RM0319 datasheet(209/368 Pages) STMICROELECTRONICS

209 / 368 page

RM0319

Controller area network ports (CAN)

Doc ID 022640 Rev 3

209/368

A given bit rate may be met by different bit time configurations, but for the proper function of

the CAN network the physical delay times and the oscillator’s tolerance range have to be

considered.

Propagation time segment

This part of the bit time is used to compensate physical delay times within the network.

These delay times consist of the signal propagation time on the bus and the internal delay

time of the CAN nodes.

Any CAN node synchronised to the bit stream on the CAN bus will be out of phase with the

transmitter of that bit stream, caused by the signal propagation time between the two nodes.

The CAN protocol’s non-destructive bitwise arbitration and the dominant acknowledge bit

provided by receivers of CAN messages require that a CAN node transmitting a bit stream

must also be able to receive dominant bits transmitted by other CAN nodes that are

synchronised to that bit stream. The example in Figure 75 shows the phase shift and

propagation times between two CAN nodes.

Figure 75.

Propagation time segment

Delay A_to_B >= node output delay(A) + bus line delay(A -> B) + node input delay(B)

Prop_Seg >= Delay A_to_B + Delay B_to_A

Prop_Seg >= 2 • [max(node output delay+ bus line delay + node input delay)]

In this example, both nodes A and B are transmitters performing an arbitration for the CAN

bus. The node A has sent its Start of Frame bit less than one bit time earlier than node B,

therefore node B has synchronised itself to the received edge from recessive to dominant.

Since node B has received this edge delay(A_to_B) after it has been transmitted, B’s bit

Phrase_Seg1

[1..8] tq

May be lengthened temporarily by synchronisation

Phase_Seg2

[1..8] tq

May be shortened temporarily by synchronisation

SJW

[1..4] tq

May not be longer than either Phase Buffer Segment

This table describes the minimum programmable ranges required by the CAN protocol

Table 62.

Parameters of the CAN Bit Time (continued)

Parameter

Range

Remark

Node B

Node A

Delay A_to_B

Delay B_to_A

Phase_Seg2

Sync_Seg

Prop_Seg

Phase_Seg1

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