
263
μPD780948, μPD78F0948, μPD780949, μPD78F0949
18.1 Protocol
CAN is an abreviation of "Controller Area Network", and is a class C high speed multiplexed commu-
nication protocol for real time communication in vehicle. CAN is being standardized in ISO (International
Organization for Standardization) and SAE (Society of Automotive Engineers). For more detailed
information please refer to Bosch, CAN specification 2.0 from September 1991.
18.1.1 Protocol mode function
(1) Standard format mode
This mode supports an 11-bit message identifier thus making it possible to differentiate between
2032 types of messages.
(2) Extension format mode
In the standard format mode, the identifier has 11 bits. However, in the extension format mode,
the identifier is extended to 29 bits (11 + 18).
When the IDE bits of the arbitration field is "recessive", it becomes the extension format mode.
When the message of the extension format mode and the remote frame of the standard format
mode are simultaneously transmitted, the node transmitting the message with the standard mode
wins arbitration.
(3) Bus values
The bus can have one of two complementary logical values: "dominant" or "recessive". During
simultaneous transmission of "dominant" and "recessive" bits, the resulting bus value will be
"dominant".
For example, in case of a wired-AND implementation of the bus, the "dominant" levelwould be
represented by a logical "0" and the "recessive" level by a logical 1.
Physical states (e.g. electrical voltage, light) that represent the logical levels are not given in this
specification.
18.1.2 Message format
The CAN protocol message supports different types of frames. The output conditions of each frame are
as follows:
Data frame:
Remote frame:
Error frame:
Overload frame: Frame output when a frame would be overwritten by the next one before the re-
ceiving mode could process it. The reception side does not finish its preparation.
Carries the data from a transmitter to the receiver.
Transmission demand frame from the requesting node.
Frame output on error detection.