COP888EB Datasheet, PDF (31/75 Pages) National Semiconductor (TI) – 8-Bit CMOS ROM Based Microcontrollers with 8k Memory, CAN Interface, 8-Bit A/D, and USART

Frame Formats (Continued)
An error frame can start anywhere in the middle of a frame.
DS012837-23
INT = Intermission
Suspend Transmission is only for error passive nodes.
DS012837-24
An overload frame can only start at the end of a frame.
FIGURE 20. CAN Control Frames
DS012837-25
FIGURE 21. CAN Frame Format
DS012837-26
ACK FIELD
The ACK field is two bits long and contains the ACK slot and
the ACK delimiter. The ACK slot is filled with a “recessive” bit
by the transmitter. This bit is overwritten with a “dominant” bit
by every receiver that has received a correct CRC se-
quence. The second bit of the ACK field is a “recessive” bit
called the acknowledge delimiter. As a consequence the ac-
knowledge flag of a valid frame is surrounded by two “reces-
sive” bits, the CRC-delimiter and the ACK delimiter.
EOF FIELD
The End of Frame Field closes a data and a remote frame. It
consists of seven “recessive” bits.
INTERFRAME SPACE
Data and remote frames are separate from every preceding
frame (data, remote, error and overload frames) by the inter-
frame space see Figure 22 and Figure 23 for details. Error
and overload frames are not preceded by an interframe
space. They can be transmitted as soon as the condition oc-
curs. The interframe space consists of a minimum of three
bit fields depending on the error state of the node.
These bit fields are coded as follows:
The intermission has the fixed form of three “recessive” bits.
While this bit field is active, no node is allowed to start a
transmission of a data or a remote frame. The only action to
be taken is signaling an overload condition. This means that
an error in this bit field would be interpreted as an overload
condition. Suspend transmission has to be inserted by error-
passive nodes that were transmitter for the last message.
This bit field has the form of eight “recessive” bits. However,
it may be overwritten by a “dominant” start-bit from another
non error passive node which starts transmission. The bus
idle field consists of “recessive” bits. Its length is not speci-
fied and depends on the bus load.
31
www.national.com