MC68908GZ8MFJE Datasheet, PDF (123/314 Pages) Freescale Semiconductor, Inc – Microcontrollers

CAN NODE 1
MCU
CAN STATION 1
CAN NODE 2
Message Storage
CAN NODE N
CAN CONTROLLER
(MSCAN08)
CAN_TX
CAN_RX
TRANSCEIVER
CAN_H
CAN_L
CAN BUS
Figure 12-2. The CAN System
Each CAN station is connected physically to the CAN bus lines through a transceiver chip. The
transceiver is capable of driving the large current needed for the CAN and has current protection against
defected CAN or defected stations.
12.4 Message Storage
MSCAN08 facilitates a sophisticated message storage system which addresses the requirements of a
broad range of network applications.
12.4.1 Background
Modern application layer software is built under two fundamental assumptions:
1. Any CAN node is able to send out a stream of scheduled messages without releasing the bus
between two messages. Such nodes will arbitrate for the bus right after sending the previous
message and will only release the bus in case of lost arbitration.
2. The internal message queue within any CAN node is organized as such that the highest priority
message will be sent out first if more than one message is ready to be sent.
Above behavior cannot be achieved with a single transmit buffer. That buffer must be reloaded right after
the previous message has been sent. This loading process lasts a definite amount of time and has to be
completed within the inter-frame sequence (IFS) 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 reacts with short latencies to
the transmit interrupt.
MC68HC908GZ16 • MC68HC908GZ8 Data Sheet, Rev. 4
Freescale Semiconductor
123