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LH75400/01/10/11 (Preliminary) User's Guide
22.2.1 Protocols
The CAN specification consists of a basic protocol and a more advanced 'full' protocol:
• The basic protocol involves a close association between the incoming data and the
CPU. With this protocol, the CPU constantly checks incoming data.
• The full protocol supports advanced filtering. Advanced filtering allows the CPU to act
upon certain data, freeing the CPU to handle other functions.
'Full' CAN also allows for two different frame formats:
• A standard frame format (referred to as 'CAN A') that is 11 bits long. Each 11-bit
identifier can have up to 2,032 unique identifiers, 16 of which are reserved.
• An extended frame format (referred to as 'CAN B') that is 29 bits long. Each 29-bit
identifier can have up to 536,870,912 unique identifiers.
Each unique identifier corresponds to a possible peripheral. The identifiers are determined
with each design of a network. CAN B networks crash if CAN A devices are used, unless
the CAN A devices can ignore the extended format found in CAN B identifiers. Devices
that can ignore this extended format known as CAN B passive devices.
22.2.2 Frame Types
The data received and transmitted on a CAN interface are referred to as frames. CAN uses
four types of frames, each with its own format.
• The message frame contains data.
• The remote frame requests data from the network.
• The error frame reports errors that occurred at a node.
• The overload frame delays transmission of a CAN frame if the receive node is not ready.
22.2.2.1 Message Frame
The contents of a message frame include:
• Start Bit — The message frame begins with a start bit. As its name implies, this bit indi-
cates the start of the message frame.
• Arbitration Field — The arbitration field follows the start bit. This field is a 29-bit Identifier
that consists of two 'recessive' bits, Self-Reception Request (SRR) and IDE, and the
Remote Transmission Request (RTR) bit.
• Data Field — The data field follows the arbitration field. The data field contains the actual
payload of data, and can range from one to eight bytes of data.
• CRC Field — The CRC field follows the data. This field consists of a 15-bit cyclic-redun-
dancy-code (CRC) that serves as a checksum of the data.
• Acknowledgement Slot — The acknowledgement slot follows the CRC field. The
acknowledgement slot verifies that the message frame was correctly received. The
transmitter checks for the presence of this acknowledge bit and retransmits the mes-
sage if no acknowledgement is found.
6/17/03
Controller Area Network
22-3
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