Advanced Embedded Systems Design

1
Advanced Embedded Systems Design

• Lecture 10 Controller Area Networks
• BAE 5030 - 003
• Fall 2004
• Instructor Marvin Stone
• Biosystems and Agricultural Engineering
• Oklahoma State University

2
Goals for Class Today

• Questions over reading / homework (Chapters 23,
  24)
• USB from a developers prospective (Patrick)
• SPI and I2C (Brody)
• Controller Area Networks introduction (Stone)
• Set assignment

3
Communications busses for embedded systems(A
very abridged list!)

• Automotive
• SAE J1850
• SAE J1708 / 1587
• CAN based
• SAE J1939 / ISO 11783
• Fault tolerant busses
• ByteFlight
• FlexRay
• TTP / TTCAN
• MOST
• Avionics
• MIL-STD-1553
• ARINC 429
• Marine
• NMEA 0183
• NMEA 2000 (IEC 61162-3 )
• Ethernet
• Ethernet
• IEEE

• LIN
• On-board busses (inter-IC busses)
• SPI
• I2C
• Microwire
• 1-Wire
• Industrial
• MODBUS
• RS485
• Ethernet
• Fieldbus
• Bitbus
• CAN
• DeviceNet
• SDS
• CiA
• Proprietary
• RS 485
• CAN

4
CAN Controller Area Network - Introduction

• History
• Developed by Bosch in mid 1980s to meet a need
  for networking ECUs (electronic control units in
  automobiles)
• Initially introduced with an 11-bit message
  identifier (CAN 1.0 1.2), later a 29 bit
  identifier introduced (CAN 2.0)
• Now
• 11 bit only parts CAN 1.2 (Also CAN2.0A)
• 2048 different CAN messages
• 11 bit compatible with 29 bit messages CAN 2.0B
  29 bit passive
• xmit and receive 11 and 29 bit ID CAN 2.0B
• 536 million CAN messages
• CAN has since been heavily adopted
• 350 million protocol controllers sold in 2003
  (Bosch / CiA)
• Broad industrial support
• Automotive
• J2284 / IDB-C 500 - kBaud automotive physical and
  data link
• J1939 Heavy duty vehicle physical/data
  link/network/app layer
• Proprietary
• Industrial
• DeviceNet Industrial physical/data link/network
  layers
• SDS - Industrial physical/data link/network
  layers

5
CAN Overview

• CAN Controller Area Network
• Serial data communications protocol for real-time
  application using a multiple access bus
• Messages have assignable priority
• most critical can dominate during heavy load
• Messages are short (controlled length)
• opportunities to insert a new message come often
• Asynchronous serial communications
• Serial bus vs. point-to-point
• Reduction in wiring complexity
• Better information sharing
• Low probability of an undetected error
• 4.7 x 10-11 x message error rate
• For a message error rate of 25/sec 1
  undetected error per 10,000 hours operation

6
Capacity Comparison
7
Capacity and Performance

• Use of bus bandwidth _at_250 kBaud by messages
• 100 messages per second (10 ms repetition) 5
• Torque/Speed control on engine
• Hitch control
• 10 messages per second (100 ms repetition) 0.5
• Throttle position
• GPS Lat/Lon data
• Implement application rate control (process data)
• 1 message per second (1s repetition) 0.05
• Display updates
• System status

8
Message latency

• 134 bit message _at_250 kBaud (bit stuffing ignored)
• _at_ 4 ms per bit 536 ms 0.5 ms per message max
• Highest priority message
• must wait no more than 0.5 ms
• Low priority messages
• must wait till higher priority messages clear
• latency may be long at high bus loads

9
Conventional Wiring (No Bus)
Serial Communication Links
10
A Simple CAN Application (Serial Bus)
11
CAN - Introduction

• Network structure
• Intended as a bus, but other physical
  implementations are possible
• Limitations No Tees

12
An overview of CAN based networking

• CAN - Elements
• CSMA/CA
• Bitwize priority access strategy
• Uses distributed synchronization of bit timing
• Non-destructive collisions
• Message components
• Identifier / Data
• Bus Access - Arbitration / Prioritization
• Error Detection / Error Confinement
• Filtering
• Other features
• In Frame Acknowledgement
• RTR

13

• Message structure
• Message frame contains
• 11/29 bit Identifier
• Data length
• 0-8 bytes data
• CRC
• In-frame acknowledgement

14
CAN Frame Format - CAN 2.0B
15
Typical ECU Components
16
ECU Connection to the bus
ECU 1
ECU 2
CAN_H
CAN_L
TBC_PWR
Terminator
Terminator
CAN_H
CAN_L
TBC_RTN
17
Physical network structure
Message
Message
18
Message Components
Message (up to 150 bits)
0 to 8 bytes (0 to 64 bits)

• 11 bits or 29 bits

Identifier
Data
Serial bit stream
Start
19
CAN Arbitration
ISO 11878
Lower numbered identifiers assume higher priority
20
Bit time control Length of a single bit

• Bit must arrive a destination before phase_seg1
  begins. Sampling at receiver will occur within
  the phase segment.
• Prop_seg. should accommodate total out and back
  propagation time plus delay in tranceivers

21
CAN synchronization / bit timing

• Definitions
• Synchronization segment (Sync_Seg) - that part of
  the bit time where bit edges are expected to
  occur
• Propagation segment (Prop_Seg) - intended to
  compensate for the physical delay times
• Phase segment (Phase_seg1, Phase_seg2) Provide
  control of the sample point position
• Synchronization Jump Width (SJW) defines how far
  a resynchronization may move the Sample Point
  inside the limits defined by the Phase Buffer
  Segments to compensate for edge phase errors.
• Synchronization
• Hard synchronization will be done to align bits
  at the beginning of a message.
• Phase_seg1 is lengthened or phase_seg2 is
  shortened to minimize phase error by up to the
  synchronization jump width (SJW). Larger phase
  errors cannot be compensated for.
• See http//www.can.bosch.com/docu/CiA99Paper.pdf

22
Physical Layer

• Many physical layer definitions
• Most ISO 11898 based
• Maximum number of ECU's
• Typical 30 (per segment)
• Typical Media
• Twisted shielded pair
• 120 W nominal impedance
• Two data lines (CAN_H, CAN_L)
• One shield
• Twisted Quad un-shielded
• 75 W nominal impedance
• Two data lines (CAN_H, CAN_L)
• Two termination supply lines
• Signal
• Compatable with ISO 11898 drivers (eg. Phillips
  80C250)

23
ISO 11898 Bus levels Microchip AN 228
24
Mapping of CAN to the OSI Microchip AN 228

• Characteristics of CAN - CAN provides Data Link
  functions

25
To form a complete communications system
ISO 7498
26
Message Filtering - Example
Accept if (ID AND MASK) XOR MATCH 0
10111100100
IDENTIFIER
1 Care, 0 Dont Care
11100000000
MASK
ID AND MASK
10100000000
10100000000
Pattern must match
MATCH
00000000000
(ID AND MASK) XOR MATCH
27
Error Detection

• 5 Error Types Detected
• Bit Error (Sent bit doesnt match monitored bit)
• Stuff Error (more than 6 successive in one state)
• CRC error
• Form Error
• Acknowledge Error
• Probability of an undetected error
• 4.7 x 10-11 x message error rate
• for a message error rate of 25/sec, 1 undetected
  error per 10,000 hours operation

28
Assignment

• Install the Pont time triggered scheduler in
  hardware and toggle the RED/GREEN LED at 1 per
  sec.
• Read Pont, Chapter 25, 28
• Tutorial 30 min
• Review SAE J1708, SAE J1850, SAE J1939
• or
• Review DeviceNET and SDS