Can

CANController Area Network

29BIT ID

Timothy E. Jackson

Overview and Application

CAN Frame

CAN Frame

IDENTIFIER DLC DATA FIELD

Identifier = 29 bits + MAC

DLC = Data Length Code 4 bits

Indicates number of data bytes

Data Field = 0 to 7 bytes

Depends on the DLC value

CAN Frame

Arbitration Field Control Field Data Field

12 bits 6 bits 0 to 64 bits

SOF

Identifier

11bits

RTR

IDE

r0

DLC

4bitsData Field

CRC

15bits

ACK

Field

2bits

EOF

7bits

Bit StuffingNo Bit

Stuffing

SOF = Start Of Frame

RTR = Remote Transmission Request

IDE = Identifier Extension Bit

DLC = Data Length Code

CRC = Cyclic Redundancy Check

ACK = Acknowledgement

EOF = End Of Frame

Standard Format, 2.0A

Max frame length with bit stuffing = 127 bits

CRC delimiter

1 bit

CAN Frame

Arbitration Field Control Field Data Field

32 bits 6 bits 0 to 64 bits

SOF

Identifier

11bitsSRR IDE

Identifier Ext.

18bitsRTR r1 r2

DLC

4bitsData Field

CRC

15bits

ACK

Field

2bits

EOF

7bits

Bit StuffingNo Bit

Stuffing

SOF = Start Of Frame

SRR = Substitute Remote Request

IDE = Identifier Extension Bit

RTR = Remote Transmission Request

DLC = Data Length Code

CRC = Cyclic Redundancy Check

ACK = Acknowledgement

EOF = End Of Frame

Extended Format, 2.0B

Max frame length with bit stuffing = 150 bits

CRC delimiter

1 bit

CAN Frame

Arbitration Field Control Field Data Field

32 bits 6 bits 0 to 64 bits

SOF

Identifier

11bitsSRR IDE

Identifier Ext.

18bitsRTR r1 r2

DLC

4bitsData Field

CRC

15bits

ACK

Field

2bits

EOF

7bits

CRC delimiter

1 bit

CAN Frame

29 Bit Identifier DLC DATA FIELD

29 bits 4bits 0 to 8 bytes

PRI PGN SA

PRI= Priority 3 bits

PGN = Parameter Group Number, 18 bits

SA = Source Address, 8 bits

Message Structure

How do I start using CAN right now?

• SAE J1939– Defined by the automotive industry– Standards defined for the Layers 1,2,3,4,

and 7

ISO Layers according to SAE J1939

Application Layer

Data Link Layer

Physical Layer

Layer 7

Layer 2

Layer 1

Presentation Layer

Session Layer

Transport Layer

Layer 6

Layer 5

Layer 4

Layer 3 Network Layer

SAE J1939/11

SAE J1939/12

SAE J1939/21

SAE J1939/31

SAE J1939/21

SAE J1939/71

SAE J1939/73

ISO Layers Used In EAMCS

Application Layer

Data Link Layer

Physical Layer

Layer 7

Layer 2

Layer 1

SAE J1939/11 – High Speed Bus Connection• Differential signal transmission• Shielded twisted pair bus lines and ground• 250k bits/sec• 30 nodes max.• Bus length = 40m max. or 131 ft. max.

Physical Layer

SAE J1939/12 – Variant Bus Connection• Bus constructed using active termination

For bus load = 1 (Using 29bit ID and 8 data bytes per frame, approx. 135bits/message)

At a bitrate of 250kbps

1850 messages per second

Node B(UQM Inverter)Node A

(EAMCS)

120Ω 120Ω

Example MSCAN TimingSAE J1939/11 requires a bitrate, R 250kbps

Using the ADAPT9S12 from Technological Arts,XTAL 16MHz

If PLL is used, always select the oscillator clock

fCANCLK XTAL

Pick Tq 16

Use the equation below to solve for the Prescale Value

bitratePrescaleValue

fCANCLKTq

250kbpsPrescaleValue

16MHz16 Tq

PrescaleValue 4

Cont. Example MSCAN Timing

16Tq = SYNC_SEG + PropagationDelaySeg + TimeSeg1 + TimeSeg2

SYNC_SEG = 1Tq

PropagationDelaySeg = 2Tq (Determined by bus characteristics)

Pick, TimeSeg2 = 5Tq

TimeSeg1 = 8Tq

This gives a SJW range 0…3 so, I picked 2

Once values are determined, then apply to MSCAN registers

29 Bit ID

PRI PGN SA

28…26 25 24 23…16 15…8 7…0

PRI r DP PF DA/GE SA

PRI= Priority 3 bits

PGN = Parameter Group Number, 18 bits

r = Reserved Bit, always 0 or dominant

DP = Data Page

PF = PDU format, (Protocol Data Unit)

DA = Destination Address

GE = Group Extension

SA = Source Address, 8 bits

Data Link Layer SAE J1939/21

Example: EAMCS

29 Bit ID

PRI r DP PDU Dest. Addr Source Addr

28…26 25 24 23…16 15…8 7…0

0x06

0x040 0 0xEF 0x02 0x01

Message sent to UQM Inverter

Priority

Defined by SAE

Reserved for future

use

Defined by SAE

PDU

Defined by SAE

DA

Defined by User

SA

Defined by User

Application LayerSAE J1939/71

Defines the bytes in the data field

SAE HANDBOOK DATA

Example: Parameter Group Definition

Electric Drive State• Transmission Repetition Rate: 100ms

• Data Length: 8 bytes

• Data Page: 0

• PDU format: 239

• PDU specific: DA

• Default Priority: 4

Byte Definitions

1 Command 0x10

2,3 Actual Speed

4 Torque

5 Voltage

6 Current

7,8 System Error

Cont. Example: Parameter Group DefinitionByte Definitions

1 Command 0x10

2,3 Actual Speed

4 Torque

5 Voltage

6 Current

7,8 System Error

Command Actual Speed Torque Voltage Current System Error

Byte 1 Byte 2 Byte 3 Byte 4 Byte 5 Byte 6 Byte 7 Byte 8

Definitions

Data Field

Cont. Example: Parameter Group Decoding Using MSCAN (The Easy Way)

Command

Actual Speed (MSB)

Actual Speed (LSB)

Torque

Voltage

Current

System Error (MSB)

System Error (LSB)

CANRXFG[4]

Define a structtypedef struct{ INT8U command; INT16U actual_speed; INT8U torque; INT8U voltage; INT8U current; INT16U system_error; }ELEC_DRIVE_STATE;

ELEC_DRIVE_STATE elec_drive_data;

Create an Instance

Take advantage of Struct copying routines

elec_drive_data = *(ELEC_DRIVE_STATE*)(CANRXFG + 4);

Now access the data fields using the corresponding struct member

outhexw(&elec_drive_data.actual_speed); /* Print the 16bit hex value */

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ReferencesMotorola Documentation

MSCAN Block Guide V02.14, S12MSCANV2.pdf

Bosch Documentation

CAN Specification Version 2.0, can2spec.pdf

Reference Texts

Konrad Etschberger,Controller Area Network. Weingarten, Germany: IXXAT Automation GmbH, 2001

1997 SAE Handbook, Volume 2 Parts and Components. Warrendale, PA: Society of Automotive Engineers, 1997