PXN20RM Datasheet, PDF (892/1376 Pages) Freescale Semiconductor, Inc – PXN20 Microcontroller

Controller Area Network (FlexCAN)
Table 29-3. Message Buffer Field Descriptions (continued)
Name
Description
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 29-2). 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 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 LPRIO_EN bit is set in CANx_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 29.4.2, 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. A many as 8 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 29-4. Message Buffer Code for Rx Buffers
Rx Code before
Rx New Frame
Description
Rx Code after
Rx New Frame
Comment
0000
0100
0010
NOT ACTIVE: MB is not active.
EMPTY: MB is active and empty.
FULL: MB is full.
—
0010
0010
0110
MB does not participate in the matching process.
MB participates in the matching process. When a
frame is received successfully, the code is
automatically updated to FULL.
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.
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 29.4.4, Matching Process, for details about
overrun behavior.
29-8
PXN20 Microcontroller Reference Manual, Rev. 1
Freescale Semiconductor