NEC V850E/CA1 ATOMIC, Preliminary User'S Manual

Page: 384 / 601

384
Chapter 13 FCAN Interface Function
Preliminary User’s Manual U14913EE1V0UM00
13.2.6 Message handling
In the FCAN system the assignment of message buffers to the CAN modules is not defined by hard-
ware. Each message buffer in the message buffer section can be assigned to any CAN module by soft-
ware. The message buffers have individual configuration registers to assign the CAN module and to
specify the message buffer type.
Basically, a message buffer can be selected as a transmit message buffer or as a receive message
buffer. For receive message buffers there are further differentiations according to the mask links.
(1) Message transmission
According to the CAN protocol the highest prior message must always gain the CAN bus access
against lower prior messages sent by other nodes at the same time (due to arbitration mechanism
of CAN protocol) and against messages waiting to be transmitted in the same node (i.e. inner prior-
ity inversion).
The FCAN system scans the message buffer section at the beginning of each message transmit to
analyse that no other message with a higher priority is waiting to be transmitted on the same CAN
bus. The FCAN system avoids inner priority inversion automatically.
Example:
5 transmit messages are waiting to be sent at the same time in the example shown in Table 13-10.
Although the priority of the transmit messages are not sorted according any scheme, the sequence
of transmits on the CAN bus is:
<1> message buffer number 15 (ID = 023H)
<2> message buffer number 1
(ID = 120H)
<3> message buffer number 22 (ID = 123H)
<4> message buffer number 14 (ID = 223H)
<5> message buffer number 2
(ID = 229H)