C515C_9711 Datasheet, PDF (140/268 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

On-Chip Peripheral Components
C515C
6.5.1 Basic CAN Controller Functions
The on-chip CAN controller combines several functional blocks (see figure 6-41) that work in
parallel and contribute to the controller’s performance. These units and the functions they provide
are described below.
The CAN controller provides storage for up to 15 message objects of maximum 8 data bytes length.
Each of these objects has a unique identifier and its own set of control and status bits. Each object
can be configured with its direction as either transmit or receive, except the last message which is
only a receive buffer with a special mask register.
An object with its direction set as transmit can be configured to be automatically sent whenever a
remote frame with a matching identifier (taking into account the respective global mask register) is
received over the CAN bus. By requesting the transmission of a message with the direction set as
receive, a remote frame can be sent to request that the appropriate object be sent by some other
node. Each object has separate transmit and receive interrupts and status bits, allowing the
microcontroller full flexibility in detecting when a remote/data frame has been sent or received.
For general purpose two masks for acceptance filtering can be programmed, one for identifiers of
11 bits and one for identifiers of 29 bits. However the microcontroller must configure bit XTD
(Normal or Extended Frame Identifier) in the message configuration register for each valid message
to determine whether a standard or extended frame will be accepted.
The last message object has its own programmable mask for acceptance filtering, allowing a large
number of infrequent objects to be handled by the system.
The object layer architecture of the CAN controller is designed to be as regular and orthogonal as
possible. This makes it easy to use and small for implementation.
The message storage is implemented in an intelligent memory which can be addressed by the CAN
controller and the microcontroller interface. The content of various bit fields in the object are used
to perform the functions of acceptance filtering, transmit search, interrupt search and transfer
completion. It can be filled with up to 15 messages of 8 bytes data.
The CAN controller offers significantly improved status information over earlier versions, enabling a
much easier diagnosis of the state of the network.
Semiconductor Group
6-77
1997-11-01