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

Afterwards it starts transmitting seven more ‘recessive’ bits. The bus will then enter an idle state
or a new Data Frame or Remote Frame will start.
3.3.4 Overload Frame
The Overload Frame can be considered a special form of the Error Frame. It is used to
ask a transmitter to delay further frames or to signal problems with the Interframe Space. The
Overload Frame has the same format as the Error Frame, but unlike the Error Frame it does not
cause the retransmission of the previous frame. If the Overload Frame is being used to delay
further transmissions on the bus, then no more than two Overload Frames can be generated
successively. Figure 3.7 shows the structure of the Overload Frame.
An Overload Frame includes a Flag Field that contains a sequence of six ‘dominant’ bits
followed by a Delimiter Field, a series of eight ‘recessive’ bits. Depending on timing of the
Error Flag, Overload Flags may overlap one on top of another up to 12 bits in total length. When
one node sends out an Overload Flag, all of the nodes on the network detect it and send out their
own Overload Flags, effectively stopping all message traffic on the CAN bus. Then, all of the
nodes listen for the sequence of eight ‘recessive’ bits. The maximum amount of time needed to
recover in a CAN network after an Overload Frame is 31 bit times (Overload Flag (6 bit times)
+ Overlapping of Overload Flag (6 bit times) + Overload Delimiter (8 bit times) + Intermission
(3 bit times) + Suspend Transmission (8 bit times) = 31 bit times max).
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