CAN

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MotoHawk Training
CAN Controller Area Network
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OUTLINE

• CAN Introduction
• CAN Physical Network
• CAN Message Format
• MotoHawk Block Walk Through
• MotoHawk Post Office
• MotoHawk Advanced CAN Blocks
• CAN Protocol Overview

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INTRODUCTION

• CAN Controller Area Network
• Communication specification implemented for
  automotive applications in the 1980s
• Often, the term CAN is misused
• CAN is a hardware definition for interoperability
  between modules
• CAN specification does not state the data content
  of a given message
• Protocols built on top of CAN state the data
  content (ex. J1939, GMLAN)
• MotoHawk doesnt define a protocol, but allows
  access to the CAN hardware to implement a
  protocol
• By itself, CAN is not difficult (I mean it)

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OUTLINE

• CAN Introduction
• CAN Physical Network
• CAN Message Format
• MotoHawk Block Walk Through
• MotoHawk Post Office
• MotoHawk Advanced CAN Blocks
• CAN Protocol Overview

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PHYSICAL NETWORK

• Two wire robust serial communication
• Up to 1Mbps data rate which is limited by wire
  length
• 100 m _at_ 250Kbps
• 30 m _at_ 1Mbps
• Termination resistance required
• Maximum bandwidth
• 2000 messages per second _at_250Kbps
• 4000 messages per second _at_500Kbps
• 8000 messages per second _at_1Mbps
• Designed bandwidth should not exceed 70 of the
  maximum bandwidth

CAN -
CAN
Wire Length
120 ?
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PHYSICAL NETWORK

• Bus arbitration is handled with a simple strategy
• All modules on the bus attempt to transmit a
  message at the same time
• The message with the lowest address wins
• This strategy is handled at the hardware level
• Dominant Bits (0) vs. Recessive Bits (1)
• Multiple modules on the same bus cannot transmit
  the same address at the same time

Module 0 continues to transmit while Module
1 waits for next opportunity to transmit
Module 0
Module 1
T0
T1
T2
T3
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OUTLINE

• CAN Introduction
• CAN Physical Network
• CAN Message Format
• MotoHawk Block Walk Through
• MotoHawk Post Office
• MotoHawk Advanced CAN Blocks
• CAN Protocol Overview

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MESSAGE FORMAT

• A CAN 2.0B Message can contain up to 131 bits
• As application developers, 3 fields are
  important
• 4 bits determine the length of the data (aka
  payload) (Range 0-8)
• Up to 64 bits of data depending on data length
• ID Format
• Extended IDs are 29 bits
• Standard IDs are 11 bits
• Extended and Standard IDs can exist on the same
  bus at the same time
• Standard IDs have less message overhead (higher
  percentage of data per message)

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OUTLINE

• CAN Physical Network
• CAN Message Format
• MotoHawk Block Walk Through
• MotoHawk Post Office
• MotoHawk Advanced CAN Blocks
• CAN Protocol Overview

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CAN DEFINITION BLOCK

• First, the CAN hardware needs to be initialized
• Baud rate and transmit queue size configured
• If the baud rate and/or MotoTune protocol
  settings are not configured properly, then
  MotoTune cant talk to the module via CAN (and
  therefore programming cannot be performed via
  CAN)
• Boot key sets CAN baud rate to 250kbps

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Send CAN Raw Block
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CAN Exercise 1

• CAN Bus Test
• Construct a calibratable CAN message transmitter
• Use CANKing to monitor bus traffic

System Diagram
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CAN Exercise 1

• Transmit a message using the Send CAN Raw Block
  on address 0x1F4.
• CAN 1
• ID Type Standard (0)
• Data Length 8
• Transmission Rate 50 ms
• Use calibrations for visibility from MotoTune
• Change the Tx ID and note that the CAN bus
  traffic changes
• Change the Data and Data length and note that the
  traffic changes
• Use CANKing to debug any failed communication or
  monitor the bus traffic

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OUTLINE

• CAN Introduction
• CAN Physical Network
• CAN Message Format
• MotoHawk Block Walk Through
• MotoHawk Post Office
• MotoHawk Advanced CAN Blocks
• CAN Protocol Overview

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POST OFFICE

• CAN is the hardware layer, so how are transmitted
  messages sorted and filtered in MotoHawk?
• A Post Office is the simplest analogy
• Mailboxes
• Letters
• Addresses
• Zip Codes

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POST OFFICE

• Many messages are transmitted on a bus at a given
  time, but a module may only be interested in a
  small subset
• Similar to a post office where the messages are
  letters and the software dispatcher is the
  postman
• A mail box in MotoHawk is called a slot

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POST OFFICE

• A slot has an address known as a CAN ID similar
  to the address on a mailbox
• The MotoHawk post office needs to deliver
  messages to these slots
• How does it happen?

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POST OFFICE

• Filtering is done with 2 masks
• ID Mask Filters a message by the address or the
  ID
• Payload Mask Filters a message by the data
  content
• A mask is similar to assigning a Dont Care or
  a Do Care to a particular number in the mask

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Filtering

• Filtering is done with 2 masks
• ID Mask Filters a message by the address or the
  ID
• Payload Mask Filters a message by the data
  content
• A 1 in the mask means that the data and the
  value must match exactly
• For example,
• if ID mask 0x7F0 (111 1111 0000)
• and ID 0x7E4 (111 1110 0100)
• If incoming ID 0x7E0, (111 1110 0000)
• ?message goes to mailbox
• If incoming ID 0x7F4, (111 1111 0100)
• x message is rejected by mailbox
• If incoming ID 0x7E1, (111 1110 0001)
• ?message goes to mailbox

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READ CAN RAW BLOCK
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POST OFFICE

• The Slot Name is the name of the mailbox at the
  post office
• The slot has default settings for the ID, ID
  Mask, Payload Value, and Payload Mask
• This is the design time mailbox configuration

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POST OFFICE

• The properties of the mailboxes can be changed at
  run time
• The filters determined at design time can be
  strengthened (more restrictive), but cannot be
  weakened
• In MotoHawk, the CAN Receive Slot Properties
  Block allows the user to change the filters at
  run time

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POST OFFICE

• What happens if the postal worker rings your
  doorbell to deliver a message?
• This is an asynchronous reception of a message
• The advantage is that processing time can be
  saved by not periodically polling this message
• How does MotoHawk handle this situation?
• Asynchronous reception processed with CAN Receive
  Slot Trigger Block
• Slot name in this block must match the slot name
  specified in Read CAN Raw block or Read CAN
  Message block

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OUTLINE

• CAN Introduction
• CAN Physical Network
• CAN Message Format
• MotoHawk Basic Transmit Block
• Exercise
• MotoHawk Post Office
• MotoHawk Advanced CAN Blocks
• CAN Protocol Overview

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MOTOHAWK ADVANCED CAN

• The Read and Send CAN blocks are nice, but
  sometimes more advanced data parsing is
  necessary. Common questions
• I have 12 bit scaled data that spans across
  multiple bytes. How do I convert it into
  engineering units?
• I have more data that can fit into 64 bits. How
  do I create multi-page messages?
• Im using a protocol that has a variable ID. How
  do I dynamically create the ID easily?
• These are valid questions and there is an answer

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MOTOHAWK ADVANCED CAN

• MotoHawk has 2 blocks Read CAN Message and Send
  CAN Message (below)
• These are very powerful blocks that allow users
  to set up multi-page documents and parse and
  scale both variables and IDs

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MOTOHAWK ADVANCED CAN

• gtgt edit motohawk_can_example.m

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LITTLE ENDIAN vs BIG ENDIAN

• Terminology originates from Gullivers Travels
• Endianness refers to the order in which bytes
  are stored in memory
• This table shows the 4 byte storage for 1025
  (0x41)

Address Big Endian Representation Little Endian Representation
00 0000 0000 0000 0001
01 0000 0000 0000 0100
02 0000 0100 0000 0000
03 0000 0001 0000 0000

• By default, the MotoHawk CAN scripts use Big
  Endian byte orderingwhich can lead to confusion

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LITTLE ENDIAN vs BIG ENDIAN
Given this CAN definition byte ordering for a 4
byte variable
Big Endian MSB
Little Endian LSB
63....56
55....48
47....40
39....32
LSB
MSB
Now, go lets back to the script definition
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OUTLINE

• CAN Introduction
• CAN Physical Network
• CAN Message Format
• MotoHawk Basic Transmit Block
• Exercise
• MotoHawk Post Office
• MotoHawk Advanced CAN Blocks
• CAN Protocol Overview

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CLASS EXERCISE

• Create a distributed control system to control
  the ETC over CAN

Pedal
Duty Cycle
PID
Analog signal from potentiometer
CAN message send
TPS
City ID 0x81
ECU 2
CAN message receive
TPS
TPS
Analog signal from ECT
CAN message send
City ID 0x0B
Duty Cycle
Duty Cycle
ECU 1
CAN message receive
PWM output
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CLASS EXERCISE
Duty Cycle message definition CAN 1 ID Type Standard (0) ID 0x1F4 Data Length 2 Bytes Transmission Rate 10 ms Data Field Duty Cycle (SIGNED) Scale 200/65,535 Offset 0 Start bit 48 Size 2 bytes TPS Percent message definition CAN 1 ID Type Standard (0) ID 0x2F4 Data Length 1 Byte Transmission Rate 15 ms Data Field TPS Percent (UNSIGNED) Scale 100/255 Offset 0 Start bit 56 Size 1 byte
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PROTOCOL OVERVIEW

• CAN gets complicated when protocols are
  considered
• J1939, SmartCraft, CCP, GMLAN, etc. are all
  examples of protocols that adhere to strict rules
• These protocols can be implemented using Simulink
  and/or Stateflow
• Some message formats (J1939) have already been
  implemented for other projects

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