynchronous % unredictable#

-pplications that react to asynchronous events and have tasks

with high run,time comple7ity 0elong to this type& -n e7ample

is intelligent real,time control systems&

C. The imortance of real time systems

-periodic or sporadic 6o0s are ones whose release times are

not known& -periodic 6o0s have either soft of no deadlines.whereas sporadic 6o0s have hard deadlines& )he processormust fit these 6o0s into idle spots9

'''& -U)*5*)'1ERE-L)'5E4:4)E54

Real time em0edded systems are critical in automo0iles& 'n

modern vehicles there could 0e up to ! electronic control

units ;ECU


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)here are many e7amples of ECU=s some common control

units are as follows>

&! 'n(ine control unit

)! Ariba( control unit *AC+!

,! oor control unit

! "owetrain control unit

/! Transmission control unit

0! Seed control unit1! Break control module *ABS or 'SC!

4ome of these ECU=s form independent su0systems 0ut for

others they need to communicate. and this communication isessential& )he way for the ECU=s to communicate is over and

em0edded network&

'1& C*55*((E)2*R?4'(-U)*5*B'LE4

)here are many different in,vehicle networks availa0le. such

as Controller -rea (etwork ;C-(


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5otorola 'nterconnect is a serial communications interface

using a single line from one 5aster to as many as ! slavedevices& )he 5' 0us is used to control 4mart 4witches.

5otors. 4ensors. and -ctuators& 't may also 0e used as

an -utomotive Busto drive 5irrors. 4eats. 2indow lifts orHead light levelers& )he 5' interface uses only ,wire to

send and receive data at a data rate of D kHK&

)he 5' 0us utiliKes a push8pull se/uence to transfer data0etween the master and the slaves& )he 5aster sends apush field to the slave devices connected to the 0us& )his

field contains data. plus the address of one of the slaves&)he 4lave addressed then responds to the received data.

transmitting the Aull field on the 5' Bus to the 5aster&)he data returned 0y the slave is likely to 0e status 0its

representing internal or e7ternal information& )he 5' 0ushas two states3 +ominant ;4tate < which is represented 0y

a ma7imum of & 1. and

Recessive ;4tate


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!igure "#$) The CAN node structure

B. Bit 'ncodin( % Synchronisation

Controller -rea (etworking is a very ro0ust and relia0le

communication network& (on Return to Gero 0it coding.

(RG. is used in C-(& 'n (RG encoding there are two signal

levels3 0it is represented 0y a high level and 0it 0y a lowlevel& )he level remains constant over the full time slot& *ne

0it is represented 0y one time slot& )o avoid a loss ofsynchroniKation in the signal due to successive =s or =s a

techni/ue called 0it stuffing is used& )he 0it stuffingtechni/ue that the C-( protocol utiliKes is a complimentary

0it is inserted after five consecutive 0its of e/ual value& -t thereceiver these 0its have to 0e un,stuffed so the original data is

processed& )his techni/ue is also a mechanism from detectingerrors at 0it level&

C. The CAN frame format

)he C-( protocol has two frame formats3 a 0ase frame

;@igure '1,


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the node is sending the message '+ it compares the 0it 0eing

sent with the 0it on the 0us& 'f at any stage this is different thenode will stop transmission to avoid collisions with another

C-( frame& )his lets the node know if the transmission of the

0it was successful or if it was overwritten3 writing a . thedominant state. overwrites a . the recessive state& Unlike

other networks. instead of producing a 6amming se/uence toprevent collisions. they are resolved 0y priority& )he 0us with

the highest priority wins ar0itration and this node can transmitthe message& )he process is constantly repeated so any

message that lost ar0itration in the last cycle will have thechance to send again. the message with the ne7t highest

priority will 0e transmitted the ne7t time around& )his entireprocess is implemented in hardware. no software intervention

is re/uired& )his mechanism is known as collision avoidancerather than collision detection as collisions do not happen in

C-(&@igure '1,F shows the C-( 0it timing structure&

)here are F segments in each 0it time3 the synchroniKationsegment. the propagation segment. the phase segment and

phase segment D& Registers can 0e set up so as to allow

certain time /uanta for each segment& Menerally the samplepoint is 0etween the st and Dnd phase segment&

!igure "#$+ CAN bit timing

'. The benefit of CAN in automotive real time systems

(early all carmakers in Europe. -merica. and @ar East useC-( high,speed networks ;e&g& " k0ps< in their power,

engine systems. which are compliant to the '4* !#!,Dphysical layer standard& -dditionally most European

passenger cars are e/uipped with C-(,0ased multiple7systems& )hese multiple7 networks link door and roof control

units as well as lighting and seat control units& 4ome of themultiple7 networks are compliant to the fault,tolerant physical

layer standard ;'4* !#!,


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The Use of CAN Based Networks in Automotive Real Time Systems