An Introduction to CAN
CAN Basics 2
Renesas Interactive
Renesas Technology America Inc.
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Key reasons to use CAN
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Scalability
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Low wiring cost
Low node connection costs
More and more chips
More off the shelf tools
Knowledge base growing
Reliability (error free)
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CAN Message Frames
• Data Frame - Carries the actual data
• Error Frame - When error is detected by any node’s
hardware
• Overload Frame - Request a delay on the bus
• Interframe Space - Provides minimum spacing between data and remote
frames
Allows error frames priority
• Remote Frame - Request transmission of Data Frame.
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Data Flow
Transmitting Node
Node configured to
receive identifier
Node not configured to
receive identifier
MCU Firmware
MCU Firmware
MCU Firmware
identifier [id_n]
Data [values_x]
Data [values_x]
Tx Mail Box [id_n]
Tx Mail Box [id_c]
Tx Mail Box [id_b]
Data [values_x]
Rx Mail Box [id_b]
Rx Mail Box [id_d]
Rx Mail Box [id_n]
Rx Mail Box [id_c]
Rx Mail Box [id_b]
Data [values_x]
Rx Mail Box [id_a]
CAN Peripheral
CAN Peripheral
CAN Peripheral
CAN Transceiver
CAN Transceiver
CAN Transceiver
Rx Mail Box [id_c]
Data Frame is broadcast to the bus [id_n][value_x]
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Data Frame
Data Field
0 to 8 bytes
CRC
CRC Delimiter
ACK slot
ACK Delimiter
Data Length
Code (4 bits)
Identifier
Remote Request
ID Extend
reserved 0
Start of Frame
Standard ID
End of
Frame
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Data Field
0 to 8 bytes
Start of Frame
Arbitration Field
Control Field
Data Field
CRC
ACK Field
End of Frame
5
CRC
CRC Delimiter
ACK slot
ACK Delimiter
Identifier
(second part)
Data Length
Code (4 bits)
Remote Request
reserved 1
reserved 0
Identifier
(first part)
Substitute RR
ID Extend
Start of Frame
Extended ID
End of
Frame
Start of Frame
• A single dominant bit while the bus is idle
• All nodes synchronize timing to leading edge
Start of Frame
All nodes
synchronize to
edge
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Identifier
7
Identifier
(second part)
Remote Request
reserved 1
Identifier
(first part)
Remote Request
ID Extend
Identifier
Substitute RR
ID Extend
Extended; 29-bits
Start of Frame
Standard; 11-bit identifier
Start of Frame
Arbitration Field
Arbitration
Handling collisions
Carrier Sense Multiple Access/Collision Resolution
= CSMA/CR
By using Dominant and Recessive Bits
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Control Field
Control Field contains the DLC
Data Length
Code (4 bits)
Remote Request
reserved 1
reserved 0
• 2 bits reserved
• 4 bits Data Length Code; number of Data Field bytes
Data Length Code
Number of
Data Bytes
DLC3
DLC2
DLC1
DLC0
0
0
0
0
0
1
0
0
0
1
2
0
0
1
0
3
0
0
1
1
4
0
1
0
0
5
0
1
0
1
6
0
1
1
0
7
0
1
1
1
8
1
0
0
0
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Data Field
Data Length
Code (4 bits)
• Data content = payload
of the frame
• 0-8 bytes
• MSB transmitted first
Data Field
0 to 8 bytes
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CRC
CRC Field
CRC
11
CRC Delimiter
ACK slot
• 15 bit CRC value followed by a recessive delimiter
• Generated by transmitter node’s HW and verified in
receiver’s HW
• CRC polynomial: x15+x14+x10+x8+x7+x4+x3+1
CRC Delimiter
ACK slot
ACK Delimiter
ACK Field
ACK FIELD = ACK SLOT + ACK
DELIMITER
All receivers that received valid
message reports with a ’dominant’
End of
bit during the ACK slot
Frame
Any receiver that disagrees votes no
after the delimiter with an error
flag
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DATA FRAME and REMOTE FRAME
delimited by ‘End of Frame’
consisting of seven recessive’ bits.
Gives a break before the next frame..
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ACK Delimiter
End of Frame
End of
Frame
Error detection
Locally detected errors
When is an error frame sent?
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Bit error -receive bit at sender not equal to transmit bit.
Bit stuffing violation -max 5 bits with same polarity
CRC error -checksum violation
Form error -bit pulse malshaped
Ack error -no dominant level in ACK slot so sender must retransmit
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What is needed?
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Firmware
CAN controller
CAN transceivers
A physical media
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Example of Renesas CAN MCUs
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SH705x series
• Automotive Powertrain
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SH7047 series
• General purpose SH2…soon updated derivative available.
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M32R/ECU series
• Automotive Powertrain
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M32C series
• General purpose (1-3 channels of CAN)
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M16C/6N series
• M16C/62 family with CAN. General purpose.
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M16C/29 series
• General Purpose M16C/Tiny with CAN.
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M16C/10 series
• Better off with M16C/Tiny or R8C/Tiny
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R8C/Tiny series
• General Purpose
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Question 1
Which is normally by far the most common frame type?
1.
2.
3.
4.
Data Frame
Error Frame
Overload Frame
Remote Frame
A: 1
B: 2
C: 3
D: 4
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Question 2
Which of these statements is false?
1.
2.
3.
4.
The data field in a data frame can consist of max 255 bytes
The data field has only 8 bytes
You must specify for each data frame how many data bytes follow
Remote frames are not necessary to use CAN
A: 1
B: 2
C: 3
D: 4
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Question 3
Which of these is false?
1.
2.
3.
The CRC field is sent by all receivers and verified by the transmitter.
Each receiver generates a CRC on the observed data frame and
compares it with the transmitted CRC check value.
Every receiver which has received a valid message correctly, reports
this to the transmitting node with a ’dominant’ bit during the ACK slot.
Any node that disagrees, votes no after the delimiter by sending an
error flag.
A: 1
B: 2
C: 3
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Question 4
When is an error frame sent?
1.
2.
3.
4.
Bit stuffing violation; > 5 bits with same polarity
CRC error; checksum violation
Form error; bit pulse malshaped
Ack error; no dominant level in ACK slot so sender must
retransmit
A: 1-3
B: 2-4
C: 2 and 4
D: All above
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Glossary
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Advanced CAN: CAN peripheral with varying numbers of buffers configurable for transmit/receive. Receive buffers have
hardware filtering on at least mask/match identifier content.
Basic CAN: CAN peripheral with no hardware filtering. Typically two receive buffers act as FIFO accepting all bus traffic. Usually
one transmit buffer.
Bit Time: Nominal time of one bit on the CAN bus. Made up of multiple segments that allows each node to synchronize to the
received bus traffic. All nodes on a bus must be configured to the same (nominal) bit time.
CAN: Controller Area Network
CAN 2.0B: Version 2.0 was the last version of CAN defined by Bosch. Part B added extended identifiers and the idea of hardware
filtering.
CIA: CAN in Automation. Group controlling the CANOpen protocol.
CANOpen: Multi-area communication protocol using CAN.
CRC: Cyclic Redundancy Check
DeviceNet: Industrial communication protocol using CAN.
Dominant/Recessive: Dominant bits on physical layer can override recessive bits.
Filters: Hardware in the CAN peripheral that can mask/match bits within the identifier field used to determine whether or not route
bus data to a mailbox.
GM LAN 3.0: GM protocol, encompasses all GM serial protocols.
Identifier: Frame field that indicates the message content. This field also is used to arbitrate the message priority on the bus;
lower identifier is higher priority.
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Standard Format; frames use 11 bit identifier.
Extended Format “29 bits
ISO 11898: ISO standardized version of CAN.
Mailbox: CAN hardware buffer that can be used to transmit or receive data. Most full CAN implementations have at least 16
mailboxes.
ODVA: Open DeviceNet Vendor Association. Group controlling DeviceNet protocol.
Time Quanta: Smallest time unit used by CAN. Multiple time quanta make up the segments of a bit time.
TT CAN: Time Triggered CAN. More deterministic CAN by assigning time slots when nodes may transmit.
FlexRay: Next generation automotive network. Time slots on the bus provide more deterministic behavior.
Vector-CanTech: Supplier of the majority of CAN software drivers and tools to North American and European automobile ECUs.
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Come back for more!
End CAN Basics 2
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