M16C Datasheet, PDF (171/262 Pages) Mitsubishi Electric Semiconductor – SINGLE-CHIP 16-BIT CMOS MICROCOMPUTER

Preliminary Specifications REV.B
Mitsubishi microcomputers
Specifications in this manual are tentative and subject to change.
M16C / 6N Group
SINGLE-CHIP 16-BIT CMOS MICROCOMPUTER
CAN Module
Basic CAN Feature
Some applications for the CAN network operate with more than 16 message types (identifiers), so an original
CAN approach (one slot corresponds to one message type) is not a feasible way for these systems. The
first approach to give system support for these applications is the sophisticated mask concept implemented
in this CAN module (refer to section 'Mask Register and Acceptance Filter').
In case there is the requirement to receive most or all messages from the CAN bus (performing further
acceptance filter by software), the CAN module provides a special slot configuration to support this kind of
system solution. In the normal operation mode, the received message is stored in the first fitting message
slot. The slots under consideration for this decision will be determined in the acceptance filter phase. In this
case many messages will be received by one slot, the CPU is heavily loaded to serve this slot without
loosing a message because of 'overwriting' (receiving the next fitting message).
By activating the 'Basic CAN' feature, the slot scheduling in 'receive' case changes for slot #14 and #15.
Received messages are stored alternately in these two message boxes.
SLOT 14
SLOT 15
Empty
Locked (empty)
Msg n
Locked (empty)
Locked (Msg n)
Msg n+2
(lost Msg n)
Msg n + 1
Locked (Msg n+1)
etc.
Msg n
Msg n+1
Msg n+2
Figure 18-8. Receive slot scheduling for implementing the 'Basic CAN' feature
The CAN module uses two different slot addresses to build the Basic CAN feature (no shadow buffer
concept). The 'lock/unlock' function will be controlled exclusively by the CAN module without any influence
of the CPU. There is no 'message protection mechanism' implemented, so the message n+2 will overwrite
the content of message n (Figure 18-7).
The following restrictions have to be kept in case of using the 'Basic CAN' feature:
- The module configuration ('Basic CAN' ON/OFF) should be selected before leaving the 'Reset/Initializa-
tion' state. The CAN module will store the first fitting message into slot #14 (in case the filtering failed for all
preceding slots). In case the 'Basic CAN' feature will be enabled or disabled 'on the fly', the slot where the
first message will be stored is undetermined.
- The CAN module never checks, whether or not the received message will be accepted by slot #15 when
slot #14 is locked (last message is stored in slot #14). Therefore it is recommended to use the same
identifier for the message slots #14 and #15 (building the 'Basic CAN' channel) and the same mask values
for both local masks. Otherwise a received message might be dropped by the CAN module, although slot
#14 could accept this message.
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