COP888EB datasheet(29/75 Pages) NSC

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Basic CAN Concepts (Continued)

•

Bus off

A unit that is “bus off” has the output drivers disabled, i.e., it

does not participate in any bus activity.

(See ERROR MANAGEMENT AND DETECTION for more

detailed information.)

Frame Formats

INTRODUCTION

There are basically two different types of frames used in the

CAN protocol.

The data transmission frames are::

data/remote frame

The control frames are::

error/overload frame

Note: This device cannot send an overload frame as a result of not being

able to process all information. However, the device is able to recog-

nize an overload condition and join overload frames initiated by other

devices.

If no message is being transmitted, i.e., the bus is idle, the

bus is kept at the “recessive” level.

Figure 19 and Figure 20

give an overview of the various CAN frame formats.

DATA AND REMOTE FRAME

Data frames consist of seven bit fields and remote frames

consist of six different bit fields:

Start of Frame (SOF)

Arbitration field

Control field (IDE bit, R0 bit, and DLC field)

Data field (not in remote frame)

CRC field

ACK field

End of Frame (EOF)

A remote frame has no data field and is used for requesting

data from other (remote) CAN nodes.

Figure 21 shows the

format of a CAN data frame.

FRAME CODING

Remote and Data Frames are NRZ codes with bit-stuffing in

every bit field which holds computable information for the in-

terface, i.e., Start of Frame arbitration field, control field, data

field (if present) and CRC field.

Error and overload frames are NRZ coded without bit stuff-

ing.

BIT STUFFING

After five consecutive bits of the same value, a stuff bit of the

inverted value is inserted by the transmitter and deleted by

the receiver.

Destuffed Bit Stream

100000x

011111x

Stuffed Bit Stream

1000001x

0111110x

Note: x = {0,1}

START OF FRAME (SOF)

The Start of Frame indicates the beginning of data and re-

mote frames. It consists of a single “dominant” bit. A node is

only allowed to start transmission when the bus is idle. All

nodes have to synchronize to the leading edge (first edge af-

ter the bus was idle) caused by SOF of the node which starts

transmission first.

ARBITRATION FIELD

The arbitration field is composed of the identifier field and the

RTR (Remote Transmission Request) bit. The value of the

RTR bit is “dominant” in a data frame and “recessive” in a re-

mote frame.

CONTROL FIELD

The control field consists of six bits. It starts with two bits re-

served for future expansion followed by the four-bit Data

Length Code. Receivers must accept all possible combina-

tions of the two reserved bits. Until the function of these re-

served bits is defined, the transmitter only sends “0” (domi-

nant) bits. The first reserved bit (IDE) is actually defined to

indicate an extended frame with 29 Identifier bits if set to “1”.

CAN chips must tolerate extended frames, even if they can

only understand standard frames, to prevent the destruction

of an extended frames on an existing network.

The Data Length Code indicates the number of bytes in the

data field. This Data Length Code consists of four bits. The

data field can be of length zero. The permissible number of

data bytes for a data frame ranges from 0 to 8.

DS012837-20

FIGURE 18. CAN Message Arbitration

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