AN1105 STMicroelectronics, AN1105 Datasheet - Page 10

• Manufacturer Part Number: AN1105
• Description: ST7 PCAN PERIPHERAL DRIVER
• Manufacturer: STMicroelectronics

ST7 pCAN PERIPHERAL DRIVER
To deal with signal phase variations on the transmission line due mostly to desynchronization
between clocks of bus participants, TSEG1 and TSEG2 are of variable length. A resynchroni-
zation of the local clock occurs on every recessive-to-dominant state change. If such a change
arises before (or after) the SYNC segment expected by the cell, TSEG1 (or TSEG2) is length-
ened (or shortened) to correct the detected variation. If the edges occur during the SYNC seg-
ment, the node assumes that it is synchronized.
The PROP segment takes into account the maximum propagation time of the signal on the
network, ensuring that all the nodes sample the same bit at the same time.
The sampling point of the bit takes place between TSEG1 and TSEG2.
1.4 ARBITRATION PHASE
1.4.1 Arbitration Phase
All the cells connected on the CAN network are likely to transmit a message at any time. A cell
can initiate a transmission only when the bus is idle. On transmission of a message, a cell si-
multaneously monitors the state of the bus to detect potential problems.
If the bus is idle, any cell that is ready for transmission begins to send its data. The other
nodes listen.
If the bus is already in use when a cell requests it, the cell waits until the end of the current
transmission. Then, all the nodes that are ready to transmit begin their transmission. If a dom-
inant and a recessive bit are sent in the meantime, the first one erases the second. This
causes the cell that is sending the recessive bit to lose the arbitration and switch to Reception
mode. This way, messages with the highest identifiers are sent before the others. This mech-
anism also prevents collisions occurring on the bus, which saves bandwidth. This is referred to
as non-destructive bitwise arbitration.
Figure 7. Arbitration example: node 1 wins
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Node 1
Node 2
Node n
CAN Bus
Recessive
Dominant