Bosch Appliances TTCAN user manual Propagation Time Segment, Brp

TTCAN User’s Manual Revision 1.6

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.

Parameter	Range	Remark
BRP	[1 … 32]	defines the length of the time quantum tq
Sync_Seg	1 tq	fixed length, synchronisation of bus input to system clock
Prop_Seg	[1 … 8] tq	compensates for the physical delay times
Phase_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 1 : Parameters of the CAN Bit Time

4.2.1.2 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 10 shows the phase shift and propagation times between two CAN nodes.

manual_about.fm

Sync_Seg		Prop_Seg	Phase_Seg1	Phase_Seg2
Node B
	Delay A_to_B		Delay B_to_A
Node A
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)]

Figure 10: The Propagation Time Segment

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 timing segments are shifted with regard to A. Node B sends an identifier with higher priority and so it will win the arbitration at a specific identifier bit when it transmits a dominant bit while node A

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