1996 Jun 27 55
Philips Semiconductors Product specification
8-bit microcontroller with on-chip CAN P8xC592
13.6.2.4 RTR bit
A CAN-controller, acting as a receiver for certain
information may initiate the transmission of the respective
data by transmitting a Remote Frame to the network,
addressing the data source via the Identifier and setting
the RTR bit HIGH (remote; recessive bus level). If the data
source simultaneously transmits a Data Frame containing
the requested data, it uses the same Identifier. No bus
access conflict occurs due to the RTR bit being set LOW
(data; dominant bus level) in the Data Frame.
13.6.2.5 Control Field
This field consists of six bits. It includes two reserved bits
(for future expansions of the CAN-protocol), transmitted
with a dominant bus level, and is followed by the Data
Length Code (4 bits).
The number of bytes (destuffed; number of data bytes to
be transmitted/received) in the Data Field is indicated by
the Data Length Code. Admissible values of the Data
Length Code, and hence the number of bytes in the
(destuffed) Data Field, are {0, 1, ...., 8}. A logic 0 (logic 1)
in the Data Length Code is transmitted as dominant
(recessive) bus level, respectively.
13.6.2.6 Data Field
The data, stored within the Data Field of the Transmit
Buffer, are transmitted according to the Data Length Code.
Conversely, data of a received Data Frame will be stored
in the Data Field of a Receive Buffer. The Data Field can
contain from 0 up to 8 bytes. The most significant bit of the
first data byte (lowest address) is transmitted/received
first.
13.6.2.7 Cyclic Redundancy Code Field (CRC)
The CRC Field consists of the CRC Sequence (15 bits)
and the CRC Delimiter (1 recessive bit). The Cyclic
Redundancy Code (CRC) encloses the destuffed bit
stream of the Start-Of-Frame, Arbitration Field, Data Field
and CRC Sequence. The most significant bit of the CRC
Sequence is transmitted/received first. This frame check
sequence, implemented in the CAN-controller is derived
from a cyclic redundancy code best suited for frames with
a total bit count of less than 127 bits, see Section 13.6.8.3.
With Start-Of-Frame (dominant bit) included in the code
word, any rotation of the code word can be detected by the
absence of the CRC Delimiter (recessive bit).
13.6.2.8 Acknowledge Field (ACK)
The Acknowledge Field consists of two bits, the
Acknowledge Slot and the Acknowledge Delimiter, which
are transmitted with a recessive level by the transmitter of
the Data Frame. All CAN-controllers having received the
matching CRC Sequence, report this by overwriting the
transmitter's recessive bit in the Acknowledge Slot with a
dominant bit. Thereby a transmitter, still monitoring the bus
level recognizes that at least one receiver within the
network has received a complete and correct message
(i.e. no error was found). The Acknowledge Delimiter
(recessive bit) is the second bit of the Acknowledge Field.
As a result, the Acknowledge Slot is surrounded by two
recessive bits: the CRC Delimiter and the Acknowledge
Delimiter.
All nodes within a CAN network may use all the information
coming to the network by all CAN-controllers (shared
memory concept). Therefore, acknowledgement and error
handling are defined to provide all information in a
consistent way throughout this shared memory. Hence,
there is no reason to discriminate different receivers of a
message in the acknowledge field. If a node is
disconnected from the network due to bus failure, this
particular node is no longer part of the shared memory. To
identify a ‘lost node’ additional and application specific
precautions are required.
13.6.2.9 End-Of-Frame
Each Data Frame or Remote Frame is delimited by the
End-Of-Frame bit sequence which consists of seven
recessive bits (exceeds the bit stuff width by two bits).
Using this method a receiver detects the end of a frame
independent of a previous transmission error because the
receiver expects all bits up to the end of the CRC
Sequence to be coded by the method of bit-stuffing, see
Section 13.6.7.3. The bit-stuffing logic is deactivated
during the End-Of-Frame sequence.
