dsPIC30F Family Reference Manual
DS70074C-page 26-14 © 2004 Microchip Technology Inc.
Full CAN devices do the whole bus protocol in hardware, including the acceptance filtering and
the message management. They contain several so called message objects which handle the
identifier, the data, the direction (receive or transmit) and the information Standard
CAN/Extended CAN. During the initialization of the device, the host CPU defines which
messages are to be sent and which are to be received. The host CPU is informed by interrupt if
the identifier of a received message matches with one of the programmed (receive-) message
objects. In this way. the CPU load is reduced. Using Full CAN devices, high baud rates and high
bus loads with many messages can be handled. These chips are more expensive than the
Basic CAN devices, though.
Many Full CAN chips provide a “Basic-CAN Feature”. One of their messages objects can be
programmed in a way that every message is stored there that does not match with one of the
other message objects. This can be very helpful in a number of applications.
B.4 ISO Model
The lSO/OSl Reference Model is used to define the layers of protocol of a communication
system, as shown in Figure B-1. At the highest end, the applications need to communicate
between each other. At the lowest end, some physical medium is used to provide electrical
signaling.
The higher levels of the protocol are run by software. Typically, only the application layer is
implemented. Within the CAN bus specification, there is no definition of the type of message or
the contents or meaning of the messages transferred. These definitions are made in systems
such as Volcano, the Volvo automotive CAN specification; J1939, the U.S. heavy truck multiplex
wiring spec; and Allen-Bradly DeviceNet and Honeywell SDS, examples of industrial protocols.
The CAN bus module definition encompasses two levels of the overall protocol.
• The Data Link Layer
- The Logical Link Control (LLC) sub-layer
- The Medium Access Control (MAC) sub-layer
• - The Physical Layer
- The Physical Signaling (PLS) sub-layer
The LLC sub layer is concerned with Message Filtering, Overload Notification and Error
Recovery Management. The scope of the LLC sub-layer is:
• To provide services for data transfer and for remote data request
• To decide which messages received by the LLC sub layer are actually to be accepted
• To provide means for error recovery management and overload notifications
The MAC sub-layer represents the kernel of the CAN protocol. The MAC sub-layer defines the
transfer protocol (i.e., controlling the Framing, Performing Arbitration, Error Checking, Error
Signalling and Fault Confinement). It presents messages received from the LLC sub-layer and
accepts messages to be transmitted to the LLC sub-layer. Within the MAC sub-layer, it is
decided whether the bus is free for starting a new transmission or whether a reception is just
starting. The MAC sub-layer is supervised by a management entity called Fault Confinement,
which is a self-checking mechanism for distinguishing short disturbances from permanent
failures. Also, some general features of the bit timing are regarded as part of the MAC
sub-layer.
The physical layer defines the actual transfer of the bits between the different nodes with
respect to all electrical properties. The PLS sub-layer defines how signals are actually
transmitted and therefore deals with the description of Bit Timing, Bit Encoding and
Synchronization.
The lower levels of the protocol are implemented in driver/receiver chips and the actual
interface, such as twisted pair wiring or optical fiber etc. Within one network, the physical layer
has to be the same for all nodes. The Driver/Receiver Characteristics of the Physical Layer are
not defined so as to allow transmission medium and signal level implementations to be
optimized for their application. The most common example is defined in the ISO11898 Road
Vehicles Multiplex Wiring specification.
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