16.2.3.8 Bit Lengthening
As a result of resynchronization, phase segment 1 may be lengthened or phase segment 2 may be shortened to
compensate for oscillator tolerances. If, for example, the transmitter oscillator is slower than the receiver oscillator, the next
falling edge used for resynchronization may be delayed. So phase segment 1 is lengthened in order to adjust the sample
point and the end of the bit time.
16.2.3.9 Bit Shortening
If, on the other hand, the transmitter oscillator is faster than the receiver one, the next falling edge used for resynchronization
may be too early. So phase segment 2 in bit N is shortened in order to adjust the sample point for bit N+1 and the end of the
bit time
16.2.3.10 Synchronization Jump Width
The limit to the amount of lengthening or shortening of the phase segments is set by the Resynchronization jump width.
This segment may not be longer than phase segment 2.
16.2.3.11 Programming the Sample Point
Programming of the sample point allows “tuning” of the characteristics to suit the bus.
Early sampling allows more time quanta in the phase segment 2 so the synchronization jump width can be programmed to
its maximum. This maximum capacity to shorten or lengthen the bit time decreases the sensitivity to node oscillator
tolerances, so that lower cost oscillators such as ceramic resonators may be used.
Late sampling allows more time quanta in the propagation time segment which allows a poorer bus topology and maximum
bus length.
16.2.3.12 Synchronization
Hard synchronization occurs on the recessive-to-dominant transition of the start bit. The bit time is restarted from that edge.
Re-synchronization occurs when a recessive-to-dominant edge doesn't occur within the synchronization segment in a
message.
16.2.4 Arbitration
The CAN protocol handles bus accesses according to the concept called “Carrier Sense Multiple Access with Arbitration on
Message Priority”.
during transmission, arbitration on the CAN bus can be lost to a competing device with a higher priority CAN Identifier. This
arbitration concept avoids collisions of messages whose transmission was started by more than one node simultaneously
and makes sure the most important message is sent first without time loss.
The bus access conflict is resolved during the arbitration field mostly over the identifier value. If a data frame and a remote
frame with the same identifier are initiated at the same time, the data frame prevails over the remote frame (c.f. RTR bit).
Figure 16-4. Bus Arbitration
Arbitration lost
Node A
TXCAN
Node A loses the bus
Node B wins the bus
Node B
TXCAN
CAN Bus
SOF ID10 ID9 ID8 ID7 ID6 ID5 ID4 ID3 ID2 ID1 ID0 RTR IDE
144
ATmega16/32/64/M1/C1 [DATASHEET]
7647O–AVR–01/15
MICROCHIP [ MICROCHIP ]
