COP87L88EB Datasheet, PDF (26/72 Pages) National Semiconductor (TI) – 8-Bit CMOS OTP Microcontrollers with 16k or 32k Memory, CAN Interface, 8-Bit A/D, and USART

Basic CAN Concepts (Continued)
MULTI-MASTER PRIORITY BASED BUS ACCESS
The CAN protocol is message based protocol that allows a
total of 2032 (= 2¹¹ −16) different messages in the standard
format and 512 million (= 2²⁹ −16) different messages in the
extended frame format.
MULTICAST FRAME TRANSFER BY
ACCEPTANCE FILTERING
Every CAN Frame is put on the common bus. Each module
receives every frame and filters out the frames which are not
required for the module’s task.
REMOTE DATA REQUEST
A CAN master module has the ability to set a specific bit
called the “remote transmission request bit” (RTR) in a
frame. This causes another module, either another master or
a slave, to transmit a data frame after the current frame has
been completed.
SYSTEM FLEXIBILITY
Additional modules can be added to an existing network
without a configuration change. These modules can either
perform completely new functions requiring new data or pro-
cess existing data to perform a new function.
SYSTEM WIDE DATA CONSISTENCY
As the CAN network is message oriented, a message can be
used like a variable which is automatically updated by the
controlling processor. If any module cannot process informa-
tion it can send an overload frame. The device is incapable
of initiating an overload frame, but will join a overload frame
initiated by another device as required by CAN specifica-
tions.
NON-DESTRUCTIVE CONTENTION-BASED
ARBITRATION
The CAN protocol allows several transmitting modules to
start a transmission at the same time as soon as they moni-
tor the bus to be idle. During the start of transmission every
node monitors the bus line to detect whether its message is
overwritten by a message with a higher priority. As soon as a
transmitting module detects another module with a higher
priority accessing the bus, it stops transmitting its own frame
and switches to receive mode. For illustration see Figure 18.
AUTOMATIC RETRANSMISSION OF FRAMES
If a data or remote frame is overwritten by either a higher-
prioritized data frame, remote frame or an error frame, the
transmitting module will automatically retransmit it. This de-
vice will handle the automatic retransmission of up to two
data bytes automatically. Messages with more than 2 data
bytes require the user’s software to update the transmit reg-
isters.
ERROR DETECTION AND ERROR SIGNALING
All messages on the bus are checked by each CAN node
and acknowledge if they are correct. If any node detects an
error it starts the transmission of an error frame.
Switching Off Defective Nodes
There are two error counters, one for transmitted data and
one for received data, which are incremented, depending on
the error type, as soon as an error occurs. If either counter
goes beyond a specific value the node goes to an error state.
A valid frame causes the error counters to decrease.
The device can be in one of three states with respect to error
handling:
• Error active
An error active unit can participate in bus communication
and sends an active (“dominant”) error flag.
• Error passive
An error passive unit can participate in bus communica-
tion. However, if the unit detects an error it is not allowed
to send an active error flag. The unit sends only a passive
(“recessive”) error flag.
• Bus off
A unit that is “bus off” has the output drivers disabled, i.e., it
does not participate in any bus activity.
(See ERROR MANAGEMENT AND DETECTION for more
detailed information.)
www.national.com
FIGURE 18. CAN Message Arbitration
DS100044-20
26