Common Extended/Standard Format Frames; Message Buffer Codes For Receive Buffers; Message Buffer Codes For Transmit Buffers - Motorola ColdFire MCF5281 User Manual

Message Buffers
Table 25-2. Common Extended/Standard Format Frames
Field
Time Stamp Contains a copy of the high byte of the free running timer, which is captured at the beginning of the
identifier field of the frame on the CAN bus.
Code Refer to Table 25-3 and Table 25-4.
Rx Length Length (in bytes) of the Rx data stored in offset 0x6 through 0xD of the buffer. This field is written by
the FlexCAN module, copied from the data length code (DLC) field of the received frame.
Tx Length Length (in bytes) of the data to be transmitted, located in offset 0x6 through 0xD of the buffer. This
field is written by the CPU, and is used as the DLC field value. If remote transmission request (RTR))
= 1, the frame is a remote frame and will be transmitted without the data field, regardless of the value
in Tx length.
Data This field can store up to eight data bytes for a frame. For Rx frames, the data is stored as it is
received from the bus. For Tx frames, the CPU provides the data to be transmitted within the frame.
Reserved This word entry field (16 bits) should not be accessed by the CPU.
Table 25-3. Message Buffer Codes for Receive Buffers
Rx Code
Before Rx
New Frame
0000 NOT ACTIVE — message buffer is not active.
0100 EMPTY — message buffer is active and empty.
0010 FULL — message buffer is full.
0110 OVERRUN — second frame was received into a full
buffer before the CPU read the first one.
1
0101 BUSY — message buffer is now being filled with a new
receive frame. This condition will be cleared within 20
1
0011
cycles.
1
0111
1
For transmit message buffers, upon read, the BUSY bit should be ignored.
Table 25-4. Message Buffer Codes for Transmit Buffers
RTR Initial Tx Code
X 1000
0 1100
1 1100
0 1010
0 1110
1
When a matching remote request frame is detected, the code for such a message buffer is changed to be 1110.
25-6
Description
Message buffer not ready for transmit.
Data frame to be transmitted once, unconditionally.
Remote frame to be transmitted once, and message buffer
becomes an Rx message buffer for data frames.
1
Data frame to be transmitted only as a response to a remote
frame.
Data frame to be transmitted only once, unconditionally, and
then only as a response to remote frame.
MCF5282 User's Manual
Description
Rx Code
After Rx
New Frame
—
0010
0110
0010
0110
0110
Description
Comment
—
—
If a CPU read occurs before
the new frame, new receive
code is 0010.
An empty buffer was filled.
A full buffer was filled.
An overrun buffer was filled.
Code After
Successful
Transmission
—
1000
0100
1010
1010
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