C515C_9711 Datasheet, PDF (157/268 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

On-Chip Peripheral Components
C515C
Arbitration Registers
The arbitration registers are used for acceptance filtering of incoming messages and to define the
identifier of outgoing messages. A received message is stored into the valid message object with a
matching identifier and DIR=”0” (data frame) or DIR=”1” (remote frame). Extended frames can be
stored only in message objects with XTD=”1”, standard frames only in message objects with
XTD=”0”. For matching, the corresponding global mask has to be considered (in case of message
object 15 also the mask of last message). If a received message (data frame or remote frame)
matches with more than one valid message object, it is stored into that with the lowest message
number.
When the CAN controller stores a data frame, not only the data bytes, but the whole identifier and
the data length code are stored into the corresponding message object (standard identifiers have
bits ID17...0 filled with “0”). This is implemented to keep the data bytes connected with the identifier,
even if arbitration mask registers are used. When the CAN controller stores a remote frame, only the
data length code is stored into the corresponding message object. The identifier and the data bytes
remain unchanged.
There must not be more than one valid message object with a particular identifier at any time. If
some bits are masked by the global mask registers (ie. “don’t care”), then the identifiers of the valid
message objects must differ in the remaining bits which are used for acceptance filtering.
If a received data frame is stored into a message object, the identifier of this message object is
updated. If some of the identifier bits are set to “don’t care” by the corresponding mask register,
these bits may be changed in the message object. If a remote frame is received, the identifier in
transmit-object remain unchanged, except for the last message object (which cannot start a
transmission). Here, the identifier bits corresponding to the “don’t care” bits of the last message
object’s mask may be overwritten by the incoming message.
Semiconductor Group
6-94
1997-11-01