DESIGN OF A CUSTOM ASIC INCORPORATING CAN™ AND 1 ...

A final proposed method is based upon an automatic node discovery process to identify
nonconfigured CAN nodes directly in a CANopen® system [41].
Every CAN node in the prototype system described in this dissertation consists of the
following components: Microchip PIC12CE674 [42] microcontroller, Microchip MCP2515
stand-alone CAN Controller [43 – 44], and a Microchip MCP2551 high-speed CAN
transceiver [45]. A simplified block diagram of the CAN components on each node is shown
in Figure 4.5. The use of the PIC® microcontroller allows each node to have a unique address,
stored in EEPROM, so a number of different devices can coexist on the same network. This
would allow communication from a 1 – Wire® device (message by address) to a specific CAN
node which would have a unique address on the CAN network. By storing the node address in
EEPROM, this allows the node to not only be re-addressed if the addition of more nodes on the
CAN bus is required, but also ensures that the node address value is retained in the event of
power loss.
Message IDs must be unique on a single CAN bus, otherwise two nodes would
continue transmission beyond the end of their arbitration field (ID) causing an error (see Figures
3.3 and 3.4). The choice of IDs for messages is often done simply on the basis of identifying the
type of data and the sending node; however, as the message ID is also used as the message
priority, this can lead to poor real-time performance. For example, consider an 11-bit ID CAN
network, with two nodes with IDs of 31 (binary representation, 00000011111) and 32 (binary
representation, 00000100000). If these two nodes transmit at the same time, each will transmit
the first five zeros of their ID with no arbitration decision being made. When the sixth bit is
transmitted, the node with the ID of 32 transmits a ‘1’ (recessive) for its ID, and the node with
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