embedded systems design

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Control Area Networks

CAN Benefits

CAN is low cost:

• Fast serial bus with two wires: good price/performance ratio

• Low cost protocol devices (controllers,transceivers) available mainly driven by high volume production in the automotive market.

CAN is reliable:

• Sophisticated error detection and error handling mechanisms results in high reliability transmission ex: 500 kbits/s,25% bus load, 2000 hours per year. One undetected error every 1000 years.
• Erroneous messages are detected and repeated.
• System-wide data consistency (every bus node is informed about an error)
• Faulty nodes automatically withdraw from bus communication
• High immunity to Electromagnetic Interference.

CAN is real-time:

• maximum data rate is 1 MBit/s @ 40m bus length (still about 40 kbit/s @ 1000m bus length)

• Short message length (0 to 8 data bytes / message)(Larger data can be split up into several messages).

• Low latency between transmission request and actual start of transmission.

• Bus access handled via CSMA/CD w/ AMP method (message with the highest priority wins arbitration without losing any time).

CAN is Flexible:

• CAN allows Multi-Master Operation(every CAN node is able to access the bus individually).

• CAN Nodes can easily be connected / disconnected (i.e. plug & play).
• Number of nodes not limited by the protocol.

CAN means Multicast / Broadcast Capability:

• CAN is not node-oriented but message-oriented.
• Message identifier specifies contents & priority of the message.
• Messages can be easily sent to multiple / all nodes simultaneously.
• All nodes simultaneously receive and work on common data.

CAN is standardized:

• ISO-11898 (high speed applications).
• ISO-11519-2 (low speed applications).

How it all began...

About Embedded Systems

An embedded system is a special-purpose computer system that is designed to perform very small sets of designated activities. Embedded systems date back as early as the late 1960s where they used to control electromechanical telephone switches. The first recognizable embedded system was the Apollo guidance computer developed by Charles Draper and his team. Later they found their way into the military, medical sciences, and the aerospace and automobile industries. Today they are widely used to serve various purposes; some examples are the following.

• Network equipment such as firewall, router, switch, and so on
• Consumer equipment such as MP3 players, cell phones, PDAs, digital cameras, camcorders, home entertainment systems, and so on
• Household appliances such as microwaves, washing machines, televisions, and so on

• Mission-critical systems such as satellites and flight control.

Following are the key factors that differentiate an embedded system from a desktop computer.

• Embedded systems are usually cost sensitive.
• Most embedded systems have real-time constraints.
• There are multitudes of CPU architectures (such as ARM®, MIPS®, PowerPC ™, etc.) that are used in embedded systems. Embedded systems employ application-specific processors. For example, the processor in your digital camera is specially tailored for image capturing and rendering.
• Embedded systems have (and require) very few resources in terms of RAM, ROM, or other I/O devices as compared to a desktop computer.
• Power management is an important aspect in most embedded systems.
• The development and debugging environment in an embedded system is very different from a desktop computer. Embedded systems generally have an inbuilt circuitry for debugging purposes.
• An embedded system is designed from both the hardware and software perspective, taking into account a specific application or set of applications. For example, your MP3 player may have a separate hardware MP3 decoder built inside it.