Freescale Semiconductor MCF5480 - Manual (Page 591)

Functional Overview
MCF548x Reference Manual, Rev. 3
Freescale Semiconductor 21-21
28–0 ID [28:18] Standard 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.
ID [17:0) Extended frame identifier: In extended frame format, all bits (both the 11 bits of the standard frame
identifier and the 18 bits of the extended frame identifier) are used for frame identification in both
receive and transmit cases.
31–24,
23–16,
15–8,
7–0
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 provides the data to be transmitted within
the frame.
Table 21-14. Message Buffer Codes for Rx Buffers
Rx Code
BEFORE
Rx New Frame
Description
Rx Code
AFTER
Rx New Frame
Comment
0000 INACTIVE: 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.
0010 FULL: MB is full. 0010 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.
0110 If the MB is FULL and a new frame should be written into this
MB before the CPU had time to read it, the MB is overwritten,
and the code is automatically updated to OVERRUN.
0110 OVERRUN: A frame
was overwritten into a
full buffer.
0010 If the code indicates OVERRUN but the CPU reads the C/S
word and then unlocks the MB, when a new frame is written
to the MB, the code returns to FULL.
0110 If the code already indicates OVERRUN, and yet another new
frame must be written, the MB will be overwritten again, and
the code will remain OVERRUN.
0XY1
1
1 Note that for transmit message buffers (see Table 21-15 ), the BUSY bit should be ignored upon read.
BUSY: Flexcan is
updating the contents
of the MB with a new
receive frame.
The CPU should not try
to access the MB.
0010 An EMPTY buffer was written with a new frame (XY was 01).
0110 A FULL/OVERRUN buffer was overwritten (XY was 11).
Table 21-13. Message Buffer Field Descriptions (Continued)
Bits Name Description