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25.3.1.2 Fields for Extended Format Frames
Table 25-5 describes the message buffer fields used only for extended identifier format
frames.
Field
ID[28:18]/[17:15] Contains the 14 most significant bits of the extended identifier, located in the ID HIGH word of the
message buffer.
Substitute
Contains a fixed recessive bit, used only in extended format. Should be set to one by the user for
Remote Request
Tx buffers. It will be stored as received on the CAN bus for Rx buffers. This is a bit in the ID HIGH
(SRR)
word of the message buffer.
ID Extended (IDE) If extended format frame is used, this field should be set to one. If zero, standard format frame
should be used. This is a bit in the ID HIGH word of the message buffer.
ID[14:0]
Bits [14:0] of the extended identifier, located in the ID LOW word of the message buffer.
Remote
This bit is located in the least significant bit of the ID LOW word of the message buffer;
Transmission
0 Data Frame
Request (RTR)
1 Remote Frame.
25.3.1.3 Fields for Standard Format Frames
Table 25-6 describes the message buffer fields used only for standard identifier format
frames.
Field
ID[28:18]
Contains bits [28:18] of the identifier, located in the ID HIGH word of the message buffer. The four
least significant bits in this register (corresponding to the IDE bit and ID[17:15] for an extended
identifier message) must all be written as logic zeros to ensure proper operation of the FlexCAN.
RTR
Remote Transmission Request. This bit is located in the ID HIGH word of the message buffer.
0 data frame
1 remote frame.
If the FlexCAN transmits this bit as a one and receives it as a zero, an "arbitration loss" is
indicated. If the FlexCAN transmits this bit as a zero and receives it as a one, a bit error is
indicated.
If the FlexCAN transmits a value and receives a matching response, a successful bit transmission
is indicated.
16-Bit Time Stamp The 16-bit time stamp, located in the ID LOW word of the message buffer, is not needed for
standard format, and is used in a standard format buffer to store the 16-bit value of the
free-running timer. The timer value is captured at the beginning of the identifier field of the frame
on the CAN bus.
25.3.2 Message Buffer Memory Map
The message buffer memory map starts at an offset of 0x80 from the FlexCAN's base
address (0x1C_0000). The 256-byte message buffer space is fully used by the 16 message
buffer structures.
MOTOROLA
Table 25-5. Extended Format Frames
Table 25-6. Standard Format Frames
Chapter 25. FlexCAN
Description
Description
Message Buffers
25-7
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