MPC5553/MPC5554 Microcontroller Reference Manual, Rev. 5
Freescale Semiconductor 22-7
Table 22-4. Message Buffer Field Descriptions
Name Description
CODE Message buffer code. This 4-bit field can be accessed (read or write) by the CPU and by the
FlexCAN2 module itself, as part of the message buffer matching and arbitration process. The
encoding is shown in Table 2 2-5 and Tabl e 22-6 . See Section 22.4, “Functional Description,” for
additional information.
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 FlexCAN2 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
IDE ID extended bit. This bit identifies whether the frame format is standard or extended.
0 Frame format is standard
1 Frame format is extended
RTR Remote transmission request. This bit is used for requesting transmissions of a data frame. If
FlexCAN2 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
FlexCAN2 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
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 MB space (see Figure 22-2). In reception, this field is written by the
FlexCAN2 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.
ID Frame identifier. In standard frame format, only the 11 most significant bits (28 to 18) 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 22-5. Message Buffer Code for RX buffers
RX Code before
RX New Frame
Description
RX Code after
RX New Frame
Comment
0000 NOT ACTIVE: MB is not active. — MB does not participate in the matching process.
0100 EMPTY: MB is active and empty.
0010 MB participates in the matching process. When a
frame is received successfully, the code is
automatically updated to FULL.
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