Freescale Semiconductor MPC5200B ユーザーズマニュアル

Freescale Semiconductor

19-27

MSCAN facilitates a sophisticated message storage system which addresses the requirements of a broad range of network applications.

Modern application layer software is built upon two fundamental assumptions: 

• Any CAN node is able to send out a stream of scheduled messages without releasing the bus between the two messages. 

Such nodes arbitrate for the bus immediately after sending the previous message and only release the bus in case of lost 
arbitration.

• The internal message queue within any CAN node is organized such that the highest priority message is sent out first, if 

more than one message is ready to be sent.

The above behavior cannot be achieved with a single transmit buffer. That buffer must be reloaded right after the previous message is sent. 
This loading process lasts a finite amount of time and has to be completed within the Inter-Frame Sequence (IFS)

1

 to be able to send an 

uninterrupted stream of messages. Even if this is feasible for limited CAN bus speeds, it requires that the CPU react with short latencies to 
the transmit interrupt. 

1. Reference the Bosch CAN 2.0A/B protocol specification dated September 1991.