16-6
MPC561/MPC563 Reference Manual
MOTOROLA
TouCAN Architecture
16.3.1.2 Fields for Extended Format Frames
Table 16-4 describes the message buffer fields used only for extended identifier format
frames.
16.3.1.3 Fields for Standard Format Frames
Table 16-5 describes the message buffer fields used only for standard identifier format
frames.
16.3.1.4 Serial Message Buffers
To allow double buffering of messages, the TouCAN has two shadow buffers called serial
message buffers. The TouCAN uses these two buffers for buffering both received messages
and messages to be transmitted. Only one serial message buffer is active at a time, and its
function depends upon the operation of the TouCAN at that time. These buffers are not
accessible or visible to the user.
Table 16-4. Extended Format Frames
Field
Description
ID[28:18]/[17:15]
Contains the 14 most significant bits of the extended identifier, located in the ID_HIGH word of the
message buffer.
Substitute Remote
Request (SRR)
Contains a fixed recessive bit, used only in extended format. Should be set to one for Tx buffers. It
will be stored as received on the CAN bus for Rx buffers.
ID Extended (IDE)
If extended format frame is used, this field should be set to one. If zero, standard format frame
should be used.
ID[14:0]
Bits [14:0] of the extended identifier, located in the ID_LOW word of the message buffer.
Remote Transmission
Request (RTR)
This bit is located in the least significant bit of the ID_LOW word of the message buffer
0 Data Frame
1 Remote Frame
Table 16-5. Standard Format Frames
Field
Description
16-bit Time Stamp The ID_LOW word, which is not needed for standard format, is used in a standard format buffer to store
the 16-bit value of the free-running timer which is captured at the beginning of the identifier field of the
frame on the CAN bus.
ID[28:18]
Contains bits [28:18] of the identifier, located in the ID_HIGH word of the message buffer. The four least
significant bits in this register (corresponding to the IDE bit and ID[17:15] for an extended identifier
message) must all be written as logic zeros to ensure proper operation of the TouCAN.
RTR
This bit is located in the ID_HIGH word of the message buffer;
0 Data Frame
1 Remote Frame
RTR/SRR Bit
Treatment
If the TouCAN transmits this bit as a one and receives it as a zero, an “arbitration loss” is
indicated. If the TouCAN transmits this bit as a zero and receives it as a one, a bit error is indicated.
If the TouCAN transmits a value and receives a matching response, a successful bit transmission is
indicated.