Transcript Vehicle Bus_finalx

Vehicle Bus
Communications Network inside a Car
By
Mike Tran
Juan Japata
Thi Nguyen
Objectives
• A Brief Introduction to Serial Bus Systems
• Common Serial Bus Systems in Motor Vehicle
• A closer look at CAN protocol
A Brief Introduction to Serial Bus
Systems
• Conventional Serial BUS
• Motivation behind the Non-conventional BUS.
• Common Bus in Vehicle: CAN, LIN, FlexRay,
MOST
Conventional Serial BUS
• RS232: One-to-one exchange, non addressing
nodes; power down to connect; Resync/timing
OH; 9-25 wires
• USB: Synchronizes bus; common clock, master to
slave connections—center hub for device to
connect; differential signal; 4 wires
• I2C: 2-wires; any device can connect; unique
address; Multi-Master/Slave structure allows any
device to send and receiver, but lead to arbitration
and synchronization problems with multiple clocks
in system; requires node to be a master node
Motivation behind the Non-conventional BUS
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Electronification
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Consumer demand for integrated controls of comfort and convenient
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Stringent government regulation on safety and exhaust emissions
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Early electronic control units (ECUs) operated independent of one another
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Coordination of these ECUs has immensely improved and extended
vehicle functionality and performance
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In conventional networking, each of the signal to be transmitted was
assigned a electrical line
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Intensive networking drove wiring cost sky high
Motivation behind the Non-conventional BUS:
Conventional Serial BUS to Vehicle BUS
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Design based on borrowed ideas from RS232, USB, and I2C
No Master node needed
2 wire twisted, differential signals, noise immunity
No power down needed to replace the module
http://canbuskit.com/what.php
Common Bus in Vehicle:
CAN, LIN, FlexRay, MOST and others
G. Leen & D.Heffernan, “In Vehicle networks: Expanding Automotive Electronic systems,” (Jan 2002)
Common Serial Bus Systems in Vehicle
• CAN
• LIN
• FLEXRAY
• RADIO FREQ
• MOST
source : www.aa1.car.com
CAN (Controller Area Network)
• CAN protocol was developed in 1980 by Robert Bosch for automotive
applications
• The protocol was official released in 1986 by Society of Automotive
Engineer (SAE).
• Since 1993 CAN was standardized and available as ISO standard (ISO
11898)
• CAN network made up of CAN nodes (ECUs) with CAN interface
• ECUs exchange data over their individual CAN interfaces and a
transmission medium (CAN bus) that interconnects all the CAN nodes
CAN (Controller Area Network)
• CAN interface is made up of two parts: communication software and
communication hardware
• Software handle higher level communication services while hardware
implemented the fundamental communication functions.
CAN (Controller Area Network)
Key benefits:
• Mature technology with over 14 years, with many products and tools
in the market.
• ISO standard protocol
• Simple transmission medium
• Excellent error checking and handling
• Fault confinement
• Transfer rate up to 1 Mbps
CAN (Controller Area Network)
Typical Applications:
• Safety: Passenger occupant detection, electronic parking brake
• Body Control: Motor control, power door, sunroof, HVAC, lighting
• Chassis: Motor control, watchdog
• Powertrain: Vacuum leak detection, electronic throttle control,
watchdog
LIN (Local Interconnect Network)
• LIN protocol was developed in the end of 1990 by group of
automotive OEMs and suppliers joined with semiconductor and tool
producer
• LIN is a universal asynchronous receiver-transmitter (UART) – based
• LIN network is a single-master, multiple-slave networking architecture
LIN (Local Interconnect Network)
Key benefits:
• LIN protocol provides a cost-effective networking option for
connecting switches, sensors and lamps in the vehicle where the
bandwidth and versatility of CAN is not required
• LIN master node can be connected to higher level of network such as
CAN to extend the communication of in-vehicle networking all the
way to individual sensor or actuator
• Complements CAN as a cost-effective sub-network
• LIN protocol can be generated by standard asynchronous
communication interfaces (SCI, UART) with no hardware required
• Synchronization mechanism means no quartz oscillator required at
slave nodes
• No protocol license fee
LIN (Local Interconnect Network)
Typical Applications:
• Steering Wheel: Cruise Control, Wiper, Turning Light, Climate
Control, Radio
• Roof: Rain Sensor, Light Sensor, Light Control, Sunroof
• Engine/Climate: Sensors, Small Motors, Control Panel (Climate)
• Door/Seat: Mirror, Central ECU, Mirror Switch, Window Lift, Door
Lock, Seat Position Motors, Occupant Sensors, Seat Control Panel
FLEXRAY
• FlexRay was originally developed around 2000 by the founding
members of the FlexRay Consortium (BMW, Volkswagen,
DaimlerChryler, NXP Semiconductors, General Motors, etc…)
• FlexRay communications protocol is designed to provide high-speed
deterministic distributed control for advanced automotive applications
• FlexRay is a dual-channel architecture offers system-wide redundancy
that meets the reliability requirements of safety-critical application
FLEXRAY
Key Features:
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High data rate up to 20 Mbps which increase network throughput
Highly deterministic response time
Dual channel redundancy
System-wide synchronized time base
Result in these benefits:
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Simplified vehicle network architectures
Increased enhanced control intelligence
Reduced wiring requirements
Reduced network subsystems
Distributed computing through a global time clock
Electromechanical system (X-by-wire) replacing hydraulic
components
FLEXRAY
Typical Applications:
• Wheel Node: Fail-Safe, Low to Medium Performance
• Body Control Module (BCM): High performance, low power, X-by
wire
• Master: Highest level of fault tolerance
• FlexRay system can also be employed a vehicle-wide network
backbone
RADIO FREQUENCY (RF)
• Radio frequency communications can provide additional vehicle
functionality and driver convenience
• RF enable a wide variety of safety and comfort features such as
Remote keyless entry (RKE)
Passive entry (PE)
Tire pressure monitoring system (TPMS)
Vehicle immobilization systems
www.freescale.com
RADIO FREQUENCY (RF)
www.freescale.com
MOST (Media Oriented System Transport)
• MOST is a high-speed multimedia network technology optimized by
the automotive industry in 1998
• MOST is a serial communication system that built for transmitting
audio, video and control data via fiber-optic cables.
• High-performance multimedia network technology based on
synchronous data communication requires professional software tools
and hardware interfaces.
Typical Applications using MOST
Autoelectronics.com
MOST used in lane departure warning
Autoelectronics.com
www.Freescale.com
LIN, CAN, FLEXRAY IN COMPARISON
Autoelectronics.com
Introduction to CAN
CAN technology has been standardized since 1994 and is described by four
ISO documents. ISO 11898-1 describes the CAN protocol.
In relation to the reference model of data communication, the CAN protocol
just covers the Data Link Layer (MAC — Medium Access Control, LLC —
Logical Link Control) and the Physical Layer (PLS — Physical Signaling).
Introduction to CAN
The two ISO documents ISO 11898-2 and ISO 11898-3 cover the two sub-layers
of the reference model for data communication:
PMA (Physical Medium Attachment) and PMS (Physical Medium Specification).
They describe two different CAN physical layers: the high-speed CAN physical
layer and the low-speed CAN physical layer. They differ primarily in their
definition of voltages and data transmission rates (data rates).
ISO 11898-3 allows data rates up to 125 kbit/s. It is primarily used in the
convenience area of the automobile. ISO 11898-2 allows data rates up to 1 Mbit/s.
Consequently, ISO 11898-2 is primarily used in the powertrain and chassis areas of
the automobile.
Introduction to CAN
ISO 11898-1 defines an event-driven communication. With a higher bus load this
may lead to delays, especially for lower priority CAN messages.
ISO 11898-4 is an extension of the Data Link Layer that adds a time triggered
communication option for CAN-based networks.
Standard and Implementation
CAN Network
A CAN network consists of a number of CAN nodes which are linked via a
physical transmission medium (CAN bus) In practice, the CAN network is usually
based on a line topology with a linear bus to which a number of electronic control
units are each connected via a CAN interface. The passive star topology may be
used as an alternative.
An unshielded twisted two-wire line is the physical transmission medium used
most frequently in applications (Unshielded Twisted Pair — UTP).
The maximum data rate is 1 Mbit/s. A maximum network extension of about 40
meters is allowed. At the ends of the CAN network, bus termination resistors
contribute to preventing transient phenomena (reflections). ISO 11898 specifies the
maximum number of CAN nodes as 32.
An ECU that performs its tasks in a CAN network is referred to as a CAN node
CAN Network
CAN Network
An electronic control unit (ECU) requires a CAN interface, this comprises a CAN
controller and a CAN transceiver. The CAN controller fulfills communication
functions prescribed by the CAN protocol, which relieves the host considerably.
The CAN transceiver connects the CAN controller to the physical transmission
medium. Usually, the two components are electrically isolated by optical or
magnetic decoupling.
The CAN nodes differ in the number of CAN messages they each send or receive.
For example, one CAN node might receive five different CAN messages, each at a
cycle of ten milliseconds, while another CAN node just needs to receive one CAN
message every 100 milliseconds, resulting two fundamental CAN controller
architectures: CAN controllers with and without object storage.
CAN controllers may be integrated, or they may a stand-alone chip component. In
this case, the microcontroller treats the CAN controller like a memory chip.
CAN Network
CAN Transceiver
A CAN transceiver always has two bus pins: one for the CAN high line (CANH)
and one for the CAN low line (CANL).
CAN Bus
Physical signal transmission in a CAN network is based on transmission of
differential voltages. This effectively eliminates the negative effects of interference
voltages induced by motors, ignition systems and switch contacts.
Twisting of the two lines reduces the magnetic field considerably. Therefore, in
practice twisted pair conductors are generally used as the physical transmission
medium.
Terminating the ends of the communication channel using termination resistors
prevents reflections in a high-speed CAN network.
The key parameter for the bus termination resistor is the so-called characteristic
impedance of the electrical line. This is 120 Ohm. In contrast to ISO 11898-2, ISO
11898-3 (low-speed CAN) does not specify any bus termination resistors due to
the low maximum data rate of 125 kbit/s.
CAN Bus
CAN Bus Levels
Physical signal transmission in a CAN network is based on differential signal
transmission. The specific differential voltages depend on the bus interface that is
used. A distinction is made here between the high-speed CAN bus interface (ISO
11898-2) and the low-speed bus interface (ISO 11898-3).
ISO 11898-2 assigns logical “1” to a differential voltage of 0 Volt. A differential
voltage of 2 Volt signifies logical “0”. High-speed CAN transceivers interpret a
differential voltage of more than 0.9 Volt as a dominant level within the common
mode operating range, typically between 12 Volt and -12 Volt.
Below 0.5 Volt, however, the differential voltage is interpreted as a recessive level.
A hysteresis circuit increases immunity to interference voltages.
ISO 11898-3 assigns a differential voltage of 5 Volt to logical “1”, and a
differential voltage of 2 Volt corresponds to logical “0”.
CAN Bus Logic
A basic prerequisite for smooth communication in a CAN network — especially
for bus access, fault indication and acknowledgement — is clear distinctions
between dominant and recessive bus levels. The dominant bus level corresponds to
logical “0”. The recessive bus level corresponds to logical “1”.
The dominant bus level overwrites the recessive bus level. When different CAN
nodes send dominant and recessive bus levels simultaneously, the CAN bus
assumes the dominant bus level. The recessive bus level only occurs if all CAN
nodes send recessive levels.
In terms of logic, such behavior is AND-logic. Physically, AND-logic is
implemented by a so-called open collector circuit. Practice with the interactive
figure “Bus Logic” to learn about the wired-AND bus logic upon which a CAN
network is based.
CAN Communication Principle
Safety-critical applications, such as those in the powertrain area, place severe
demands on a communication system’s availability. Decentralized bus access, so
that each bus node has the right to access the bus.
That is why a CAN network is based on a combination of multi-master architecture
and line topology: essentially each CAN node is authorized to place CAN
messages on the bus in a CAN network. The transmission of CAN messages does
not follow any predetermined time sequence, rather it is event-driven.
The communication channel is only busy if new information actually needs to be
transmitted, and this allows for very quick bus accesses.
A method of receiver-selective addressing is used in a CAN network. Every CAN
message is available for every CAN node to receive (broadcasting). A prerequisite
is that it must be possible to recognize each CAN message by a message identifier
(ID) and node-specific filtering. Although this increases overhead, it allows
integration of additional CAN nodes without requiring modification of the CAN
network.
Frame Types
For transmitting user data, ISO 11898-1 prescribes the so-called data frame. A data
frame can transport a maximum payload of eight bytes. For that there is the socalled data field, which is framed by many other fields that are required to execute
the CAN communication protocol. They include the message address (identifier or
ID), data length code (DLC), checksum (cyclic redundancy check sequence —
CRC sequence) and RX acknowledgement located in the acknowledgement field.
While the generator ECU of relevant information takes the initiative in sending
data frames, there also is the remote frame — a frame type with which user data,
i.e. data frames, can be requested from any other CAN node. Except for its missing
data field, a remote frame has the same structure as a data frame.
The error frame is available to indicate errors detected during communication. An
ongoing erroneous data transmission is terminated and an error frame is issued. It
consists of just two parts: The error flag and the error delimiter.
Principle of Bus Access
Each node in the CAN network has the right to access the CAN bus without
requiring permission and without prior coordination with other CAN nodes.
Although bus access based on an event-driven approach enables very quick
reactions to events, there is the inherent risk that several CAN nodes might want to
access the CAN bus at the same time.
CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) method is
used hereto ensure that CAN nodes wishing to send do not access the CAN bus
until it is available.
In case of simultaneous bus access, the CSMA/CA method based on bitwise bus
arbitration ensures that the highest priority CAN message among the CAN nodes
prevails.
Wired-AND bus logic and arbitration logic ensure that the priority of the CAN
message increases with decreasing identifier value: The smaller an identifier is, the
higher the priority of the CAN message
Principle of Data Protection
Reliable data transmission is a prerequisite for the safety and reliability of
electronic systems in motor vehicles. Therefore, CAN not only has to satisfy strict
real time requirements, but must always provide for reliable data transmission.
• Bit coding: helps to reduce emissions significantly. NRZ bit coding (NRZ: Non
Return to Zero) was chosen for CAN. NRZ coding is that consecutive bits of
the same polarity exhibit no level changes.
• Twisted Pair: In symmetrical signal transmission, external noise acts equally on
both lines. Ideally, the magnetic fields will exhibit opposite directions in each
sub-segment, which leads to the mutual cancellation of any induced voltages or
inductive effects. The effectiveness of twisting increases with the number of
wraps. At least 30 wraps per meter yields good results.
• Termination : On high data rates lines, reflections can be expected on the CAN
bus due to finite signal propagation. The bus ends in High-Speed CAN
networks must be terminated with the characteristic impedance of the physical
transmission medium. The characteristic impedance of the communication
channel is 120 Ohm.
Logical Error Detection
To detect corrupted messages, the CAN protocol defines five mechanisms:
Bit monitoring,
Monitoring of the message format (Form Check),
Monitoring of the bit coding (Stuff Check),
Evaluation of the acknowledgement (ACK Check) and
Verifying the checksum (Cyclic Redundancy Check).
References
www.vector.com
www.ni.com
www.freescale.com
http://www.canbuskit.com/what.php
http://www.eetimes.com/discussion/murphy-s-law/4024614/A-short-trip-on-the-CAN-bus
http://www.vector.com/vi_flexray_solutions_en.html
http://autoelectronics.com/body_electronics/data_buses/modeling-tomorrows-networks0501/
Peckol J., On EmbeddedSystems: A Contemporary Design Tool (New Jersey: Wiley,2008)
693-720
G. Leen & D.Heffernan, “In Vehicle networks: Expanding Automotive Electronic systems,”
(Jan 2002)
CAN tutorial at www.computer-solutions.co.uk
Atmel Microcontrollers CAN Tutorial
Distributed Embedded Systems (Philip Koopman) October 5, 2011
Questions?