PXR40RM Datasheet, PDF (819/1434 Pages) Freescale Semiconductor, Inc – PXR40 Microcontroller

FlexCAN Module
Table 24-4. Message Buffer Structure Field Descriptions
Field
Description
CODE
Message Buffer 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 is shown in Table 24-5 and Table 24-6. See Section 24.4,
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 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
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 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
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 24-2). 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 Free-Running Counter Time Stamp
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 LPRIO_EN bit is set in FLEXCAN_x_MCR 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 24.4.3, 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.
Freescale Semiconductor
PXR40 Microcontroller Reference Manual, Rev. 1
24-9