CAN bus - Computer Busses - page 327

• Error frame - transmitted by a unit on a bus error.
• Overload frame- provides for an extra delay between the preceding and the succeeding
data or remote frames.

Data frames and remote frames are separated from preceding frames by an interframespace.
There is no explicit address in the messages. Instead, messages are contents-addressed ,
so that their contents implicitly determine their address.
In order to wake up other nodes of the system, which are in sleep-mode, a special wake-
up message with the dedicated, lowest possible identifier (rrr rrrd rrrr; r= recessive, d =
dominant) may be used.
21.4.1 Data frame

Data frames have seven different bit fields, as illustrated in Figure 21.3, these are:

• Start of frame - this defines the start of a data frame or a remote frame. It consists of a
single dominant bit. All units on the bus synchronise to the leading edge of the bit.
• Arbitration field - this field consists of the identifier and the RTR bit. The identifier
length is 11 bits (from ID-10 to ID-0). The seven most-signification bits (ID-10 to ID-4)
must not be all recessive. The RTR bit (remote transmission request bit) is dominant in a
dataframe, and recessive in a remote frame. Note that CAN 2.0B (extended CAN), uses a
29-bit Identifier (which also contains two recessive bits: SRR and IDE) and the RTR bit.
• Control field - this field has six bits and includes a 4-bit data length code(DLC) and two
bits which are reserved for future expansion (dominant, at present). The codes for the
DLC are dddd (for 0 bytes), dddr (for 1 byte), ddrd (for 2 bytes), up to rddd (for 8 bytes).
Thus up to 8 bytes can be defined in the data field.
• Data field - this field consists of up to eight bytes of data (MSB first).
• CRC field - this field contains the CRC sequence, followed by a CRC delimiter (which is
a single recessive bit).
• ACK slot - a receiver which correctly receives a message, reports an acknowledgement
by sending a message back to the transmitter with a dominant bit in the ACK slot. After
this field is the ACK delimiter field which is a single recessive bit.
• End of frame - this delimits a data frame and remote frame and consists of seven reces-
sive bits.

Interframe
Space

Data frame

Start of
frame

Arbitration
field

Control
field

Data
field

CRC
field

ACK
field

End of
frame

Figure 21.3 Data frame format

The 15-bit CRC calculation uses the message fields as a polynomial which is divided by a
defined CRC generator polynomial (using modulo-2 division), which is: