XC908AS60ACFU Motorola Semiconductor Products, XC908AS60ACFU Datasheet - Page 383

• Manufacturer Part Number: XC908AS60ACFU
• Description: MC68HC908AZ60A, MC68HC908AS60A Hcmos Microcontroller Unit Technical Data
• Manufacturer: Motorola Semiconductor Products

23.5 Message Storage
23.5.1 Background
MC68HC908AZ60A — Rev 2.0
MOTOROLA
MSCAN08 facilitates a sophisticated message storage system which
addresses the requirements of a broad range of network applications.
Modern application layer software is built under two fundamental
assumptions:
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.
A double buffer scheme would de-couple the re-loading of the transmit
buffers from the actual message being sent and as such reduces the
reactiveness requirements on the CPU. Problems may arise if the
sending of a message would be finished just while the CPU re-loads the
second buffer. In that case, no buffer would then be ready for
transmission and the bus would be released.
At least three transmit buffers are required to meet the first of the above
requirements under all circumstances. The MSCAN08 has three
transmit buffers.
1. Any CAN node is able to send out a stream of scheduled
2. The internal message queue within any CAN node is organized as
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.
such that the highest priority message will be sent out first if more
than one message is ready to be sent.
MSCAN Controller (MSCAN08)
MSCAN Controller (MSCAN08)
Message Storage
Technical Data
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