MikroElektronika PIC Microcontrollers PIC18 Manuel d'utilisateur télécharger pdf (Page 4)

removed from the bus, no configuration data needs to be changed (i.e., the bus is

“hot pluggable”).



CAN bus offers remote transmit request (RTR), which means that one node on

the bus is able to request information from the other nodes. Thus instead of

waiting for a node to continuously send information, a request for information

can be sent to the node. For example, in a vehicle, where the engine temperature

is an important parameter, the system can be designed so the temperature is

sent periodically over the bus. However, a more elegant solution is to request

the temperature as needed, since it minimizes the bus traffic while maintaining

the network’s integrity.



CAN bus communication speed is not fixed. Any communication speed can be

set for the devices attached to a bus.



All devices on the bus can detect an error. The device that has detected an error

immediately notifies all other devices.



Multiple devices can be connected to the bus at the same time, and there are no

logical limits to the number of devices that can be connected. In practice, the

number of units that can be attached to a bus is limited by the bus’s delay time

and electrical load.

The data on CAN bus is differential and can be in two states: dominant and recessive.

Figure 9.3 shows the state of voltages on the bus. The bus defines a logic bit 0 as a

dominant bit and a logic bit 1 as a recessive bit. When there is arbitration on the bus, a

Voltage

level

Vdiff

Recessive

CANL

CANH

Dominant

3.5

2.5

1.5

Time

Figure 9.3: CAN logic states

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MikroElektronika PIC Microcontrollers PIC18 Manuel d'utilisateur Page 4