Freescale Semiconductor MPC5200B - Manual (Page 666)

Functional Description
MPC5200B Users Guide, Rev. 1
Freescale Semiconductor 19-27
19.7.2 Message Storage
Figure 19-3. User Model for Message Buffer Organization
MSCAN facilitates a sophisticated message storage system which addresses the requirements of a broad range of network applications.
19.7.2.1 Message Transmit Background
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.
MSCAN
Rx0
Rx1
CPU bus
MSCAN
Tx2 TXE2
PRIO
Receiver
Transmitter
RxBG
TxBG
Tx0 TXE0
PRIO TxB
G
Tx1
PRIO
TXE1
TxFG
CPU bus
Rx2
Rx3 Rx3
RXF
Rx
FG