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Renesas Technology America Inc.

An Introduction to CANAn Introduction to CAN

CAN Basics 2CAN Basics 2Renesas InteractiveRenesas Interactive

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Key reasons to use CAN

• Scalability• 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’shardware

• 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 Frame is broadcast to the bus [id_n][value_x]

Transmitting Node

MCU Firmware

identifier [id_n]

Data [values_x]

CAN Peripheral

CAN Transceiver

Node configured to receive identifier

MCU Firmware

CAN Transceiver

CAN Peripheral

Rx Mail Box [id_n]

Rx Mail Box [id_b]

Tx Mail Box [id_c]

Data [values_x]

Data [values_x]

Tx Mail Box [id_n]

Rx Mail Box [id_c]

Rx Mail Box [id_b]

Data [values_x]

Node not configured to receive identifier

MCU Firmware

CAN Transceiver

CAN Peripheral

Rx Mail Box [id_d]

Tx Mail Box [id_b]

Rx Mail Box [id_c]

Rx Mail Box [id_a]

Data Flow

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Data Frame

• Start of Frame

• Arbitration Field

• Control Field

• Data Field

• CRC

• ACK Field

• End of Frame

re

se

rve

d 0

re

se

rve

d 0

AC

K D

elim

ite

r

AC

K D

elim

ite

r

Sta

rt

of

Fra

me

Re

mo

te R

eq

ue

st

ID E

xte

nd

Da

ta L

en

gth

C

od

e (

4 b

its)

Data Field0 to 8 bytes

CR

C D

elim

ite

rA

CK

slo

t

End of Frame

Sta

rt

of

Fra

me

Su

bstitu

te R

RID

Exte

nd

Da

ta L

en

gth

C

od

e (

4 b

its)

Data Field0 to 8 bytes

CR

C D

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ite

rA

CK

slo

t

End of Frame

Re

mo

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eq

ue

st

Standard ID

Extended ID

CRC

CRC

Identifier

Identifier(second part)

re

se

rve

d 1

Identifier(first part)

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Start of Frame

• A single dominant bit while the bus is idle• All nodes synchronize timing to leading edge

All nodes synchronize to

edge

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Arbitration Field

Standard; 11-bit identifier

Extended; 29-bits

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Arbitration

Carrier Sense Multiple Access/Collision Resolution= CSMA/CR

By using Dominant and Recessive Bits

Handling collisions

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Control Field

Control Field contains the DLC• 2 bits reserved• 4 bits Data Length Code; number of Data Field bytes

Number of Data Bytes

Data Length Code

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 content = payload

of the frame• 0-8 bytes• MSB transmitted first

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CRC Field

• 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

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ACK Field

ACK FIELD = ACK SLOT + ACK DELIMITER

All receivers that received valid message reports with a ’dominant’ bit during the ACK slot

Any receiver that disagrees votes no after the delimiter with an error flag

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End of Frame

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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Error detection

Locally detected errorsWhen is an error frame sent?

• 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?

• Firmware• CAN controller • CAN transceivers• A physical media

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Example of Renesas CAN MCUs• SH705x series

• Automotive Powertrain• SH7047 series

• General purpose SH2…soon updated derivative available.• M32R/ECU series

• Automotive Powertrain• M32C series

• General purpose (1-3 channels of CAN)• M16C/6N series

• M16C/62 family with CAN. General purpose.• M16C/29 series

• General Purpose M16C/Tiny with CAN.• M16C/10 series

• Better off with M16C/Tiny or R8C/Tiny• R8C/Tiny series

• General Purpose

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Which is normally by far the most common frame type?

1. Data Frame

2. Error Frame

3. Overload Frame

4. Remote Frame

A: 1

B: 2

C: 3

D: 4

Question 1

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Which of these statements is false?

1. The data field in a data frame can consist of max 255 bytes

2. The data field has only 8 bytes

3. You must specify for each data frame how many data bytes follow

4. Remote frames are not necessary to use CAN

A: 1

B: 2

C: 3

D: 4

Question 2

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Which of these is false?

1. The CRC field is sent by all receivers and verified by the transmitter.

2. Each receiver generates a CRC on the observed data frame and compares it with the transmitted CRC check value.

3. 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

Question 3

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When is an error frame sent?

1. Bit stuffing violation; > 5 bits with same polarity

2. CRC error; checksum violation

3. Form error; bit pulse malshaped

4. 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

Question 4

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Glossary• 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.• 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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End CAN Basics 2End CAN Basics 2