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A
CAN PROTOCOL
Rev. 3
MOTOROLA
A-3
THE MOTOROLA CAN (MCAN) MODULE
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A.1.2
TBF – transmit buffer
The transmit buffer is an interface between the CPU and the Bit Stream Processor (BSP) and is
able to store a complete message. The buffer is written by the CPU and read by the BSP. The CPU
may access this buffer whenever TRANSMIT BUFFER ACCESS is set to released. On requesting
a transmission, by setting TRANSMISSION REQUEST in the CAN COMMAND REGISTER =
present, TRANSMIT BUFFER ACCESS is set to locked, giving the BSP exclusive access to this
buffer. The transmit buffer is released after the message transfer has been completed or aborted.
The TBF is 10 bytes long and holds the Identifier (1 byte), the Control Field (1 byte) and the Data
Field (maximum length 8 bytes). The buffer is implemented as a single-ported RAM, with mutual
exclusive access by the CPU and the BSP.
A.1.3
RBF – receive buffer
The receive buffer is an interface between the BSP and the CPU and stores a message received
from the bus line. Once filled by the BSP and allocated to the CPU by the IML, the receive buffer
cannot be used to store subsequent received messages until the CPU has acknowledged the
reading of the buffer’s contents. Thus, unless the CPU releases an RBF within a protocol defined
time frame, future messages to be received may be lost.
To reduce the requirements on the CPU, two receive buffers (RBF0 and RBF1) are implemented.
While one receive buffer is allocated to the CPU, the BSP may write to the other buffer. RBF0 and
RBF1 are each 10 bytes long and hold the Identifier (1 byte), the Control Field(1 byte) and the Data
Field(maximum length 8 bytes). The buffers are implemented as single-ported RAMs with mutual
exclusive access from the CPU and the BSP. The BSP only writes into a receive buffer when the
message being received or ransmitted has an Identifier which passes the Acceptance Filter. Note
that a message being transmitted will be written to the receive buffer if its Identifier passes the
Acceptance Filter, as it cannot be known until after the first byte has been stored whether or not
the message will lose arbitration to another transmitter.
A.1.4
BSP – bit stream processor
This is a sequencer controlling the data stream between the transmit and receive buffers (parallel
data) and the bus line (serial data). The BSP also controls the Transceive Logic (TCL) and the Error
Management Logic (EML) such that the processes of reception, arbitration, transmission and error
signalling are performed according to the protocol and the bus rules. The BSP also provides
signals to the IML indicating when a receive buffer contains a valid message and also when the
transmit buffer is no longer required after a successful transmission. Note that the automatic
retransmission of messages which have been corrupted by noise or other external error conditions
on the bus line is effectively handled by the BSP.
TPG
F
.
Freescale Semiconductor, Inc.