CAN Bus Video Lecture - Embedded Systems (Web) - Computer Science Engineering (CSE)

hello my name is Jim Lawrence of a
senior design engineer at Akron egg and
I'll be giving an introduction to the
canvas ken is an acronym that stands for
controller area network is a form of
multiplex wiring designed by Bosch
corporation that allows the linking of a
number of control systems together
normally in a vehicle so they could
share information and has been used in
automobiles for over 20 years the first
can controller chips were produced by
Intel and Phillips and it came on the
market in 1987 can is a very reliable
data transmission that satisfies the
real-time requirements of many
applications can is a multi drought
multi master serial bus that provides
communications between controllers
sensors and actuators that is very
reliable technology that has been well
proven it's been around since the mid
1980s CANbus is available on many low
cost micro processors and as a result of
that very inexpensive to use and designs
of sensors and actuators there are two
versions of can that are in use can 2.0
a and can point to point zero B now the
difference between the two standards is
the V has an extended identifier they
use 29 identifiers instead of 11 so it
allows for more types of messages to be
identified
there are two international standards
iso 11 8 9 8 - 2 is the high speed can
one negatives per second and then
there's iso 11 8 98 - 3 which is
low-speed and applications at 125
kilobits per second can is well-known in
the automotive application area but it
is also seeing extensive use in other
areas as well such as military vehicles
industrial machinery medical systems
agricultural machinery marine control
navigation and elevator control systems
are some examples of other can
application areas this slide shows the
model of a network layer representation
of can or any network application and
can defines the lower two levels the
data link layer and the physical layer
there are higher layer protocols that
are built up on topic and and some of
these are device net can open can
aerospace milk and SAE j1939 is the
automotive standard and there's also iso
1192 which is used for heavy trucks and
to communicate between the truck and
trailer so the characteristics that can
that all messages are broadcast messages
and any note on the bus can broadcast a
message at any time it wants to each
message contains an identifier that
identifies the source of the content of
message and each receiver on the bus
listens to or receives every message
that sent and depending on the
identifier or the count
the message it decides on whether that
information is useful and they can
choose to process that information or
ignore it the maximum length of the
canvas is dependent on the bitrate that
is being used on that bus one megabit
per second bus would being able to have
a maximum length up to 40 meters and a
slower implementation of can such as the
125k bit per second baud rate but we
would be able to go up to 500 meters now
the physical median used by kin is a
single twisted pair of wire and it's
terminated on each end with 120 ohm
termination network the slide shows two
nodes connected to the twisted pair this
slide shows a display of what you would
see on an oscilloscope if you were
looking at the can high and can low
signals on a can bus the two logic
states of the bus
not only notice like logic high or low
or one or zero are turned dominant and
recessive by the canvas specification
the dominant is a logic low and when the
bus is in the dominant state both can
high and can low more driven can't
highest driven two three and a half
volts can low as well down to one and a
half and when we're in the recessive
state neither one of the can't higher
kandala two driven in the bus floats to
its midpoint level of about two and a
half volts now this by using these two
the dominant and recessive state if two
nodes we're transmitting simultaneously
and one node attempted to put the
recessive state on in the bus while the
other one for the dominant state out of
course the dominant state would be
visible to all nodes and the recessive
state would be
written by the dominant and that is the
mechanism that is used to canvas to
arbitrate between two transmitters
trying to access the bus at the same
time the transmitter that's transmitting
the dominant statement when the
arbitration and the transmitter that was
attempting to transmit the recessive
state would be the loser of the
arbitration and it would have to be
about a clock it's transmitting and wait
for another available time to transmit
each can note has its own local
oscillator
there's no timing information well
there's no clock information embedded in
the cans signal so at the beginning of
each bit time that has a recessive to
dominant transition all receivers on the
canvas must synchronize to the
transmitter - when this gadget curves
now each of the local oscillators is
running at some multiple of the bit rate
and it has a counters pea counters
internal to them that will determine the
optimum time to sample the can bus a
signal level during the bit period and
that is adjusted by or I should say
tuned by the system engineer to optimize
the timing of the can receivers the can
specification doesn't they do not
specify a maximum number of nodes that
are allowed on a canvas the networks are
really limited by the electrical loading
of the revolving notes and the basically
the drive strength of the
visit transmitters but typically
sixty-four nodes it's normal for the
canvas up to 64 there are four message
types defined in the canvas the data
frame remote frame error frame and
overload frame almost all the
information is sent using the data frame
and that is when a transmitter broadcast
a message that is the message type that
we use in frequently used is the remote
frame and that is message sent by
transmitter requesting another device to
transmit data but is not frequently used
the other two as the error frame and
that is sent by the receiver no Kenny
doubt that detects an error and the
message can send an error frame and
there's also the overload frame and that
is information that is sent by the
receiver that to notify us under that
it's not ready to receive additional
data this slide shows the components of
a message in standard format
it starts out Oh messages start with a
start of frame bit and they are followed
by the identifier bit in this case in
the standard message that's 11 bits
really identifier if it were an extended
format message then the identifier
length would be 29 bits
eleven following the identifier is a
single built that is used to distinguish
this message from the remote
transmission message the next bid is the
ID extension which would distinguish the
standard format message from the
extended format message we have one
reserved bit and the next four bits will
tell we identify the length of the
message the can be 0 8 megabytes in the
message and following the data is a
cyclic redundancy check there's 15 bits
of CRC followed by one CRC delimiter and
there's a bit period for the receivers
of a message to acknowledge they've
received the message and there's one bit
time for a delimiter and then seven bit
times to identify the end of the frame
there are five different error detection
capabilities designed into can - first -
our error detection that are that is it
done by the transmitter of a message and
the last three are errors that are
detected by the receiver of a message
now the first the bit error is detected
by transmitter when it is attempting to
output a dominant or a recessive state
on the bus and it detects something
opposite of what is expecting so it can
detect a bit error as done by the
transmitter and the transmitter also
will expect it acknowledged from each
receiver or from at least one receiver
each time it sends a message and if it
does not detect it acknowledged for a
transmission that it sends then that
would be an acknowledged error that
means that one of the truth either none
of the transceivers receive the message
or none of them received it without air
okay the third what the last three
errors are detected by a receiver on the
canvas the first error is a form error
and that would be a receiver that
detects that a delimiter is not in its
correct position would be an example of
a form error so the transmitter of a
message will calculate a CR a checksum
for the whole message and transmit that
checksum at the end of the data section
of the message a receiver will also
calculate a checksum as it receives the
data and it will compare its calculate a
checksum with the checksum that set by
the transmitter and if they are not the
same then they will detect that error
has occurred the transmission there's
also a stuffier that can be detected by
a receiver the in a canvas specification
no more than five consecutive bits of
the same value can be sent and if
receiver detects a six bit
that's the same logical value then that
would be a stuff error the since the
receivers have to resynchronize
occasionally they can't tolerate a
longer period than five bit times going
between synchronizations thank you for
viewing this video I hope you found it
helpful and informative if you would
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