COP87L84BC Datasheet, PDF (27/54 Pages) National Semiconductor (TI) – 8-Bit CMOS OTP Microcontrollers with 16k Memory, Comparators, and CAN Interface

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 22 and Figure 23
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 24 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
Stuffed Bit Stream
100000x
011111x
1000001x
0111110x
Note: x = {0,1}
FIGURE 21. CAN Message Arbitration
DS101137-53
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