|CAN Bus Controller Area Networking|
Since 2003, some vehicle makes and models started using a new diagnostic communication protocol called CANbus. By 2008 all vehicles sold for the United States market must use the CANbus protocol.
5.1.5 The CAN Bus
The two-wire CAN bus represents the most popular implementation of CAN. The
two-wire CAN bus uses non-return-to-zero (NRZ) signaling with bit stuffing. The
term NRZ means that the transmission of two successive 1 bits does not result in
the signal first being lowered to zero after the first 1 bit.
The left portion of Figure 5.2 illustrates the connection of a CAN controller
to a two-wire CAN bus. Note that a CAN transceiver has two connections to the
bus. The first connection (CANh) is used to transmit a differential signal, while the
second connection (CANl) is used to monitor the CAN bus, which also provides
for the receipt of the receiver signal by the CAN controller.
The CAN controller is usually integrated on a digital signal processor (DSP)
chip, which in turn is built into an electronic control unit (ECU). Thus, the CAN
controller provides the mechanism whereby one ECU can communicate with
another to check its status or exchange information. The left portion of Figure 5.2
illustrates the NRZ signaling method used on the CAN bus as well as the relationship
between the transmit and receive signaling state and dominant and recessive
signals on the bus.
Now that we have an appreciation for the term NRZ and the connection of a
CAN controller to the two-wire CAN bus, let us turn our attention to the term
bit stuffing. Bit stuffing prohibits the transmission of a string of six consecutive zero
(000000) or six consecutive one (111111) bits by inserting an opposite bit in the
data stream to prevent the transmission of six bits set to a 0 or 1. At the receiver an
opposite action occurs, with the receiver removing the stuffed bit. Under CAN a
bit stuffing violation in which six consecutive bits of the same type are received is
considered to represent an error.
CAN uses a modified carrier sense multiple access with collision detection
(CSMA/CD) method for nodes to gain access to the bus, in addition to an arbitration
process. In a CAN each device listens to the bus to determine if the message
flowing on the bus is the same as it is trying to transmit. If it is different, the device
Figure 5.2 The CAN two-wire physical layer and signal on the bus.
will immediately release the bus. This process ensures that one master will always
win and results in no messages lost due to a collision.
184.108.40.206 Signaling States
Under CAN there are two different signaling states, referred to as dominant (logical
0) and recessive (logical 1). These signaling states correspond to certain electrical
levels, which depend upon the physical layer used. As we will shortly note, there
are several different physical layers that can be used by CAN.
At the CAN transceiver the connection to the bus represents a wired-AND
function. This means that if just one node is driving the bus to a dominant state,
then the entire bus will be in that state regardless of the number of nodes transmitting
a recessive state.
220.127.116.11 The Physical Layer
Currently there are several different physical layers defined under the CAN specification.
The most common physical layer specification is the one defined in the ISO
11898-2 specification for a two-wire balanced signaling method. This specification
is also referred to as high-speed CAN.
Under the ISO 11898-3 specification another two-wire balanced signaling
scheme is defined, for lower bus speeds. This is a fault-tolerant specification, which
enables signaling to continue even if one bus wire should become cut or shorted to
ground or battery. This specification is referred to as low-speed CAN.
A third common physical layer is defined by the Society of Automotive Engineers
(SAE) in the J2411 specification. This specification defines the use of a singlewire-
plus-ground physical layer, which is used primarily in certain vehicles, such
as GM automobiles.
18.104.22.168 Data Transmission
Under CAN, arbitration-free transmission is used to place data on the bus. That
is, a CAN message transmitted with the highest priority will satisfy the arbitration
while nodes transmitting lower-priority messages will sense the higher priority and
back off and wait for access to the bus.
Arbitration-free transmission is supported by the use of dominant (logical 0)
and recessive (logical 1) bits. This means that if one node transmits a dominant bit
while another node transmits a recessive bit, then the dominant bit wins the arbitration.
Table 5.1 indicates the bus state for two nodes transmitting as well as the
value of a logical AND between the two.
Based upon the entries in the truth tables shown in Table 5.1, let us assume one
node is transmitting a recessive bit (logical 1) when another node transmits a dominant
bit (logical 0). The node transmitting the recessive bit (0) sees the dominant
bit (which creates a voltage across the bus while the recessive bit is not asserted on
the bus) and determines a collision occurred. Thus, the node transmitting a recessive
bit will back off. Then, instead of transmitting, it will wait six bit durations
after the end of the dominant message prior to attempting to retransmit.
During the arbitration process each transmitting node will monitor the state of
the CAN bus, comparing the received bit with the transmitted bit. If a dominant
bit is received when a recessive bit is transmitted the node will stop transmitting.
The actual arbitration process commences during the transmission of the identifier
field in the message frame. Each node that commences transmission at the
same time places an identifier field with a dominant bit as binary 0, beginning with
the high-order bit. As soon as the node ID is a larger number, which indicates a
lower priority, the node will transmit a binary 1 (recessive) and observe a binary 0
(dominant), which provides the indication to back off and wait. At the end of the
transmission of the identifier field all nodes except the node with the highest-priority
message will have backed off, while the node with the highest priority continues
its transmission. This action results in the higher-priority message gaining access
to the bus, while lower-priority messages will automatically retransmit in the next
bus cycle or in a subsequent bus cycle, assuming that there are other higher-priority
messages waiting to gain access to the bus.
22.214.171.124 Interoperability Issues
Because different physical layers as a rule are not interoperable, the cost of CAN
components, such as transceivers, cannot be amortized over a very large number
because different transceivers are used with different physical layers. This is turn
drives the cost of CAN upward and results in the use of LIN as a mechanism to
provide a lower-cost communications capability for groups of up to 16 slave nodes.
Table 5.1 Truth Tables for Dominant/Recessive
and Logical AND
Bus State with Two Nodes Transmitting Logical AND
Dominant Recessive 01
Dominant Dominant Dominant 000
Recessive Dominant Recessive 101
126.96.36.199 Bus Speed
As previously mentioned in this section, the maximum speed of a CAN bus
(ISO11898-2) is 1 Mbps, while a low-speed CAN (ISO11898-3) has a data rate up
to 125 kbps. In addition, a single-wire CAN has the ability to transmit at a data rate
up to approximately 50 kbps in its standard mode of operation, while its high-speed
mode allows a data transfer capability of up to approximately 100 kbps. Because
the type of transceiver used also governs the obtainable data rate, it is possible that
transmissions may have both an upper and lower boundary, as some transceivers
cannot transmit below a certain data rate.
188.8.131.52 Cable Length
At a data rate of 1 Mbps a maximum cable length of 40 m, or 130 ft, can be supported.
Because the pulse width is inversely proportional to the data rate, slowing
the transmission rate widens pulses transmitted on the bus, which in turn enables
the transmission distance to be extended. Thus, at a data rate of 500 kbps the
maximum cable length is increased to 100 m (330 ft), while at a data rate of 250
kbps the maximum cable length is extended to 500 m (1600 ft). From a practical
standpoint, the lower-speed versions of CAN are more suitable for the factory floor
where extended cable lengths are required.
184.108.40.206 Bus Termination
Under the ISO 11898 CAN standard the CAN bus must be terminated. The termination
of the ends of the bus is accomplished through the use of a 120-ohm
resistor at each end of the bus. The use of 120-ohm resistors removes potential
signal reflections at the end of the bus as well as ensures that the correct DC level
flows on the bus.
Figure 5.3 illustrates the structure of a typical CAN bus. Note that each end
has a 120-ohm resistor to remove signal reflections. The actual bus length will vary
based upon the data rate of the bus, with higher data rates reducing the length of
Device 2 • • • Device n
Figure 5.3 A typical CAN bus.
220.127.116.11 Cable and Cable Connectors
Under the ISO 11898 specification a twisted-pair cable that can be either shielded
or unshielded is acceptable. Under the SAE J2411 specification a single wire is
defined for use.
Although there is presently no standard defined for CAN connectors, the higher
layer of the protocol stack defines a few preferred connectors. Three of those connectors
are the nine-pin D-sub, five-pin Mini-C, and six-pin Deutsch.
The top portion of Figure 5.4 illustrates the pin positions and assignments for
the nine-pin D-sub connector. This illustration represents a male connector viewed
from the connector side or a female connector viewed from the sodering side. The
power portion of Figure 5.4 contains a table indicating the pins and pin assignments
of the connector.
Although the nine-pin D-sub connector is the most popularly used in a CAN,
both the five-pin Mini-C and six-pin Deutch connectors are also used. The five-pin
Mini-C connector resembles two concentric circles with five pins spaced within the
inner concentric circle and is compatible with both standard and extended CAN.
The six-pin Deutsch connector is primarily used for mobile hydraulic applications.
5.2 Message Frames
CAN has the ability to support the transmission of four different message types,
with each message broadcast on the bus. This means that all nodes literally hear
each transmission, requiring hardware to provide local filtering that enables a node
to react to messages of interest to the node. The four types of messages that can flow
on a CAN bus include:
Figure 5.4 Nine-pin DSUB connector and pin assignments: 1 = reserved; 2
CAN_L = CAN_L bus line (dominant low); 3 CAN_GND = CANZ Ground; 4 =
reserved; 5 CAN_SHLD = optional CAN shield; 6 GND = optional CAN ground;
7 CAN_H = CAN_H bus line (dominant high); 8 = reserved (error line); 9
CAN_V+ = optional power.
5.2.1 Data Frame
The CAN data frame represents the most common type of message transmitted on
the CAN bus. The first version of CAN, which is defined by the ISO 11519 specification,
uses an 11-bit identifier field that, when combined with a one-bit remote
transmission request (RTR) field, is used to determine the priority of messages
when two or more nodes are contending for access to the common bus. This version
of CAN operates at data rates up to 125 kbps and is referred to as standard CAN.
A second CAN data frame uses a 29-bit identifier formed by adding an 18-bit
identifier field to the standard CAN frame as well as incorporating three modifications
to the frame, which we will shortly discuss. This type of frame is referred to
as an extended frame and can operate at data rates up to 1 Mbps. For an extended
CAN data frame the arbitration field, which is employed to determine the priority
of messages when two or more nodes contend for access to the bus, consists of a
29-bit identifier field formed by separate 11-bit and 18-bit identifier fields and the
RTR bit. Now that we have a basic appreciation for the two types of data frames,
let us examine their composition in detail.
18.104.22.168 Standard Data Frame
Figure 5.5 illustrates the fields in the standard CAN data frame. Both the low-speed
CAN, defined by the ISO 11519 specification, and CAN 2.0A, defined by the ISO
11898 specification, are compatible with the use of an 11-bit identifier field. The
primary difference between the two ISO specifications is the fact that the original
standard CAN operates at 125 kbps while CAN 2.0A operates at 1 Mbps.
In examining both Figure 5.5, which illustrates the standard CAN data frame
format, and Figure 5.6, which shows the extended data frame format, you will note
the absence of an address field. Because CAN messages are broadcast on the bus,
there is no need for an address field. Instead, CAN messages can be considered to
be content addressed because the contents of a message determine if a node acts
upon a message.
Another item worth noting is the fact that the presence of an ACK bit does not
indicate that any of the intended nodes have received the message. This bit can be
set by any controller that was able to correctly receive the message, by sending an
ACK bit at the end of the message. Thus, the ACK bit only informs us that one or
more nodes on the bus correctly received the message.
22.214.171.124 Extended Data Frame
The extended CAN data frame, as previously noted in this chapter, uses a 29-bit
identifier. To extend the identifier, the extended CAN frame added an 18-bit identifier
field, which is separated from the original standard CAN 11-bit identifier field
by two fields, a substitute remote request (SRR) field and an identifier extension
Figure 5.6 illustrates the format of the extended CAN message frame. In comparing
Figure 5.4 to Figure 5.6, note that other than the use of an 18-bit identifier
field to extend the identifier to 29 bits, the extended CAN data frame only differs
from the standard CAN data frame by the addition of three fields:
11-bit identifier 18-bit
identifier r0 r1 0 ... 8 data bytes
Figure 5.6 The extended CAN message frame.
11-bit identifier r0 0 ... 8 data bytes
Figure 5.5 Standard CAN data frame format. SOF, a bit that marks the start of
a frame and is used to synchronize nodes on a bus after being idle; identifier,
an 11-bit identifier that establishes the priority of a message, where a lower
value indicates a higher priority; RTR, the remote transmission request bit is
set when information is required from another node (although all nodes on
the bus receive the request, the identifier determines the node that responds);
IDE, the single identifier extension bit is set to define a standard CAN identifier
without an extension; R0, a reserved bit for future use; DLC, a 4-bit data
length code that indicates the number of bytes of data transmitted; data, 0 to 64
bits (8 bytes); CRC, a 15-bit cyclic redundancy check containing the checksum
(number of bits transmitted) of the preceding application data for error detection
(in actuality the CRC field consists of a 15-bit CRC and a recessive delimiter
bit that indicates the end of the field); ACK, each node that receives an accurate
message overwrites this bit position with a dominant bit, which indicates
the message was received error-free (if a receiving node detects an error, the
message is discarded and the sending node repeats the message; the ACK field
is two bits in length, with the first bit used for acknowledgment and the second
functioning as a delimiter); EOF, a seven-bit end-of-frame (EOF) field marks the
end of a CAN message; IFS, a seven-bit inter-frame separator (IFS) represents
the amount of time required by a controller to move a correctly received frame
into its message buffer area.
SRR — Substitute remote request bit, which replaces the RTR bit in the standard
message location as a placeholder in the extended frame
IDE — Identifier extension (IDE) bit, which indicates that an 18-bit extension
R1 — An additional reserved bit
For both the standard and extended CAN frames the arbitration field that is used
to determine the priority of a message when two or more nodes contend for use of
the bus can be considered to represent a pseudo-field. This field under standard
CAN contains an 11-bit identifier and the RTR bit, which is dominant for data
frames. Under extended CAN the arbitration field consists of a 29-bit identifier,
two recessive bits (SRR and IDE), and the RTR bit.
126.96.36.199 Bit Stuffing
For both standard and extended CAN frames bit stuffing results in the insertion of
a bit of opposite polarity after a sequence of five bits of the same polarity occurs. Bit
stuffing covers both standard and extended frames from the start-of-frame bit field
through the 15-bit cyclic redundancy code field.
5.2.2 Remote Frame
A third type of message that can be transmitted on a CAN bus is the remote
frame. The remote frame is similar to the standard and extended CAN data frames.
However, there are two key differences between the remote frame and each type of
data frame. First, the remote frame has no data field. Second, the remote frame is
explicitly marked by the RTR bit being set recessive.
Remote frames can be used to invoke a request–response type of bus traffic. For
example, if Node A transmits a remote frame with its arbitration field set to a value
of 246, then a node that determines the request frame requires a response would
respond with a data frame with its arbitration field similarly set to a value of 246.
Unlike data frames that commonly flow on a CAN bus, remote frames are not
commonly used. However, when used, the data length code field must be set to the
length of the expected response. If not, arbitration will not work.
5.2.3 Error Frame
A fourth type of frame supported by CAN is the error frame. This frame is transmitted
by any node detecting an error. In actuality, the error frame represents a
special message that violates the rules of a CAN message. The error frame is transmitted
when a node detects an error in a message, causing all other nodes in the
network to similarly transmit an error frame. The original node that transmitted
the error frame automatically retransmits the message. Through the use of error
counters in the CAN controller, which will be reviewed in the next section in this
chapter, a node is prevented from continuously transmitting error frames, which in
effect would lock up the bus.
The error frame consists of two fields. The first field is the error flags, which is
created by the superposition of error flags contributed by different nodes on the bus.
There are two types of error flags: active and passive. An active error flag is transmitted
by a node that detects an error on the network that is in the “error active”
error state. In comparison, a passive error flag is transmitted by a node that detects
an active error frame on the network that is in the “error passive” error state.
5.2.4 Overload Frame
The fifth type of frame that can flow on the CAN bus is the overload frame. This
frame is transmitted by a node that becomes too busy to process additional data. Thus,
the purpose of this frame is to provide for an additional delay between messages.
5.3 Error Handling
In concluding our examination of the operation of the Controller Area Network
we will turn our attention to one of the more important aspects of the technology:
the manner by which error handling occurs. However, prior to doing so, let us first
briefly review how conventional communications technology detects and corrects
errors, as this will provide a frame of reference for comparing CAN error handling
to common communications error handling.
5.3.1 Communications Error Handling
In a modern communications environment error handling occurs through either
the use of parity, when bytes are transmitted independently of one another, or the
use of a checksum, when bytes are grouped into a block for transmission.
5.3.2 Parity Checking
Under parity checking an extra bit, referred to as a parity bit, is added to each byte
to be transmitted. Parity bit checking can be either odd or even. Under even parity
bit checking the parity bit is set to a binary 0 if the number of set bits in the byte to
be transmitted is even. If the number of set bits is odd, then the parity bit is set to
a binary 1 so that the sum of all set bits is even. Under odd parity bit checking the
parity bit is set to a binary 1 if the number of bits set in the byte are an even number
and to a binary 0 if the number of bits set in the byte are an odd number.
Under parity checking only a single bit error can be detected. In addition, there
is no easy way to correct a byte with a bit error other than visually or by retransmission
of an entire document. Due to these problems, most error detection and correction
methods evolved through the blocking of bytes and the addition of a checksum
to the block that is computed based upon the use of a predefined algorithm.
5.3.3 Block Checking
Under a communications block checking method a fixed number of bytes are used
to generate a block. For example, one common communications protocol that
employs block checking is the Xmodem protocol. Under the Xmodem protocol
128 bytes are used to form a block. If the last block is only partially filled with data,
then the remainder of the block is filled with pad characters (ASCII 127) until the
block is filled with 128 characters.
Under block checking an algorithm is applied to each block to generate a checksum
that is appended to the block. Thus, the block and its checksum are transmitted.
At the receiver the same algorithm is applied to the received data block and a
locally generated checksum is computed. The locally generated checksum is then
compared to the transmitted checksum. If they match, the data block is assumed
to have been received error-free. Then the checksum is removed and the block is
sent from the receiver’s buffer for processing on the local computer. In addition,
the receiver transmits an acknowledement to the sender, which informs the sender
that it is okay to send the next data block. If the two checksums do not match, one
or more bit errors are assumed to have occurred. Thus, the receiver will transmit
a negative acknowledgment to the sender and place the received data block and
checksum into the great bit bucket in the sky. The negative acknowledgment serves
to inform the sender to resend the data block to include its checksum. Thus, errors
are corrected by retransmission.
Although the use of a checksum lowers the probability of an undetected error,
such errors can occur when the algorithm used to create the checksum is relatively
simple. To reduce the probability of an undetected error, modern communications
systems use a polynomial approach to error checking. That is, the bytes in the
data block to be transmitted are assumed to represent a long polynomial, which
is divided by a fixed polynomial. The resulting quotient is discarded while the
remainder becomes the checksum. However, when a polynomial approach is used,
the remainder is referred to as a cyclic redundancy check (CRC), which is placed
into a CRC field.
Now that we have an appreciation for the manner by which conventional communications
systems perform error handling, let us turn our attention to CAN
5.3.4 CAN Error Handling
Similar to conventional communications systems, an error handling capability is
included in the CAN protocol. Under CAN there are five ways that an error can
be detected. Two ways operate at the bit level, while the other three operate at the
message level. In general, detecting errors in a message appearing on the CAN bus
will result in the controller that detected the error transmitting an error flag. The
error flag informs other controllers on the bus to discard the current message, in
effect eliminating bus traffic. Then, the originating transmitter will retransmit the
previously erroneous message. Thus, the error flag can be thought of as similar to a
negative acknowledgment, while errors are corrected via retransmission.
5.3.5 Node Removal
One of the more interesting aspects of CAN error handling is the ability of a node
to remove itself from the CAN bus under certain conditions. To obtain the ability
to determine if a node should leave the bus, each node maintains two error counters.
One error counter increments when a transmit error occurs and logically has
the name transmit error counter. The second error counter is incremented when a
receive error occurs and has the name receive error counter. Because it is logical to
expect that a transmitter detecting an error increments its transmit error counter
faster than the listening nodes on the bus will increment their receive error counter,
because there is a high probability that the transmitter caused a detected error, the
transmit error counter value can be used as a threshold for action. That is, once the
transmit error counter value reaches a predefined value, the node associated with
the counter will first go into an error passive state. When in an error passive state
the node will not actively transmit an error flag when an error occurs. Next, the
node will then go into a “bus off” state, which means that the node will not participate
in any bus traffic.
5.3.6 Error Detection Methods
As mentioned earlier in this section, the CAN protocol defines five methods whereby
errors can be detected. Those methods can be categorized as error detection at the
bit level and at the message level. In this section we will turn our attention to the
manner by which the CAN protocol detects errors. Those methods used by CAN
for error detection are summarized in Table 5.2.
188.8.131.52 Bit Monitoring
Bit monitoring is one of two bit-level error detection methods used by the CAN
protocol. Under bit monitoring each transmitter on the CAN bus “reads” the transmitted
signal level. If the bit level differs from the one transmitted, the node connected
to the bus generates a bit error signal.
184.108.40.206 Bit Stuffing
The second error condition to occur at the bit level results from the bit stuffing
process. As discussed earlier in this chapter, when five consecutive bits of the same
level (0s or 1s) have been transmitted by a node, the node will add a sixth bit of
the opposite level to the transmitted bit stream. This process is similar to the zero
insertion method used by the High-level Data-Link Control (HDLC) protocol.
That method prevents a sequence of six consecutive binary 1 bits from appearing
between two flags that define the beginning and ending of a communications
frame. When five consecutive 1 bits occur in any part of a frame other than the
beginning and ending flag, the sending station inserts an extra 0 bit. When the
receiving station detects five 1 bits followed by a 0 bit, it will remove the previously
inserted 0 bit, restoring the bit stream to its original value. Thus, under HDLC a
false frame is precluded from occurring due to the zero insertion process.
The bit stuffing method utilized by the CAN protocol is used as a mechanism
to prevent excessive DC voltage bus buildup. That is, under CAN data is transmitted
using non-return-to-zero (NRZ) coding. This coding method means that a
sequence of binary 1s results in a high voltage level for the bit duration of all bits,
CAN Error Detection Methods
Cyclic redundancy check
while a sequence of binary 0s would result in no voltage for the bit duration of the
sequence of zero bits. Because a long string of binary 1s could result in DC voltage
buildup, while a long string of binary 0s could result in a loss of synchronization,
bit stuffing under CAN treats sequences of consecutive bits of each polarity the
same. This explains why a sixth bit of the opposite polarity is added to the outgoing
bit stream when five consecutive bits of 1s or 0s are transmitted by a node. Because
bit stuffing changes any long sequence of binary 0s or binary 1s, if more than five
consecutive bits of the same polarity occur on a bus, this represents an error condition.
The error condition that is signaled is referred to as a stuff error.
220.127.116.11 Frame Check
The frame check is one of three message errors that can occur under the CAN protocol.
Because the CAN message has a number of fixed fields that have a range of
predefined values, a single value, or a computed value, it becomes possible to check
certain CAN message fields. For example, the CRC delimiter, ACK delimiter, and
end-of-frame fields have values that can be easily checked. If the CAN controller
detects an invalid value in one or more of these fixed fields it will initiate a form
18.104.22.168 Acknowledgment Check
A second error message that can occur at the message level is referred to as an
acknowledgment error. As a review, all nodes that correctly receive a message
regardless of the message destination under the CAN protocol are expected to send
a dominant level in the acknowledgment slot in the message, while the transmitter
places a recessive level in the slot. If the transmitter does not detect a dominant level
in the acknowledgment slot, then an acknowledgment error is signaled.
22.214.171.124 Cyclic Redundancy Check
Each CAN message includes a 15-bit cyclic redundancy check (CRC). This CRC is
similar to a checksum, but computed by treating the message as a long polynomial,
which is then divided by a fixed polynomial. This results in a quotient and remainder,
with the quotient discarded and the remainder becoming the 15-bit CRC.
Each node performs a similar computation on the transmitted message, using the
same fixed polynomial. If a node’s computed CRC does not match the transmitted
CRC, a CRC error will be signaled.
5.3.7 CAN Controller Operations
Previously, we noted that a CAN controller can increment two counters, one of
which corresponds to recognition that a transmitter error occurred, while the second
counter is incremented in recognition that a receive error occurred. In actuality,
a CAN controller has its mode of operation controlled by the two error counters. In
this section we will examine the states that a controller can be in and how the values
of the two error counters are used to move the controller from one state to another.
126.96.36.199 Controller States
A CAN controller can be placed into one of three defined error states: error active,
error passive, and bus off. The error active state enables messages to be transmitted
and received. Thus, this state represents the normal operating mode of a controller.
When an error is detected, an error flag is transmitted by the controller.
The second CAN controller error state is the error passive state. A node enters
the error passive state when a controller experiences frequent problems when transmitting
and receiving messages. Although the controller can transmit and receive
messages in an error passive state, it will transmit an error flag when it detects an
error when receiving data.
The third CAN controller state is bus off. A controller enters this state if it
experiences significant problems when transmitting messages. Once the controller
enters the bus off state it cannot transmit or receive messages until it is reset by the
host microcontroller or processor.
188.8.131.52 Mode Control
The actual mode of operation of a CAN controller is determined by the contents of
the transmit error counter and the receive error counter. The CAN controller will
be in an error active mode when both the transmit error counter and receive error
counter contents are each less than or equal to 127. If the transmit error counter
value is greater than 127 but less than or equal to 255, the CAN controller will be
placed into an error passive mode of operation. Only if the contents of the transmit
error counter exceed 255 will the CAN controller be placed into a bus off state of
operation. When this situation occurs, the CAN controller must be reset by the
host microcontroller or processor to be able to resume an operational capability.
184.108.40.206 Counter Updating
Because the contents of the transmit error counter and receive error counters are the
mechanism by which a CAN controller resides in a particular state, let us discuss
how those counters are updated.
220.127.116.11 Receiver Error Counter
When a receiver detects an error it will normally increment the value of the counter
by 1. There are two exceptions to this. One exception occurs when the detected
error was a bit error when an error flag or an overload flag was transmitted. If the
receiver detects a dominant bit as the first bit after sending an error flag, its receive
error counter will be increased by 8.
The second exception occurs when a node detects 14 consecutive dominant bits
after sending an active error flag or an overload flag, or after detecting 8 consecutive
dominant bits following a passive error flag and after each sequence of 8 additional
consecutive dominant bits. When any of these conditions occurs, every receiver on
the bus will increment its receiver error counter value by 8.
18.104.22.168 Transmit Error Counter
The operational setting of the transmit error counter is slightly different from that
of the receive error counter. Those differences include decrementing the counter
value by 1 after the successful reception of a message unless the value was already
0. In addition, the counter value will also be decreased by 1 if it is between 1 and
127 upon the successful reception of a message, to include the successful sending of
the acknowledgment bit. In this situation, if the counter value was 0, it will remain
at 0, while a counter value greater than 127 will be set to a value between 119 and
127. Another key series of differences between the transmitter error counter and the
receive error counter are two situations that cause the transmit error counter to be
incremented by 8:
When a transmitter sends an error flag
When a transmitter detects a bit error while sending either an active error flag
or an overload flag
For both of the above situations the transmitter error counter value will be
incremented by 8.
22.214.171.124 Error Signaling
Previously we noted the different types of bit and message errors. However, in doing
so we deferred until now the details concerning the manner by which different error
signals are formed. Thus, let us turn our attention to this important topic.
When a node detects an error it will place an error flag on the bus as a mechanism
to prevent other nodes from accepting the erroneous message. The active error
flag consists of a sequence of six low or dominant bits. This sequence of six consecutive
low bits represents an intentional bit stuffing violation, which will be detected
by all other nodes on the bus. Each of the nodes will respond with its own error
flag. Once this occurs, the nodes that need to transmit, to include the node that
originated the active error flag, will begin their transmissions. This will result in the
occurrence of the CAN arbitration process, where the message with the highest priority
wins the arbitration process and obtains the ability to transmit its message.
When the CAN controller is in an error passive mode, the error frame will be in
the reverse state of an active error flag. That is, it will consist of six passive or high
bits. Because the error flag now consists of passive bits, the bus is not affected. Thus,
if no other nodes on the bus detect an error, the message will reach its destination
without interruption. Note that the error flag is used when a node has recognized
a receiving problem that does not require the bus to be affected. Because error
handling is automatically performed by the CAN controller, there is no need for
the host microcontroller to perform any error handling operations. Thus, the error
handling performed by the CAN controller enables the microcontroller to perform
|Last Updated on Sunday, 02 January 2011 20:52|