2000 Microchip Technology Inc.
CAN MESSAGE FRAMES
The MCP2510 supports Standard Data Frames,
Extended Data Frames, and Remote Frames (Standard
and Extended) as defined in the CAN 2.0B specification.
Standard Data Frame
The CAN Standard Data Frame is shown in Figure 2-1.
In common with all other frames, the frame begins with
a Start Of Frame (SOF) bit, which is of the dominant
state, which allows hard synchronization of all nodes.
The SOF is followed by the arbitration field, consisting
of 12 bits; the 11-bit ldentifier and the Remote Trans-
mission Request (RTR) bit. The RTR bit is used to dis-
tinguish a data frame (RTR bit dominant) from a remote
frame (RTR bit recessive).
Following the arbitration field is the control field, con-
sisting of six bits. The first bit of this field is the Identifier
Extension (IDE) bit which must be dominant to specify
a standard frame. The following bit, Reserved Bit Zero
(RB0), is reserved and is defined to be a dominant bit
by the can protocol. the remaining four bits of the con-
trol field are the Data Length Code (DLC) which speci-
fies the number of bytes of data contained in the
After the control field is the data field, which contains
any data bytes that are being sent, and is of the length
defined by the DLC above (0-8 bytes).
The Cyclic Redundancy Check (CRC) Field follows the
data field and is used to detect transmission errors. The
CRC Field consists of a 15-bit CRC sequence, followed
by the recessive CRC Delimiter bit.
The final field is the two-bit 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). The recessive acknowledge delimiter com-
pletes the acknowledge field and may not be overwrit-
ten by a dominant bit.
Extended Data Frame
In the Extended CAN Data Frame, shown in Figure 2-2,
the SOF bit is followed by the arbitration field which
consists of 32 bits. The first 11 bits are the most signif-
icant bits (Base-lD) of the 29-bit identifier. These 11 bits
are followed by the Substitute Remote Request (SRR)
bit which is defined to be recessive. The SRR bit is fol-
lowed by the lDE bit which is recessive to denote an
extended CAN frame.
It should be noted that if arbitration remains unresolved
after transmission of the first 11 bits of the identifier,
and one of the nodes involved in the arbitration is send-
ing 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 send-
ing a standard CAN remote frame.
The SRR and lDE bits are followed by the remaining 18
bits of the identifier (Extended lD) and the remote trans-
mission 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 lDE bit can remain at the same bit
position in both standard and extended frames.
Following the arbitration field is the six-bit control field.
the first two bits of this field are reserved and must be
dominant. the remaining four bits of the control field are
the Data Length Code (DLC) which specifies the num-
ber of data bytes contained in the message.
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 (see Section 2.1).
Normally, data transmission is performed on an auton-
omous basis by the data source node (e.g. a sensor
sending out a data frame). It is possible, however, for a
destination node to request data from the source. To
accomplish this, the destination node sends a remote
frame with an identifier that matches the identifier of the
required data frame. The appropriate data source node
will then send a data frame in response to the remote
There are two differences between a remote frame
(shown in Figure 2-3) and a data frame. First, the RTR
bit is at the recessive state, and second, there is no
data field. In the event of a data frame and a remote
frame with the same identifier being transmitted at the
same time, the data frame wins arbitration due to the
dominant RTR bit following the identifier. In this way, the
node that transmitted the remote frame receives the
desired data immediately.