Datasheet

20.3.3.9. 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.
20.3.3.10. 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.
20.3.3.11. 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.
20.3.3.12. 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.
20.3.4. 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 20-4. Bus arbitration.
node A
TXCAN
node B
TXCAN
ID10 ID9 ID8 ID7 ID6 ID5 ID4 ID3 ID2 ID1 ID0
SOF
SOF
RTR IDE
CAN bus
- - - - - - - - -
Arbitration lost
Node A loses the bus
Node B wins the bus
Atmel ATmega16M1/32M1/64M1 [DATASHEET]
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