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FlexCAN Module
MPC5644A Microcontroller Reference Manual, Rev. 6
1398
Freescale Semiconductor
32.4.3
Message buffer structure
The Message Buffer structure used by the FlexCAN module is represented in
. Both Extended
and Standard Frames (29-bit Identifier and 11-bit Identifier, respectively) used in the CAN specification
(Version 2.0 Part B) are supported. The buffer is a 4-word (128-bit) structure summarized in
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
0x0
CODE
SRR
IDE
RTR
LENGTH
TIME STAMP
0x4
PRIO
ID (Standard/Extended)
ID (Extended)
0x8
Data Byte 0
Data Byte 1
Data Byte 2
Data Byte 3
0xC
Data Byte 4
Data Byte 5
Data Byte 6
Data Byte 7
= Unimplemented or Reserved
Figure 32-4. Message Buffer Structure
Table 32-4. Message Buffer Structure (Word 0—0x0)
Field
Description
CODE
This 4-bit field can be accessed (read or write) by the CPU and by the FlexCAN module itself, as part
of the message buffer matching and arbitration process. The encoding for Rx buffers is shown in
and the encoding for Tx buffers is shown in
See
Section 32.5, Functional description
SRR
Substitute Remote Request
Fixed recessive bit, used only in extended format. It must be set to ‘1’ by the user for transmission (Tx
Buffers) and will be stored with the value received on the CAN bus for Rx receiving buffers. It can be
received as either recessive or dominant. If FlexCAN receives this bit as dominant, then it is interpreted
as arbitration loss.
1: Recessive value is compulsory for transmission in Extended Format frames
0: Dominant is not a valid value for transmission in Extended Format frames
IDE
ID Extended Bit
This bit identifies whether the frame format is standard or extended.
1: Frame format is extended
0: Frame format is standard
RTR
Remote Transmission Request
This bit is used for requesting transmissions of a data frame. If FlexCAN transmits this bit as ‘1’
(recessive) and receives it as ‘0’ (dominant), it is interpreted as arbitration loss. If this bit is transmitted
as ‘0’ (dominant), then if it is received as ‘1’ (recessive), the FlexCAN module treats it as bit error. If the
value received matches the value transmitted, it is considered as a successful bit transmission.
1: Indicates the current message buffer has a Remote Frame to be transmitted
0: Indicates the current message buffer has a Data Frame to be transmitted
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