Physical Layer - CAN System Engineering - page 69

2.2.3.1 Ringing at the Transition from Dominant to Recessive

For signal quality evaluations of CAN network topologies, it is crucial to know
where disturbances occur and which properties of the network architecture or prop-
erties of the CAN node architecture amplify these disturbances. In case of signal
integrity evaluation of CAN networks, the state change from dominant to recessive
is of particular importance. Abstracted, it could be said that during the state change
an energy source driving the differential signal is taken from the bus (which results
in the recessive state) and instead a high ohmic receiver circuit is added to the bus.
The existing energy inside the system degrades over the passive components of
the network. Implemented capacitive and inductive energy storages discharge and
influence each other. Recharging between inductive and capacitive stored energy
may occur which results in oscillations of the bus signals which is called ringing.
These overshoots and undershoots occur at different strengths dependent on the
characteristics of the different used components. These interactions can be clarified
by formula [2.2] of the fundamental field of the electro techniques:

uL i
t

= · ∆
∆

(2.2)

First, to assume a constant stray inductance, which is operative in the differential
mode, it is easy to see using formula [2.2] that the overshoots and undershoots will
be more intense in case of a transceiver which produces faster slow rates; thus,
the Δi/Δt is higher. Furthermore, different CMCs have different stray inductances.
CMCs with higher stray inductances (smaller coupling coefficients) invoke high-
er overshoots and undershoots. CMCs with sector-based winding have in general
smaller coupling coefficients and approximately higher stray inductances. CMCs
with bifilar winding have in general higher coupling coefficients.
Figure 2.25 compares both extreme cases of these interactions of different com-
ponents. The solid curve refers to a CAN node with a transceiver with a fast slew
rate, corresponding to a high Δi/Δt, and with a CMC with sector winding, thus a
high stray inductance. The dotted line belongs to the same CAN node but in the
case with a transceiver with a slower slew rate while applying a CMC with a higher
coupling coefficient. It is easy to see that the ringing is less intense with the second
equipped case (dotted line). The measured CAN node is located inside a symmetric
CAN topology with two stars where all stubs have the same length to the star points.
This topology is typical for a lot of ringing at the state change from dominant to
recessive caused by the symmetric characteristics which leads to an adding of par-
ticular reflection parts.
As already mentioned, the energy distribution in the system is of importance.
Therefore, two relevant scenarios shall be taken into account: On the one hand, the
signal flow and behaviour at the sending node are important to consider because this
is the point of the energy input. On the other hand, the consideration of the ACK
bit is important. In that case, all receiving nodes start transmitting the dominant