cop888eb National Semiconductor Corporation, cop888eb Datasheet - Page 29

• Manufacturer Part Number: cop888eb
• Description: 8-bit Cmos Rom Based Microcontrollers With 8k Memory, Can Interface, 8-bit A/d, And Usart
• Manufacturer: National Semiconductor Corporation

Basic CAN Concepts
A unit that is “bus off” has the output drivers disabled, i.e., it
does not participate in any bus activity.
Frame Formats
INTRODUCTION
There are basically two different types of frames used in the
CAN protocol.
The data transmission frames are::
The control frames are:: error/overload frame
Note: This device cannot send an overload frame as a result of not being
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:
1. Start of Frame (SOF)
2. Arbitration field
3. Control field (IDE bit, R0 bit, and DLC field)
4. Data field (not in remote frame)
5. CRC field
6. ACK field
7. 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.
• Bus off
able to process all information. However, the device is able to recog-
nize an overload condition and join overload frames initiated by other
devices.
(Continued)
data/remote frame
FIGURE 18. CAN Message Arbitration
29
(See ERROR MANAGEMENT AND DETECTION for more
detailed information.)
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.
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.
Destuffed Bit Stream
Stuffed Bit Stream
100000x
1000001x
DS012837-20
Note: x = {0,1}
011111x
0111110x
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