CAN Based Products
CAN Stack
In todays automotive communication applications like data / message exchange between two or more electronic control units (ECU), demand highly reliable communication. In particular for automotive industry, to control the complete activity of the vehicle, an efficient, reliable and near real-time communication is required between the ECUs.
CAN - Control Area Network is the serial communication protocol developed mainly for automotive industry to meet the sophisticated nature of communication. And J1939 is the standard on which the automotive communication is built. TriozTechnologies in spite of providing RTOS - Specially designed for automotive applications-now provide CAN communication stack for automotive based applications.
The CAN communication could also be used for the any kind of data transfer where the physical media and the time to transfer data both becomes critical.
What is CAN?
Controller Area Network (CAN) is a serial network that was originally designed for the automotive industry, but has also become a popular bus in industrial automation as well as other applications. The CAN bus is primarily used in embedded systems, and as its name implies, is the network established among microcontrollers. It is a two-wire, half duplex, high-speed network system and is well suited for high speed applications using short messages. Its robustness, reliability and the large following from the semiconductor industry are some of the benefits with CAN.
CAN can theoretically link up to 2032 devices (assuming one node with one identifier) on a single network. However, due to the practical limitation of the hardware (transceivers), it can only link up to110 nodes (with 82C250, Philips) on a single network. It offers high-speed communication rate up to 1 Mbits/sec thus allows real-time control. In addition, the error confinement and the error detection feature make it more reliable in noise critical environment
How does CAN work?
Introduction
As stated earlier, CAN is a multimaster network. It uses CSMA/CD+AMP (Carrier Sense Multiple Access/Collision Detection with Arbitration on Message Priority). Before sending a message the CAN node checks if the bus is busy. It also uses collision detection. In these ways it is similar to Ethernet. However, when an ethernet network detects collision both sending nodes stop transmitting. They then wait a random time before trying to send again. This make ethernet networks very sensitive to high bus loads. CAN solves this problem with the very clever principle of arbitration.
Principle
Data messages transmitted from any node on a CAN bus do not contain addresses of either the transmitting node, or of any intended receiving node.
Instead, the content of the message is labelled by an identifier that is unique throughout the network. All other nodes on the network receive the message and each performs an acceptance test on the identifier to determine if the message, and thus its content, is relevant to that particular node. If the message is relevant, it will be processed; otherwise it is ignored.
Identifiers & arbitration
The unique identifier also determines the priority of the message. The lower the numerical value of the identifier, the higher the priority. This allows arbitration if two (or more) nodes compete for access to the bus at the same time.
The higher priority message is guaranteed to gain bus access as if it were the only message being transmitted. Lower priority messages are automatically re-transmitted in the next bus cycle, or in a subsequent bus cycle if there are still other, higher priority messages waiting to be sent.
Each CAN message has an identifier which is 11 bits (CAN specification part A) or 29 bits (part B). This identifier is the principle part of the CAN arbitration field, which is located in the beginning of each CAN message. The identifier identifies the type of message, but is also the message priority.
The bits in a CAN message can be sent as either high or low. The low bits are always dominant, which means that if one node tries to send a low and another node tries to send a high, the result on the buse will be a low. A transmitting node always listens on the bus while transmitting. A node that sends a high in the arbitration field and detects a low knows that it has lost arbitration. It stops transmitting, letting the other node, with a higher priority message, continue uninterupted.
Two nodes on the network are not allowed to send messages with the same id. If two nodes try to send a message with the same id at the same time arbitration will not work. Instead, one of the transmitting nodes will detect that his message is distorted outside of the arbitration field. The nodes will then use the error handling of CAN, which in this case ultimately will lead to one of the transmitting node being switched off (bus-off mode).
Remote frames
There are two kinds of frames in CAN - remote frames and data frames. Data frames are used when a node wants to transmit data on the network, and are the "normal" frame type.
Remote frames can be described as a request for information. A frame with the RTR bit set (see description of the CAN message format) means the transmitting node is asking for information of the type given by the identifier. A node which has the information available should then respond by sending the information onto the network.
Depending on the implementation of the CAN controller the answer may be sent automatically. Simpler CAN controllers (BasicCAN) can not respond automatically. In this case the host microcontroller is made aware of the remote request and has to send the data.
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