VLSI and Embedded System Technical Library: CAN Protocol

What is CAN?

CAN Stands for Controller Area Network
CAN is an extremely robust standardized communication protocol.
CAN protocol was developed by Bosch in the 1980s to create standardization and reduce the cost and weight in vehicle wiring harnesses
Primary uses today are automotive and Industrial control applications

Controller Area Network (CAN)

Controller Area Network (CAN) is a fast serial bus that is designed to provide an efficient,reliable and very economical link between sensors and actuators.
CAN uses a twisted pair cable to communicate at speeds up to 1Mbit/s with up to 40 devices.
Originally developed to simplify the wiring in automobiles.
CAN field buses are now used in machine and factory automation products as well.

CAN features

Any node can access the bus when the bus is quiet
Non-destructive bit-wise arbitration to allow 100% use of the bandwidth without loss of data
Variable message priority based on 11-bit (or 29 bit) packet identifier
Peer-to-peer and multi-cast reception
Automatic error detection, signaling and retries
Data packets 8 bytes long

Tradeoff: CAN bus versus point-to-point connections

By introducing one single bus as the only means of communication as opposed to the point-to-point network, we traded off the channel access simplicity for the circuit simplicty
Since two devices might want to transmit simultaneously, we need to have a MAC protocol to handle the situation.
CAN manages MAC issues by using a unique identifier for each of the outgoing messages
Identifier of a message represents its priority.

Other applications of CAN

Due to its simplicty and real-time capabilities, CAN is also used industrial field bus systems.

Layered Architecture of CAN according to the OSI Reference Model

Physical Layer

The physical layer defines how signals are actually transmitted .
therefore deals with the description of Bit Timing , Bit Encoding, and Synchronization.
Within this specification the Driver/Receiver Characteristics of the Physical Layer are not defined so as to allow trans-mission medium and signal level implementations to be optimized for their application

MAC sublayer

The MAC sublayer represents the kernel of the CAN protocol.
It presents messages received from the LLC sublayer and accepts messages to be transmitted to the LLC sublayer.
The MAC sublayer is responsible for Message Framing, Arbitration, Acknowledgement, Error Detection and Signalling.
The MAC sublayer are supervised by a management entity called Fault Confinement which is self-checking mechanism for distinguishing short disturbances from permanent failures
The LLC sublayer is concerned with Message Filtering, Overload Notification and Recovery Management

CAN Networks

Physical CAN connection

What does a CAN ‘Node’ Do?

A node is any sub system that is connected to the CAN bus
A node may be very simple or very complex
A node may continually send messages, such as a motor speed control node
A node may only transmit a message when a system failure has occurred, such as a temperature monitoring node
A node may only take action when another node instructs it to do so, such as an electronic valve control

Data rate