Transcript CAN Communication Protocol

What is a Controller Area Network?
 History of CAN
 CAN communication protocol
 Physical layer
 ISO 11898
 CiA
 CANopen
 DeviceNet
 Applying CAN

CAN = controller area network
 ECU = electronically controlled unit
 Dominant = 0
 Recessive = 1
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CAN implements a vehicle bus to which
all CAN devices are connected
 Allows all devices to communicate with
one another
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Cars, trucks, buses, off-road vehicles
 Maritime electronics
 Aircraft/aerospace electronics
 Medical equipment and devices
 Coffee machines
 Elevators
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Developed in Germany in the 1980's by
engineers working at Robert Bosch
 More functionality while reducing the
weight and complexity of the wiring
between controllers.
 Multi-processor system which has better
performance, increased reliability and is
more maintainable
 No communication protocols at the time
met the speed and reliability requirements
so Bosch developed CAN specification 2.0
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Physical and data link layer
are implemented in the
CAN controllers
Standard CAN bypasses
layers 3-6
(1) Physical layer = actual
hardware
(2) Data link layer =
connects data to the
protocol
(7) Application layer =
interacts with OS of CAN
device
Since CAN is so widely used CAN chips
are cheap
 Reliability and error resistance of CAN
calculated in mathematical model
shown to have only 1 undetected error in
1000 years
 Has world wide acceptance
 Higher layer protocols defined for data
intensive applications
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Data frame – sends data
 Remote frame – requests data
 Error frame – reports an error
 Overload frame – reports a node is
overloaded
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SOF – 1 dominant bit
 Arbitration Field – message ID + RTR
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Control Field
-Length of the
data field to
follow
-For remote
frame DLC is
ignored, data
field is always
zero
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CRC Field – CRC segment + delimiter bit
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ACK Field – acknowledgement bit +
delimiter bit
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SOF – 1 bit
Arbitration field – 12 or 32 bits
Control field – 6 bits
Data field – up to 8 bytes
CRC field – 16 bits
ACK field – 2 bits
EOF -7 bits
IFS – 3 bits
Off-road vehicles require more messages
 11 bit message ID -> 29 bit
 Use IDE to determine format:
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› 0 = 11 bit
› 1 = 29 bit
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Extended frame has trade-offs:
› Bus latency time is longer
› More bandwidth required
› Error detection performance decreased
Error occurs: bit stuffing, CRC incorrect,
etc.
 Error flag set followed by recessive
delimiter and interframe
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Errors are caught quickly and have short
recovery times:
Error Frame Length
Baud Rate
Total error recovery time
(error frame + interframe)
14 bits
1 Mbit/sec
14 + 3 uSec
14 bits
500 kBit/sec
28 + 6 uSec
14 bits
250 kBit/sec
56 + 12 uSec
20 bits
1 Mbit/sec
20 + 3 uSec
20 bits
500 kBit/sec
40 + 6 uSec
20 bits
250 kBit/sec
80 + 12 uSec
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Overload flag – 6 dominant bits so all
nodes detect overload, messaging
ceased
Producer/consumer
 All messages are acknowledged for
consistency (error if inconsistent or
unacknowledged)
 Nodes can accept or decline message
filitering
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CAN uses message ID of the node to
prevent collisions – high priority first
 Uses bitwise arbitration as follows:
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› Node sends SOF with dominant level
 Non-sending nodes begin listening
› Node sends next bit and compares output
signal with actual bus level
 If sent recessive, detects dominant, then
begins listening, else sends next bit
› Transmits the rest of the message if it
transmits all arbitration bits
Node A = 1000101100 (high priority)
 Node B = 1110110000
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1st arbitration bit
2nd arbitration bit
3rd arbitration bit
Node A
1
0
0
Node B
1
1
listen
Bus level
1
0
0
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B send a recessive, detects dominant,
then listens
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Errors detected by:
› Bit monitoring
› Checksum Check
› Variable bit stuffing with a stuff width of 5
› Frame check
› Acknowledge Check
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Defective frames are aborted and
retransmitted
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Malfunctions that would disrupt the
system:
› Transmitting Error from malfunctioning node
› Receiving Error from malfunctioning node
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CAN determines permanent vs.
temporary malfunctions
› Transmit error counter
› Receive error counter
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Based on the counters, a node is in one
of the following states:
› Error Active
 Transmit and receive counters <128
› Error Passive
 Transmit or receive counters >127
› Bus-Off
 Transmit error counter >255
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ISO 11898-2 is the most common standard for
the physical layer
Nodes connected by 2 wires: CAN_H and
CAN_L
Bus level determined by:
› Vdiff = Vcan_h – Vcan_l
› 2V for recessive, 0V for dominant
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Bus terminated by 120 Ω resistors to suppress
electrical reflections on the bus
Electromagnetic interferences are minimized
with the differential voltage between CAN_H
and CAN_L
At a maximum baud rate of 1 Mbit/sec a bus
can be up to 40m long
› At <10 kbit/sec can have up to 5000 m bus
“Road vehicles – Controller area network
(CAN)” 1993
 Contains:
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› Data link layer and physical signaling
› High speed medium access unit
› Low speed fault tolerant medium
dependant interface
› Time triggered communication
Users + manufacturers develop and
support CAN
 Based on participation and initiative
 Represented at ISO and IEC committees
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Higher layer protocol adds:
› Network management
› Device monitoring
› Communication between nodes
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Requires CANopen nodes to have:
› Communication unit
› State machine
› Object dictionary
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Application layer protocol adds 4
required objects:
› Identity object
› Connection object
› Message router object
› DeviceNet object
Hewlett-Packard
 Lockheed Martin
 Boeing
 NASA
 GE Medical
 Siemens Medical
 John Deere
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Vehicle Spy (Vspy)
 NETCAR-Analyzer
 Volcano Network Architect (VNA)
 Ability to:
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› Read bus signals
› Transmit messages
› Log data
› Write to ECU memory
ECU
 TCU
 CCU
 SSM15
 SSM25
 CMU
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LIN-Bus (Local Interconnect Network)
 Ethernet
 RS232
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Universal serial bus
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CAN specification 2.0
› A = standard format
› B = extended format
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ISO 11898
› Specific requirements on the sub layers of the
data link layer and physical layer
CANopen specification
 Book: A comprehensible guide to CAN
 Book: Embedded networking with CAN and
CANopen
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[1]
Wilfried Voss. (2005). A Comprehensible Guide to Controller Area Network. Greenfield: Copperhill
Technologies Corporation.
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[2]
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[3]
CiA. (2001). Can physical layer. Retrieved 1 24, 2012, from http://www.cancia.de/index.php?id=systemdesign-can-physicallayer
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[4]
International Standard ISO 11898. First edition 2003-12-01. Road vehicles - Controller area
network (CAN).
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[5]
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[6]
National Instruments. (2012). Controller Area Network (CAN) Overview. Retrieved 2 2, 2012, from
http://zone.ni.com/devzone/cda/tut/p/id/2732
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[7]
Vehicle Spy. (2005-2012). Vehicle Spy Professional. Retrieved 2 4, 2012 from
http://intrepidcs.com/VehicleSpy/
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[8]
CANopen. CANopen USA. Retrieved 2 4, 2012 from http://www.canopen.us/
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[9]
Real Time Automation. DeviceNet Introduction. Retrieved 2 4, 2012 from
http://www.rtaautomation.com/devicenet/#2
Pazul Keith. (2002). Controller Area Network (CAN) Basics. In Microchip.
Robert Bosch. (1991). CAN Specification. Robert Bosch GmbH.