Datasheet

External Pins
MC68HC908GZ60 • MC68HC908GZ48 • MC68HC908GZ32 Data Sheet, Rev. 6
Freescale Semiconductor 137
12.3 External Pins
The MSCAN08 uses two external pins, one input (CAN
RX
) and one output (CAN
TX
). The CAN
TX
output
pin represents the logic level on the CAN: 0 is for a dominant state, and 1 is for a recessive state.
A typical CAN system with MSCAN08 is shown in Figure 12-2.
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
C A N BUS
CAN CONTROLLER
(MSCAN08)
TRANSCEIVER
CAN NODE 1
CAN STATION 1
CAN NODE 2 CAN NODE N
CAN_L
CAN_H
CAN
TX
CAN
RX
MCU