Controller Area Network (CAN) - The HCS12/9S12: An Introduction to Software and Hardware Interfacing - page 671

Bit Rate (kbps) Bus Length

1000
500
250
125
62.5

40
100
250
500
1000

Table 13.3 ■ CAN bus bit rate/bus length relation

Bus Length/Number of Nodes 32 64 100

0.25 mm 2 or AWG 24
0.34 mm 2 or AWG 22
0.75 mm 2 or AWG 18

0.25 mm 2 or AWG 24
0.50 mm 2 or AWG 20
0.75 mm 2 or AWG 18

0.25 mm 2 or AWG 24
0.50 mm 2 or AWG 20
0.75 mm 2 or AWG 18

100 m
250 m
500 m

Table 13.4 ■ Minimum recommended bus wire cross section for the trunk cable

shown in Table 13.3. The types and the cross sections suitable for the CAN bus trunk cable that
has more than 32 nodes connected and spans more than 100 meters are listed in Table 13.4.

13.11 Setting the CAN Timing Parameters

The nominal bit rate of a CAN network is uniform throughout the network and is given
by the equation
f NBT 5 1/ t NBT (13.1)
where t NBT is the nominal bit time . As described earlier, a bit time is divided into four separate
nonoverlapping time segments: sync_seg , prop_seg , phase_seg1 , and phase_seg2 .
The purpose of the sync_seg segment is to inform all nodes that a bit time has just started.
The existence of the propagation delay segment, prop_seg, is due to the fact that the CAN
protocol allows for nondestructive arbitration between nodes contending for access to the bus
and the requirement for in-frame acknowledgement . In the case of nondestructive arbitration,
more than one node may be transmitting during the arbitration field. Each transmitting node
samples data from the bus in order to determine whether it has won or lost the arbitration and
also to receive the arbitration field in case it loses the arbitration. When each node samples a
bit, the value sampled must be the logical superposition of the bit values transmitted by each
of the nodes arbitrating for bus access. In the case of the acknowledge field, the transmitting
node transmits a recessive bit but expects to receive a dominant bit; in other words, a dominant
value must be sampled at the sample time. The length of the prop_seg segment must be se-
lected for the earliest possible sample of the bit by a node until the transmitted bit values from
all the transmitting nodes have reached all of the nodes. When multiple nodes are arbitrating
for the control of the CAN bus, the node that transmitted the earliest must wait until the node
that transmitted the latest in order to find out if it won or lost. That is, the worst-case value for
t PROP_SEG is given by

t PROP_SEG 5 t PROP(A, B) 1 t PROP(B, A) (13.2)