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FlexCAN Module
MPC5644A Microcontroller Reference Manual, Rev. 6
Freescale Semiconductor
1399
LENGTH
Length of Data in Bytes
This 4-bit field is the length (in bytes) of the Rx or Tx data, which is located in offset 0x8 through 0xF of
the message buffer space (see
). In reception, this field is written by the FlexCAN module,
copied from the DLC (Data Length Code) field of the received frame. In transmission, this field is written
by the CPU and corresponds to the DLC field value of the frame to be transmitted. When RTR=1, the
Frame to be transmitted is a Remote Frame and does not include the data field, regardless of the
Length field.
TIME
STAMP
Free-Running Counter Time Stamp
This 16-bit field is a copy of the Free-Running Timer, captured for Tx and Rx frames at the time when
the beginning of the Identifier field appears on the CAN bus.
PRIO
Local priority
This 3-bit field is only used when MCR[LPRIO_EN] is set and it only makes sense for Tx buffers. These
bits are not transmitted. They are appended to the regular ID to define the transmission priority. See
Section 32.5.3, Arbitration process
.
ID
Frame Identifier
In Standard Frame format, only the 11 most significant bits (3 to 13) are used for frame identification in
both receive and transmit cases. The 18 least significant bits are ignored. In Extended Frame format,
all bits are used for frame identification in both receive and transmit cases.
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 prepares the data field to be transmitted within the frame.
Table 32-5. Message Buffer Code for Rx buffers
Rx Code
BEFORE
Rx New Frame
Description
Rx Code
AFTER
Rx New Frame
Comment
0000
INACTIVE: buffer
is not active.
—
MB does not participate in the matching process.
0100
EMPTY: buffer is
active and empty.
0010
MB participates in the matching process. When a frame
is received successfully, the code is automatically
updated to FULL.
0010
FULL: buffer is
full.
0010
The act of reading the C/S word followed by unlocking
the MB does not make the code return to EMPTY. It
remains FULL. If a new frame is written to the MB after
the C/S word was read and the MB was unlocked, the
code still remains FULL.
0110
If the MB is FULL and a new frame is overwritten to this
MB before the CPU had time to read it, the code is
automatically updated to OVERRUN. Refer to
Section 32.5.5, Matching process
for details about
overrun behavior.
Table 32-4. Message Buffer Structure (Word 0—0x0)
Field
Description
Summary of Contents for MPC5644A
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