AN1077 View Datasheet(PDF) - STMicroelectronics
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MFG CO.
'AN1077' PDF : 14 Pages View PDF
OVERVIEW OF ENHANCED CAN CONTROLLERS FOR ST7 AND ST9 MCUS
ciently. Therefore software solutions are usually implemented, consuming CPU resources and
making the application less independent from the CAN bus traffic.
The Overall Tasks of the CAN Interface
Considering the receiver part of the CAN interface the following tasks have to be performed:
s Message checking, error handling and acknowledgement. This task is fully handled by the
CAN protocol implemented in all CAN controllers
s Message filtering indicates receptions addressed to the application and discards the other
ones. This task is partially done by hardware but software filtering is often needed. The CPU
resources consumed for software filtering depends on the CAN bus traffic. This means that
the ECU performance is influenced by events not related to the application. This can make
the modification of an entire system or the integration of the ECU in another network very
critical.
s Identifier recognition. Once a message has passed through the filters its content must be
identified in order to compute the destination address the data must be stored to.
s Signal extraction. To optimize the bandwidth usage of the CAN bus, the signals are
concatenated to have more data transmitted in each message. Thus on message reception
the application must extract the signals it is interested in.
s Signal storage in RAM
2 CAN CONTROLLER SOLUTIONS
2.1 TRADE-OFF BETWEEN COSTS AND EFFICIENCY
Nowadays CAN controllers can be classified in two main families:
s BasicCAN
This approach provides only few mailboxes for message transmission and reception. This so-
lution allows very compact implementations on silicon.
Advantages:
– Very cost-effective and therefore well suited to 8-bit microcontrollers
Drawbacks:
– Requires CPU resources for filtering, identifier recognition and signal storage
– High software real-time constraints on message reception
s FullCAN This approach provides more mailboxes than messages to handle. In the past, 16
mailboxes were sufficient, today 32 or 64 are standard. Advantages:
s Autonomous message filtering
s One static identifier per mailbox
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