Image Processing Reference
In-Depth Information
FIGURE .
VNA screen.
The priority bits are the most significant bits. hey indicate priority and uniquely identify a frame.
The number of priority bits must be large enough to uniquely identify a frame in a given network
coniguration. he priority bits for a given frame are set by the relative urgency (or deadline) of the
frame.hisisderivedfromhowurgentlyeachsubscriberofasignalintheframeneedsthesignal(as
described earlier). In most systems - priority bits are sufficient.
The filter bits are the remaining least-significant bits and are used to indicate the destination ECUs
foragivenframe.Treatingthemasa“targetmask”doesthis:EachECU(orgroupofECUs)isassigned
a single filter bit. he filtering for a CAN controller in the ECU is set up to accept only frames where
the corresponding filter bit in the identifier is set. This can give “perfect filtering”: an interrupt is
raised if and only if the frame is needed by the ECU. Perfect filtering can dramatically reduce the
CPU load compared to filtering in software. Indeed, perfect filtering is essential if the system inte-
grator needs to connect ECUs with slow bit CPUs to high-speed CAN networks (if filtering were
implementedinsotwaretheCPUwouldspendmostofitsavailableprocessingtimehandlinginter-
rupts and discarding unwanted frames). The filtering scheme also allows for broadcast of a frame
to an arbitrary set of ECUs. This can reduce the traffic on the bus since frames do not need to be
transmitted several times to different destinations (Figure .).
Because the system integrator is able to define the configuration data and because that data defines
thecompletenetworkbehaviorofanECU,thein-vehiclenetworksareunderthecontrolofthesystem
integrator.
19.16.8 Multiprotocol Support
The existing version of VNA supports the complementary, contemporary network protocols of CAN
and LIN. As network technology continues to advance into other protocols, VNA will also move
to support these advances. The philosophy behind is that communications has to be managed in
