DESIGN OF A CUSTOM ASIC INCORPORATING CAN™ AND 1 ...

Figure 3.9 Structure of the Interframe Space (error-passive nodes) [19].
The Intermission bit field consists of 3 ‘recessive’ bits. If a dominant level is detected
during an Intermission, an Overload Frame must be transmitted. However, if the third bit of an
Intermission is of a ‘dominant’ level, the Intermission is recognized as the Start-Of-Frame. The
Bus Idle bit field may be of arbitrary length. The bus is recognized to be free and any node
having something to transmit can access the bus and start sending a message. Similar to the
Intermission bit field, the detection of a ‘dominant’ bit on the bus is interpreted as a Start-Of-
Frame. For error-passive nodes which have been the transmitter of the previous message, there
is an extra bit field called Suspend Transmission. This field consists of eight ‘recessive’ bits
following the Intermission bit field [20]. This bit field is included in only an Interframe Space if
the immediately preceding message transmitting node was in an error-passive state.
3.4 Error Detection and Handling
Error detection and error handling are important for the performance of CAN. Because
of complementary error detection mechanisms, the probability of having an undetected error is
small. The Data Link Layer in CAN systems is very efficient in detecting errors. Every frame
is simultaneously accepted by every node in the network or rejected by every node. If a node
detects an error it transmits an error flag to every other node in the network and destroys the
transmitted frame. The CAN protocol supports five defined error conditions: Bit Error, Form
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