Image Processing Reference
In-Depth Information
classed as “fixed.” Frame packing can also be performed manually if desired. Should changes to the
design be required at a later time, the process allows rapid turnaround of design changes, rerunning
the frame compiler, and regenerating the configuration files.
he VNA tool can be used to design network solutions that are later realized by embedded software
from any provider. However, the VNA tool is designed with the volcano embedded software (volcano
target package [VTP]) in mind, which implements the expected behavior into the different nodes.
To get the full benefits of the tool chain, VNA and VTP should be used together.
19.16.7 Volcano Filtering Algorithm
A crucial aspect of network configuration is how to choose identifiers so that the load on a CPU
related to handling of interrupts generated by frames of no interest for the particular node is
minimized—most CAN controllers have only limited filtering capabilities. The Volcano filtering
algorithm is designed to achieve this.
An identifier is split into two parts: the priority bits and the filter bits. All frames on a network must
have unique priority bits; for real-time performance the priority setting of a frame should reflect the
relative urgency of the frame. he filter bits are used to determine if a CAN controller should accept
or reject a frame. Each ECU that needs to receive frames by interrupts is assigned a single filter bit;
thehardwareilteringintheCANcontrollerissetto“mustmatch”fortheilterbit,and“don'tcare”
forallotherbits.
The filter bits of a frame are set for each ECU to which the frame needs to be seen. So a frame that
is broadcast to all ECUs on the network is assigned filter bits all set to “.” For a frame sent to a single
ECU on the network just one filter bit is set. Figure . below illustrates this; the frame shown is sent
to four ECUs.
Figure . shows an example of a CAN identifier on an extended CAN network. The network
clause has defined the CAN identifiers to have priority bits, and filter bits. he least-significant
bit of the value corresponds with the bit of the identifier transmitted last. Only legal CAN identifiers
can be specified: identifiers with the seven most-significant bits equal to “” are illegal according to
the CAN standard.
If an ECU takes an interrupt for just the frames that it needs, then the filtering is said to be
perfect
.
In some systems there may be more ECUs needing to receive frames by interrupt than there are filter
bits in the network; in this case, some ECUs will need to share a bit. If this happens then Volcano
will filter the frames in software, using the priority bits to uniquely identify the frame and discarding
unwanted frames.
ID bit
28
24
20
16
12
8
4
0
1
6
0
0
0
4
C
2
1
0
1
1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
0
0
1
1
0
0
0
0
1
0
7
8
0
4
C
2
num filter bits = 13
num priority bits = 7
Unused bits (0)
FIGURE .
CAN ID filtering scheme in volcano.
