Motorola MCF5282 Common Extended/Standard Format Frames, Message Buffer Codes for Receive Buffers

Message Buffers

	Table 25-2. Common Extended/Standard Format Frames
Field	Description
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 25-3and 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		Rx Code
Before Rx	Description	After Rx	Comment
New Frame		New Frame
0000	NOT ACTIVE — message buffer is not active.	—	—
0100	EMPTY — message buffer is active and empty.	0010	—
0010	FULL — message buffer is full.		If a CPU read occurs before
		0110	the new frame, new receive
0110	OVERRUN — second frame was received into a full
			code is 0010.
	buffer before the CPU read the first one.
0101 1	BUSY — message buffer is now being filled with a new	0010	An empty buffer was filled.
	receive frame. This condition will be cleared within 20
00111		0110	A full buffer was filled.
	cycles.
01111		0110	An overrun buffer was filled.

1For transmit message buffers, upon read, the BUSY bit should be ignored.

Table 25-4. Message Buffer Codes for Transmit Buffers

			Code After
RTR	Initial Tx Code	Description	Successful
			Transmission
X	1000	Message buffer not ready for transmit.	—
0	1100	Data frame to be transmitted once, unconditionally.	1000
1	1100	Remote frame to be transmitted once, and message buffer	0100
		becomes an Rx message buffer for data frames.
0	1010 1	Data frame to be transmitted only as a response to a remote	1010
		frame.
0	1110	Data frame to be transmitted only once, unconditionally, and	1010
		then only as a response to remote frame.

1When a matching remote request frame is detected, the code for such a message buffer is changed to be 1110.

25-6 MCF5282 User’s Manual MOTOROLA