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Online course on Embedded Systems

 

MODULE - 9

Communication Interfaces Continued:

Controller Area Network (CAN) interface in embedded systems:

History:

CAN or Controller Area Network or CAN-bus is an ISO standard computer network protocol and bus standard, designed for microcontrollers and devices to communicate with each other without a host computer. Designed earlier for industrial networking but recently more adopted to automotive applications, CAN have gained widespread popularity for embedded control in the areas like industrial automation, automotives, mobile machines, medical, military and other harsh environment network applications.

Development of the CAN-bus started originally in 1983 at Robert Bosch GmbH. The protocol was officially released in 1986. and the first CAN controller chips, produced by Intel and Philips, introduced in the market in the year of 1987.


Introduction:

The CAN is a "broadcast" type of bus. That means there is no explicit address in the messages. All the nodes in the network are able to pick-up or receive all transmissions. There is no way to send a message to just a specific node. To be more specific, the messages transmitted from any node on a CAN bus does not contain addresses of either the transmitting node, or of any intended receiving node. Instead, an identifier that is unique throughout the network is used to label the content of the message. Each message carries a numeric value, which controls its priority on the bus, and may also serve as an identification of the contents of the message. And each of the receiving nodes performs an acceptance test or provides local filtering on the identifier to determine whether the message, and thus its content, is relevant to that particular node or not, so that each node may react only on the intended messages. If the message is relevant, it will be processed; otherwise it is ignored.

How do they communicate?
If the bus is free, any node may begin to transmit. But what will happen in situations where two or more nodes attempt to transmit message (to the CAN bus) at the same time. The identifier field, which is unique throughout the network helps to determine the priority of the message. A "non-destructive arbitration technique" is used to accomplish this, to ensure that the messages are sent in order of priority and that no messages are lost. The lower the numerical value of the identifier, the higher the priority. That means the message with identifier having more dominant bits (i.e. bit 0) will overwrite other nodes' less dominant identifier so that eventually (after the arbitration on the ID) only the dominant message remains and is received by all nodes.
As stated earlier, CAN do not use address-based format for communication, instead uses a message-based data format. Here the information is transferred from one location to another by sending a group of bytes at one time (depending on the order of priority). This makes CAN ideally suited in applications requiring a large number of short messages (e.g.: transmission of temperature and rpm information). by more than one location and system-wide data consistency is mandatory. (The traditional networks such as USB or Ethernet are used to send large blocks of data, point-to-point from node A to node B under the supervision of a central bus master).
Let us now try to understand how these nodes are interconnected physically, by pointing out some examples. A modern automobile system will have many electronic control units for various subsystems (fig1-a). Typically the biggest processor will be the engine control unit (or the host processor). The CAN standard even facilitates the subsystem to control actuators or receive signals from sensors. A CAN message never reaches these devices directly, but instead a host-processor and a CAN Controller (with a CAN transciever) is needed between these devices and the bus. (In some cases, the network need not have a controller node; each node can easily be connected to the main bus directly.)
The CAN Controller stores received bits (one by one) from the bus until an entire message block is available, that can then be fetched by the host processor (usually after the CAN Controller has triggered an interrupt). The Can transciever adapts signal levels from the bus, to levels that the CAN Controller expects and also provides a protective circuitry for the CAN Controller. The host-processor decides what the received messages mean, and which messages it wants to transmit itself.

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Fig 1-a
It is likely that the more rapidly changing parameters need to be transmitted more frequently and, therefore, must be given a higher priority. How this high-priority is achieved? As we know, the priority of a CAN message is determined by the numerical value of its identifier. The numerical value of each message identifier (and thus the priority of the message) is assigned during the initial phase of system design. To determine the priority of messages (while communication), CAN uses the established method known as CSMA/CD with the enhanced capability of non-destructive bit-wise arbitration to provide collision resolution and to exploit the maximum available capacity of the bus. "Carrier Sense" describes the fact that a transmitter listens for a carrier wave before trying to send. That is, it tries to detect the presence of an encoded signal from another station before attempting to transmit. If a carrier is sensed, the node waits for the transmission in progress to finish before initiating its own transmission. "Multiple Access" describes the fact that multiple nodes send and receive on the same medium. All other nodes using the medium generally receive transmissions by one node. "Collision Detection" (CD) means that collisions are resolved through a bit-wise arbitration, based on a preprogrammed priority of each message in the identifier field of a message.


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Fig 1-b

Let us now try to understand how the term "priority" becomes more important in the network. Each node can have one or more function. Different nodes may transmit messages at different times (Depends how the system is configured) based on the function(s) of each node. For example:
1) Only when a system failure (communication failure) occurs.
2) Continually, such as when it is monitoring the temperature.
3) A node may take action or transmit a message only when instructed by another node, such as when a fan controller is instructed to turn a fan on when the temperature-monitoring node has detected an elevated temperature.
Note:
When one node transmits the message, sometimes many nodes may accept the message and act on it (which is not a usual case). For example, a temperature-sensing node may send out temperature data that are accepted & acted on only by a temperature display node. But if the temperature sensor detects an over-temperature situation, then many nodes might act on the information.
CAN use "Non Return to Zero" (NRZ) encoding (with "bit-stuffing") for data communication on a "differential two wire bus". The two-wire bus is usually a twisted pair (shielded or unshielded). Flat pair (telephone type) cable also performs well but generates more noise itself, and may be more susceptible to external sources of noise.
Main Features:

a) A two-wire, half duplex, high-speed network system mainly suited for high-speed applications using "short messages". (The message is transmitted serially onto the bus, one bit after another in a specified format).
b) The CAN bus offers a high-speed communication rate up to 1 M bits / sec, for up to 40 feet, thus facilitating real-time control. (Increasing the distance may decrease the bit-rate).
c) With the message-based format and the error-containment followed, it's possible to add nodes to the bus without reprogramming the other nodes to recognize the addition or changing the existing hardware. This can be done even while the system is in operation. The new node will start receiving messages from the network immediately. This is called "hot-plugging"
d) Another useful feature built into the CAN protocol is the ability of a node to request information from other nodes. This is called a remote transmit request, or RTR.
e) The use of NRZ encoding ensures compact messages with a minimum number of transitions and high resilience to external disturbance.
f) CAN protocol can link up to 2032 devices (assuming one node with one identifier) on a single network. But accounting to the practical limitations of the hardware (transceivers), it may only link up to 110 nodes on a single network.
g) Has an extensive and unique error checking mechanisms.
h) Has High immunity to Electromagnetic Interference. Has the ability to self-diagnose & repair data errors.
i) Non-destructive bit-wise arbitration provides bus allocation on the basis of need, and delivers efficiency benefits that can not be gained from either fixed time schedule allocation (e.g. Token ring) or destructive bus allocation (e.g. Ethernet.)
j) Fault confinement is a major advantage of CAN. Faulty nodes are automatically dropped from the bus. This helps to prevent any single node from bringing the entire network down, and thus ensures that bandwidth is always available for critical message transmission.
k) The use of differential signaling (a method of transmitting information electrically by means of two complementary signals sent on two separate wires) gives resistance to EMI & tolerance of ground offsets.
l) CAN is able to operate in extremely harsh environments. Communication can still continue (but with reduced signal to noise ratio) even if:
1. Either of the two wires in the bus is broken
2. Either wire is shorted to ground
3. Either wire is shorted to power supply.

CAN protocol Layers & message Frames:

Like any network applications, Can also follows layered approach to the system implementation. It conforms to the Open Systems Interconnection (OSI) model that is defined in terms of layers. The ISO 11898 (For CAN) architecture defines the lowest two layers of the seven layers OSI/ISO model as the data-link layer and physical layer. The rest of the layers (called Higher Layers) are left to be implemented by the system software developers (used to adapt and optimize the protocol on multiple media like twisted pair. Single wire, optical, RF or IR). The Higher Level Protocols (HLP) is used to implement the upper five layers of the OSI in CAN.

CAN use a specific message frame format for receiving and transmitting the data. The two types of frame format available are:

a) Standard CAN protocol or Base frame format
b) Extended Can or Extended frame format

The following figure (Fig 2) illustrates the standard CAN frame format, which consists of seven different bit-fields.

a) A Start of Frame (SOF) field - indicates the beginning of a message frame.
b) An Arbitration field, containing a message identifier and the Remote Transmission Request (RTR) bit. The RTR bit is used to discriminate between a transmitted Data Frame and a request for data from a remote node.
c) A Control Field containing six bits in which two reserved bits (r0 and r1) and a four bit Data Length Code (DLC). The DLC indicates the number of bytes in the Data Field that follows.
d) A Data Field, containing from zero to eight bytes.
e) The CRC field, containing a fifteen-bit cyclic redundancy check-code and a recessive delimiter bit.
f) The Acknowledge field, consisting of two bits. The first one is a Slot bit which is transmitted as recessive, but is subsequently over written by dominant bits transmitted from any node that successfully receives the transmitted message. The second bit is a recessive delimiter bit.
g) The End of Frame field, consisting of seven recessive bits.

An Intermission field consisting of three recessive bits is then added after the EOF field. Then the bus is recognized to be free.

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(Fig 2)

The Extended Frame format provides the Arbitration field with two identifier bit fields. The first (the base ID) is eleven (11) bits long and the second field (the ID extension) is eighteen (18) bits long, to give a total length of twenty nine (29) bits. The distinction between the two formats is made using an Identifier Extension (IDE) bit. A Substitute Remote Request (SRR) bit is also included in the Arbitration Field.

Error detection & correction:

This mechanism is used for detecting errors in messages appearing on the CAN bus, so that the transmitter can retransmit message. The CAN protocol defines five different ways of detecting errors. Two of these works at the bit level, and the other three at the message level.
1. Bit Monitoring.
2. Bit Stuffing.
3. Frame Check.
4. Acknowledgement Check.
5. Cyclic Redundancy Check

1. Each transmitter on the CAN bus monitors (i.e. reads back) the transmitted signal level. If the signal level read differs from the one transmitted, a Bit Error is signaled. Note that no bit error is raised during the arbitration process.

2. When five consecutive bits of the same level have been transmitted by a node, it will add a sixth bit of the opposite level to the outgoing bit stream. The receivers will remove this extra bit. This is done to avoid excessive DC components on the bus, but it also gives the receivers an extra opportunity to detect errors: if more than five consecutive bits of the same level occurs on the bus, a Stuff Error is signaled.

3. Some parts of the CAN message have a fixed format, i.e. the standard defines exactly what levels must occur and when. (Those parts are the CRC Delimiter, ACK Delimiter, End of Frame, and also the Intermission). If a CAN controller detects an invalid value in one of these fixed fields, a Frame Error is signaled.

4. All nodes on the bus that correctly receives a message (regardless of their being "interested" of its contents or not) are expected to send a dominant level in the so-called Acknowledgement Slot in the message. The transmitter will transmit a recessive level here. If the transmitter can't detect a dominant level in the ACK slot, an Acknowledgement Error is signaled.

5. Each message features a 15-bit Cyclic Redundancy Checksum and any node that detects a different CRC in the message than what it has calculated itself will produce a CRC Error.

Error confinement:

Error confinement is a technique, which is unique to CAN and provides a method for discriminating between temporary errors and permanent failures in the communication network. Temporary errors may be caused by, spurious external conditions, voltage spikes, etc. Permanent failures are likely to be caused by bad connections, faulty cables, defective transmitters or receivers, or long lasting external disturbances.

Let us now try to understand how this works.

Each node along the bus will be having two error counters namely the transmit error counter (TEC) and the receive error counter (REC), which are used to be incremented and/or decremented in accordance with the error detected. If a transmitting node detects a fault, then it will increments its TEC faster than the listening nodes increments its REC because there is a good chance that it is the transmitter who is at fault.
A node usually operates in a state known as "Error Active" mode. In this condition a node is fully functional and both the error count registers contain counts of less than 127. When any one of the two error counters raises above 127, the node will enter a state known as "Error Passive". That means, it will not actively destroy the bus traffic when it detects an error. The node which is in error passive mode can still transmit and receive messages but are restricted in relation to how they flag any errors that they may detect. When the Transmit Error Counter rises above 255, the node will enter the Bus Off state, which means that the node doesn't participate in the bus traffic at all. But the communications between the other nodes can continue unhindered.

To be more specific, an "Error Active" node will transmit "Active Error Flags" when it detects errors, an "Error Passive" node will transmit "Passive Error Flags" when it detects errors and a node, which is in "Bus Off" state will not transmit "anything" on the bus at all. The transmit errors give 8 error points, and receive errors give 1 error point. Correctly transmitted and/or received messages cause the counter(s) to decrease. The other nodes will detect the error caused by the Error Flag (if they haven't already detected the original error) and take appropriate action, i.e. discard the current message.

Confused? Let us try to get slightly simplified.

Let's assume that whenever node-A (for example) on a bus tries to transmit a message, it fails (for whatever reason). Each time this happens, it increases its Transmit Error Counter by 8 and transmits an Active Error Flag. Then it will attempt to retransmit the message and suppose the same thing happens again. When the Transmit Error Counter rises above 127 (i.e. after 16 attempts), node A goes Error Passive. It will now transmit passive error flags on the bus. A Passive Error Flag comprises 6 recessive bits, and will not destroy other bus traffic - so the other nodes will not hear the node-A complaining about bus errors. However, A continues to increase its TEC. When it rises above 255, node-A finally stops and goes to "Bus Off" state.
What does the other nodes think about node A? - For every active error flag that A transmitted, the other nodes will increase their Receive Error Counters by 1. By the time that A goes Bus Off, the other nodes will have a count in their Receive Error Counters that is well below the limit for Error Passive, i.e. 127. This count will decrease by one for every correctly received message. However, node A will stay bus off. Most CAN controllers will provide status bits and corresponding interrupts for two states: "Error Warning" (for one or both error counters are above 96) and "Bus Off".

Bit Timing and Synchronization:

The time for each bit in a CAN message frame is made up of four non-overlapping time segments as shown below.

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The following points may be relevant as far as the "bit timing" is concerned.

1. Synchronization segment is used to synchronize the nodes on the bus. And it will always be of one quantum long.
2. One time quanta (which is also known as the system clock period) is the period of the local oscillator, multiplied by the value in the Baud Rate Pre-scaler (BRP) register in the CAN controller.
3. A bit edge is expected to take place during this synchronization segment when the data changes on the bus.
4. Propagation segment is used to compensate for physical delay times within the network bus lines. And is programmable from one to eight time quanta long.
5. Phase-segment1 is a buffer segment that can be lengthened during resynchronization to compensate for oscillator drift and positive phase differences between the oscillators of the transmitting and receiving nodes. And is also programmable from one to eight time quanta long.
6. Phase-segment2 can be shortened during resynchronization to compensate for negative phase errors and oscillator drift. And is the maximum of Phase-segment1 combined with the Information Processing Time.

7. The Sample point will always be at the end of Phase-seg1. It is the time at which the bus level is read and interpreted as the value of the current bit.
8. The Information Processing Time is less than or equal to 2 time quanta.


This bit time is programmable at each node on a CAN Bus. But be aware that all nodes on a single CAN bus must have the same bit time regardless of transmitting or receiving. The bit time is a function of the period of the oscillator local to each node, the value that is user-programmed into BRP register in the controller at each node, and the programmed number of time quanta per bit.

How do they synchronize:

Suppose a node receives a data frame. Then it is necessary for the receiver to synchronize with the transmitter to have proper communication. But we don't have any explicit clock signal that a CAN system can use as a timing reference. Instead, we use two mechanisms to maintain synchronization, which is explained below.

Hard synchronization:
It occurs at the Start-of-Frame or at the transition of the start bit. The bit time is restarted from that edge.

Resynchronization:

To compensate for oscillator drift, and phase differences between transmitter and receiver oscillators, additional synchronization is needed. The resynchronization for the subsequent bits in any received frame occurs when a bit edge doesn't occur within the Synchronization Segment in a message. The resynchronization is automatically invoked and one of the Phase Segments are shortened or lengthened with an amount that depends on the phase error in the signal. The maximum amount that can be used is determined by a user-programmable number of time quanta known as the Synchronization Jump Width parameter (SJW).
Higher Layer Protocols:
Higher layer protocol (HLP) is required to manage the communication within a system. The term HLP is derived from the OSI model and its seven layers. But the CAN protocol just specifies how small packets of data may be transported from one point to another safely using a shared communications medium. It does not contain anything on the topics such as flow control, transportation of data larger than CAN fit in an 8-byte message, node addresses, establishment of communication, etc. The HLP gives solution for these topics.
Higher layer protocols are used in order to
1. Standardize startup procedures including bit rate setting
2. Distribute addresses among participating nodes or kinds of messages
3. Determine the layout of the messages
4. Provide routines for error handling on system level
Different Higher Layer Protocols
There are many higher layer protocols for the CAN bus. Some of the most commonly used ones are given below.
1. Can Kingdom
2. CAN open
3. CCP/XCP
4. Device Net
5. J1939
6. OSEK
7. SDS
Note:
Lot of recently released microcontrollers from Freescale, Renesas, Microchip, NEC, Fujitsu, Infineon, and Atmel and many such leading MCU vendors are integrated with CAN interface.

 

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