HI-3110
BIT WISE ARBITRATION
The CAN standard resolves data contention on the bus
using a scheme called Carrier Sense Multiple
Access/Collision Detection-Carrier Resolution (CSMA/CD-
CR).
Carrier Sense:
Each node waits for a period without bus
activity (bus idle state) before attempting transmission.
Multiple Access:
Every node on the bus has equal access
to the bus for transmitting.
Collision Detection:
Collisions occur if two nodes attempt
to transmit at the same time.
Collision Resolution:
Collisions are resolved by bitwise
arbitration.
Highest priority messages (lowest binary
identifiers) are sent first without delay and lower-priority
messages are automatically re-transmitted later. A dominant
bit (logic 0) has priority over a recessive bit (logic 1).
Bitwise arbitration works by comparing each node’s
transmitted data bit by bit. All nodes are synchronized by
adjusting individual bit times as a function of bit time quanta
(see section on bit timing). Synchronization takes place on
recessive to dominant edges. A Hard Synchronization at the
start of each frame and subsequent re-synchronizations
during a message frame ensures corresponding bits match
in time during a given transmission cycle. When a node
transmits a recessive bit on the bus, but detects a dominant
bit on it’s receiver, it realizes arbitration is lost and it
immediately ceases transmission and becomes a receiver.
It will then wait for the next bus idle state and attempt to re-
transmit. Eventually, lower priority messages will gain
access to the bus. Figure 7 shows an example of how this
works for a frame with a standard identifier.
Start-OF-Frame
Standard ID Arbitration Field
ID28
ID18
Node 2 loses arbitration
TXCAN: Node 1
1
0
TXCAN: Node 2
1
0
TXCAN: Node 3
1
0
CAN Bus differential
signal (not to scale)
Node 3 loses arbitration
Figure 7. Bitwise Arbitration.
HOLT INTEGRATED CIRCUITS
13
RTR
HOLTIC [ HOLT INTEGRATED CIRCUITS ]
