
MOTOROLA
CAN 2.0B CONTROLLER MODULE (TouCAN)
MC68336/376
13-4
USER’S MANUAL
Figure 13-4 Standard ID Message Buffer Structure
13.4.1.1 Common Fields for Extended and Standard Format Frames
Table 13-1 describes the message buffer fields that are common to both extended and
standard identifier format frames.
NOTES:
1. For TX message buffers, upon read, the BUSY bit should be ignored.
15
8 7
4 3
0
$0
TIME STAMP
CODE
LENGTH
CONTROL/STATUS
$2
ID[28:18]
RTR
0
ID_HIGH
$4
16-BIT TIME STAMP
ID_LOW
$6
DATA BYTE 0
DATA BYTE 1
$8
DATA BYTE 2
DATA BYTE 3
$A
DATA BYTE 4
DATA BYTE 5
$C
DATA BYTE 6
DATA BYTE 7
$E
RESERVED
Table 13-1 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
RX Length
Length (in bytes) of the RX data stored in offset $6 through $D of the buffer. This field is written by
the TouCAN module, copied from the DLC (data length code) field of the received frame.
TX Length
Length (in bytes) of the data to be transmitted, located in offset $6 through $D of the buffer. This
field is written by the CPU32, and is used as the DLC field value. If RTR (remote transmission re-
quest) = 1, the frame is a remote frame and will be transmitted without 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 re-
ceived from the bus. For TX frames, the CPU32 provides the data to be transmitted within the frame.
Reserved
This word entry field (16 bits) should not be accessed by the CPU32.
Table 13-2 Message Buffer Codes for Receive Buffers
RX Code
Before RX
New Frame
Description
RX Code
After RX
New Frame
Comment
0000
NOT ACTIVE — message buffer is not active.
—
0100
EMPTY — message buffer is active and empty.
0010
—
0010
FULL — message buffer is full.
0110
If a CPU32 read occurs be-
fore the new frame, new re-
ceive code is 0010.
0110
OVERRUN — second frame was received into a full
buffer before the CPU read the first one.
0XY11
BUSY — message buffer is now being filled with a new
receive frame. This condition will be cleared within 20
cycles.
0010
An empty buffer was filled
(XY was 10).
0110
A full/overrun buffer was
filled (Y was 1).
336376UMBook Page 4 Friday, November 15, 1996 2:09 PM