CAN Controller Area Network 29BIT ID
Overview and Application
Timothy E. Jackson
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
Standard Format, 2.0A Max frame length with bit stuffing = 127 bits
Arbitration Field Control Field Data Field
CRC delimiter 1 bit
12 bits
S O F Identifier 11bits R T R I D E
6 bits
r 0 DLC 4bits
0 to 64 bits
Data Field CRC 15bits ACK Field 2bits No Bit Stuffing EOF 7bits
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
CAN Frame
Extended Format, 2.0B Max frame length with bit stuffing = 150 bits
Arbitration Field Control Field Data Field
CRC delimiter 1 bit
32 bits
S O F Identifier 11bits SRR IDE Identifier Ext. 18bits RTR r1
6 bits
r2 DLC 4bits
0 to 64 bits
Data Field CRC 15bits ACK Field 2bits No Bit Stuffing EOF 7bits
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
CAN Frame
CRC delimiter 1 bit
Arbitration Field
Control Field
Data Field
32 bits
S O F Identifier 11bits SRR IDE Identifier Ext. 18bits RTR r1
6 bits
r2 DLC 4bits
0 to 64 bits
Data Field CRC 15bits ACK Field 2bits EOF 7bits
CAN Frame
Message Structure 29 Bit Identifier 29 bits PRI PGN SA DLC 4bits DATA FIELD 0 to 8 bytes
PRI= Priority 3 bits PGN = Parameter Group Number, 18 bits SA = Source Address, 8 bits
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
Layer 7 Layer 6 Layer 5 Layer 4 Layer 3 Layer 2 Layer 1
Application Layer Presentation Layer Session Layer Transport Layer
SAE J1939/71 SAE J1939/73
SAE J1939/21
Network Layer Data Link Layer
Physical Layer
SAE J1939/31
SAE J1939/21
SAE J1939/11 SAE J1939/12
ISO Layers Used In EAMCS
Layer 7
Application Layer
Layer 2
Data Link Layer Physical Layer
Layer 1
Physical Layer
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.
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 A (EAMCS)
Node B (UQM Inverter)
120
120
Example MSCAN Timing
SAE J1939/11 re quire s a bitrateR 250kbps , Using the ADAPT9S12 from Technological Arts, AL 16MHz XT If PLL is used, always sele ct the oscillator clock
fCANCLK XT AL
Pick Tq 16 Use the equation be low to solv e for the Prescale Value
bit rat e PrescaleValue fCANCLK Tq
250kbps
PrescaleValue 16MHz
16Tq
PrescaleValue 4
Cont. Example MSCAN Timing
16Tq = SYNC_SEG + PropagationDelaySeg + TimeSeg1 + TimeSeg2
SYNC_SEG = 1Tq PropagationDelaySeg = 2Tq Pick, TimeSeg2 = 5Tq TimeSeg1 = 8Tq This gives a SJW range 03 so, I picked 2
(Determined by bus characteristics)
Once values are determined, then apply to MSCAN registers
Data Link Layer SAE J1939/21
29 Bit ID
PRI
2826 PRI 25 r 24 DP
PGN
2316 PF 158 DA/GE
PRI= Priority 3 bits
SA
70 SA
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
Example: EAMCS
Message sent to UQM Inverter 29 Bit ID
PRI
2826 0x06 0x04
r
25 0
DP
24 0
PDU
2316 0xEF
Dest. Addr
158 0x02
Source Addr
70 0x01
Priority
Defined by SAE
Reserved for future use Defined by SAE
PDU
DA
SA
Defined by SAE
Defined by User
Defined by User
Application Layer SAE 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 Byte Data Page: 0 PDU format: 239 1 PDU specific: DA 2,3 Default Priority: 4 4
5 6
Definitions Command 0x10
Actual Speed
Torque Voltage Current
7,8
System Error
Cont. Example: Parameter Group Definition
Byte 1 2,3 4 5 6 7,8 Actual Speed Torque Voltage Current System Error Definitions Command 0x10
Definitions
Command Byte 1 Actual Speed Byte 2 Byte 3 Torque Byte 4 Voltage Byte 5 Current Byte 6 System Error Byte 7 Byte 8
Data Field
Cont. Example: Parameter Group Decoding Using MSCAN
(The Easy Way)
Define a struct
typedef struct{ INT8U command; INT16U actual_speed; INT8U torque; INT8U voltage; INT8U current; INT16U system_error; }ELEC_DRIVE_STATE;
CANRXFG[4]
Command
Actual Speed (MSB) Actual Speed (LSB) Torque Voltage
D
a t a F i e l d
Create an Instance
ELEC_DRIVE_STATE elec_drive_data;
Current System Error (MSB) System Error (LSB)
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 */
References
Motorola 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
