dsPIC30F Family Reference Manual
DS70074C-page 26-18 © 2004 Microchip Technology Inc.
B.6 Frame Types
B.6.1 Standard Data Frame
A data frame is generated by a node when the node wishes to transmit data. The Standard CAN
Data Frame is shown in Figure B-2. In common with all other frames, the frame begins with a
Start-Of-Frame bit (SOF – dominant state) for hard synchronization of all nodes.
The SOF is followed by the Arbitration field consisting of 12 bits, the 11-bit ldentifier (reflecting
the contents and priority of the message) and the RTR bit (Remote Transmission Request bit).
The RTR bit is used to distinguish a data frame (RTR – dominant) from a remote frame.
The next field is the Control field, consisting of 6 bits. The first bit of this field is called the lDE bit
(Identifier Extension) and is at dominant state to specify that the frame is a standard frame. The
following bit is reserved, RB0, and defined as a dominant bit. The remaining 4 bits of the Control
field are the Data Length Code (DLC) and specify the number of bytes of data contained in the
message.
The data being sent follows in the Data field, which is of the length defined by the DLC above
(1-8 bytes).
The Cyclic Redundancy field (CRC) follows and is used to detect possible transmission errors.
The CRC field consists of a 15-bit CRC sequence, completed by the recessive CRC Delimiter
bit.
The final field is the Acknowledge field. During the ACK Slot bit, the transmitting node sends out
a recessive bit. Any node that has received an error free frame acknowledges the correct
reception of the frame by sending back a dominant bit (regardless of whether the node is
configured to accept that specific message or not). From this, it can be seen that CAN belongs
to the “in-bit-response” group of protocols. The recessive Acknowledge Delimiter completes the
Acknowledge slot and may not be overwritten by a dominant bit.
B.7 Extended Data Frame
In the Extended CAN Data frame, shown in Figure B-3, the Start-Of-Frame bit (SOF) is followed
by the Arbitration field consisting of 38 bits. The first 11 bits are the 11 most significant bits of the
29-bit identifier (“Base-lD”). These 11 bits are followed by the Substitute Remote Request bit,
SRR, which is transmitted as recessive. The SRR is followed by the lDE bit, which is recessive
to denote that the frame is an extended CAN frame. It should be noted from this, that if
arbitration remains unresolved after transmission of the first 11 bits of the identifier, and one of
the nodes involved in arbitration is sending a standard CAN frame (11-bit identifier), then the
standard CAN frame will win arbitration due to the assertion of a dominant lDE bit. Also, the
SRR bit in an extended CAN frame must be recessive to allow the assertion of a dominant RTR
bit by a node that is sending a standard CAN remote frame. The SRR and lDE bits are followed
by the remaining 18 bits of the identifier (“lD-Extension”) and the Remote Transmission Request
bit.
To enable standard and extended frames to be sent across a shared network, it is necessary to
split the 29-bit extended message identifier into 11-bit (most significant) and 18-bit (least
significant) sections. This split ensures that the Identifier Extension bit (lDE) can remain at the
same bit position in both standard and extended frames.
The next field is the Control field, consisting of 6 bits. The first 2 bits of this field are reserved
and are at dominant state. The remaining 4 bits of the Control field are the Data Length Code
(DLC) and specify the number of data bytes.
The remaining portion of the frame (Data field, CRC field, Acknowledge field, End-Of-Frame
and lntermission) is constructed in the same way as for a standard data frame.
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