HI-3110
generated bit level is constant during the total bit time and
consecutive bits do not return to a neutral or rest condition.
This means that a bit stream of “1s” or “0”s appears
continuous on the bus. A logic “0” is called a dominant bit
and a logic “1” is called a recessive bit.
Bit stuffing is used to ensure frequent enough transitions
occur to achieve synchronization. Every time a transmitter
detects five consecutive bits of the same polarity in the bit
stream to be transmitted, it inserts a bit of opposite polarity
into the actual transmitted bit stream.
This bit stuffing rule applies to the Start-of-Frame field,
arbitration field, control field, data field and CRC sequence.
The CRC delimiter, ACK field and End-Of-Frame fields are of
fixed form and not stuffed (see below for definition of these
fields). Furthermore, Error frames and Overload frames are
also of fixed form and not stuffed.
An example of how the bits in a stuffed bit stream might look
is shown below.
00101011111
O
0000
I
1100000
I
11000
0 = dominant bit,
O
= dominant stuffed bit.
1 = recessive bit,
I
= recessive stuffed bit.
The data field is followed by the 16-bit Cyclic Redundancy
Check (CRC) field. This is used to check transmission
errors by computing a 15-bit CRC sequence from the
previous bit stream (SOF, arbitration field, control field and
data field, excluding stuff bits). The last bit in the CRC field is
the CRC delimiter bit (always recessive).
After the CRC field is the Acknowledge Field (ACK Field).
The first bit is the ACK Slot bit. A transmitting node sends a
recessive bit (logic 1) during the ACK slot. Any node which
receives the message error-free acknowledges the
reception by placing a dominant bit (logic 0) in the ACK slot,
over-writing the recessive bit of the transmitter. The final bit
in the ACK field is a recessive ACK delimiter bit. Therefore,
the dominant ACK slot bit is surrounded on each side by a
recessive bit.
Each data frame is delimited by an End-Of-Frame field
(EOF). The EOF consists of seven recessive bits.
Following the EOF, there is a gap to the next frame called the
Interframe Space (IFS) . The IFS consists of two bit fields,
Intermission and Bus-Idle. The Intermission consists of
three recessive bits, however the following notes apply:
a) detection of a dominant bit on the bus at the third slot is
interpreted as a SOF,
b) detection of a dominant bit in either the first or second
slots results in generation of an overload frame (see below).
The bus idle period is of arbitrary length and consists of
recessive bits. A dominant bit detected during this period is
interpreted as a SOF.
EXTENDED DATA FRAME
The extended data frame is shown in figure 3. In this frame
format, SOF is followed by a 32-bit arbitration field consisting
of a 29-bit identifier, ID28 - ID0. The first 11 most significant
bits of the ID are know as the base identifier. This is followed
by the Substitute Remote Request (SRR) bit, which is
defined as recessive. Following the SRR bit is the IDE bit,
which is defined as recessive for extended data frames.
Note that the SRR bit is in the same slot as the RTR bit of the
standard frame and the IDE bits are also in corresponding
slots. This means if standard and extended identifier data
frames with identical base identifiers are transmitted
simultaneously, the standard identifier data frame will win
arbitration (see Bitwise Arbitration section below).
The SRR and IDE bits are followed by the remaining 18 bits
of the identifier (extended ID) and the last bit of the
arbitration field is the RTR bit. The RTR bit has the same
function as in the standard frame format.
Following the arbitration field is the 6-bit control field. The
first two bits, r1 and r0, are specified by the CAN protocol as
reserved bits for future expansion. Both these bits must be
transmitted dominant, but receivers must be able to receive
all combinations of dominant or recessive bits. The final 4
bits of the control field is the data length code (DLC). The
MESSAGE FRAMES
STANDARD DATA FRAME
The standard data frame is shown in figure 2. The frame
starts with a Start-of-Frame (SOF) bit. This is a dominant bit
that identifies the start of the data frame on the bus.
The SOF is followed by the 12-bit arbitration field. The
arbitration field consists of an 11-bit identifer, ID28 - ID18,
and the Remote Transmission Request (RTR) bit. The RTR
bit is used to distinguish between a data frame (RTR bit
dominant, logic 0) and a remote frame (RTR bit recessive,
logic 1).
Following the arbitration field is the 6-bit control field. The
first bit of the control field is the Identifier Extension flag bit
(IDE). This is used to distinguish between standard and
extended identifiers and must be dominant (logic 0) for
standard data frames. The next bit, r0, is specified by the
CAN protocol as a reserved bit for future expansion. This bit
must be transmitted dominant, but receivers must be
capable of receiving either a dominant or recessive bit. The
final 4 bits of the control field make up the data length code
(DLC). The binary value of this 4-bit field specifies the
number of data bytes in the data payload (0 - 8 bytes).
Note:
All binary combinations greater than or equal to <1 0 0 0>
specify 8 bytes of data.
After the control field is the data field, which contains a data
payload equal to the number of bytes specified by the DLC
(see note above).
HOLT INTEGRATED CIRCUITS
6
HOLTIC [ HOLT INTEGRATED CIRCUITS ]
