FlexCAN
MCF5213 ColdFire® Integrated Microcontroller Reference Manual, Rev. 3
25-18
Freescale Semiconductor
Table 25-11. Message Buffer Field Descriptions
Field
Description
31–28
Reserved, must be cleared.
27–24
CODE
Message buffer code. 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 is shown in
and
. See
Section 25.3.11, “Functional Overview,”
for additional information.
23
Reserved, must be cleared.
22
SRR
Substitute remote request. Fixed recessive bit, used only in extended format. It must be set by the user for
transmission (Tx Buffers) and is stored with the value received on the CAN bus for Rx receiving buffers. It can be
received as recessive or dominant. If FlexCAN receives this bit as dominant, then it is interpreted as arbitration loss.
0 Dominant is not a valid value for transmission in Extended Format frames
1 Recessive value is compulsory for transmission in Extended Format frames
21
IDE
ID extended bit. Identifies whether the frame format is standard or extended.
0 Standard frame format
1 Extended frame format
20
RTR
Remote transmission request. 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.
0 Indicates the current MB has a data frame to be transmitted
1 Indicates the current MB has a remote frame to be transmitted
19–16
LENGTH
Length of data in bytes. Indicates the length (in bytes) of the Rx or Tx data; data is located in offset 0x8 through
0xF of the MB space (see
). In reception, this field is written by the FlexCAN module, copied from the
DLC (data length code) field of the received frame. DLC is defined by the
CAN Specification
and refers to the data
length of the actual frame before it is copied into the message buffer. In transmission, this field is written by the CPU
and is used as the DLC field value of the frame to be transmitted.
When RTR is set, the frame to be transmitted is a remote frame and is transmitted without the DATA field, regardless
of the LENGTH field.
15–-0
TIME
STAMP
Free-running counter time stamp. Stores the value of the free-running timer which is captured when the beginning
of the identifier (ID) field appears on the CAN bus.
31–29
Reserved, must be cleared.
28–0
ID
Standard frame identifier: In standard frame format, only the 11 most significant bits (28 to 18) are used for frame
identification in receive and transmit cases. The 18 least significant bits are ignored.
Extended frame identifier: In extended frame format, all bits (the 11 bits of the standard frame identifier and the 18
bits of the extended frame identifier) are used for frame identification in receive and transmit cases.
31–24,
23–16,
15–8, 7–0
DATA
Data field. Up to eight bytes can be used for a data frame. For Rx frames, the data is stored as it is received from
the CAN bus. For Tx frames, the CPU provides the data to be transmitted within the frame.
