MPC555
/
MPC556
CAN 2.0B CONTROLLER MODULE
MOTOROLA
USER’S MANUAL
Rev. 15 October 2000
16-6
16.3.1.2 Fields for Extended Format Frames
describes the message buffer fields used only for extended identifier for-
mat frames.
16.3.1.3 Fields for Standard Format Frames
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 re-
ceived 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. At no time does the user have access to or visibility of these two buffers.
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 by the user 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 iden-
tifier 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 is receives it as a one, a bit error is indi-
cated.
If the TouCAN transmits a value and receives a matching response, a successful bit transmission
is indicated.
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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