Image Processing Reference
In-Depth Information
S
O
F
R
T
R
I
D
E
DLC
EOF
IMS
r
0
11 bit
Identifier
0−8 byte
Data field
CRC
Field
ACK
Field
Arbitration field
Control field
(a) Standard (base) frame format
S
O
F
S
R
R
I
D
E
R
T
R
r
1
r
0
DLC
0−8 byte
Data field
ACK
Field
EOF
IMS
11 bit
Base ID.
18 bit
ID. Extension
CRC
Field
Control field
Arbitration field
(b) Extended frame format
FIGURE .
Format of data frames.
15.2.2.1 Data Frame
Data frames are used to send information over the network. Each CAN data frame begins with
an SOF bit at the dominant level, as shown in Figure .. Its role is to mark the beginning of
the frame, as in serial transmissions carried out by means of conventional universal asynchronous
receiver/transmitters (UARTs). he SOF bit is also used to synchronize the receiving nodes.
Immediately after the SOF bit the arbitration field follows, which includes both the identifier field
and the remote transmission request (RTR) bit. As the name suggests, the identifier field identifies the
content of the frame which is being exchanged uniquely on the whole network. he identifier is also
used by the MAC sublayer to detect and manage the priority of the frame, which is used whenever a
collision occurs (the lower the numerical value of the identifier, the higher the priority of the frame).
The identifier is sent starting from the most significant bit up to the least significant one. The size
of the identifier is different for the standard (base) and the extended frames. In the latter case the
identifier has been split into an bit base identifier and an bit identifier extension, so as to provide
compatibility with the standard frame format.
The RTR bit is used to discriminate between data and remote frames. Since a dominant value of
RTR denotes a data frame whilst a recessive value stands for a remote frame, a data frame has a higher
priority than a remote frame having the same identifier.
Next to the arbitration field comes the control field. In the case of base frames, it includes the
identifier extension (IDE) bit, which discriminates between base and extended frames, followed by
the reserved bit
r
. Instead, in extended frames the IDE bit effectively belongs to the arbitration field,
as well as the substitute remote request (SRR) bit—a placeholder that is sent at recessive value to
preserve the structure of the frames. In this case the IDE bit is followed by the identifier extension
and then by the control field, that begins with the two reserved bits
r
and
r
. After the reserved bits
the data length code (DLC) follows, which specifies the length (in number of bytes encoded on
bits) of the data field. Since the IDE bit is dominant in base format frames whilst it is recessive in the
extended ones, standard frames have precedence over their extended counterparts when the same
base identifier is considered.
Reserved bits
r
and
r
must be sent by the transmitting node at the dominant value. Receivers,
however, will ignore the value of these bits. For the DLC field, values ranging from to are allowed.
According to the last specification, higher values (from to ) can be used for application-specific
purposes. In this case, however, the length of the data field is meant to be .
The data field is used to store the effective payload of the frame. In order to ensure a high degree of
responsiveness and minimize the priority inversion phenomenon, the size of the data field is limited
to bytes at most.
