MikroElektronika PIC Microcontrollers PIC18 Manual de usuario descargar pdf (Pagina 20)

communicating with each other. The baud rate can be determined by testing

different values until valid messages are received. The listen-only mode cannot

transmit messages.

9.9.5 Loop-Back Mode

In the loop-back mode, messages can be directed from internal transmit buffers to

receive buffers without actually transmitting messages on the CAN bus. This mode

is useful during system developing and testing.

9.9.6 Error Recognition Mode

The error recognition mode is used to ignore all errors and receive all messages. In

this mode, all messages, valid or invalid are received and copied to the receive buffer.

9.9.7 CAN Message Transmission

The PIC18F258 microcontroller implements three dedicated transmit buffers: TXB0,

TXB1, and TXB2. Pending transmittable messages are in a priority queue. Before

the SOF is sent, the priorities of all buffers queued for transmission are compared.

The transmit buffer with the highest priority is sent first. If two buffers have the

same priority, the one with the higher buffer number is sent first. There are four

levels of priority.

9.9.8 CAN Message Reception

Reception of a message is a more complex process. The PIC18F258 microcontroller

includes two receive buffers, RXB0 and RXB1, with multiple acceptance filters

for each (see Figure 9.13). All received messages are assembled in the message

assembly buffer (MAB). Once a message is received, regardless of the type of

identifier and the number of data bytes, the entire message is copied into the MAB.

Received messages have priorities. RXB0 is the higher priority buffer, and it has two

message acceptance filters, RXF0 and RXF1. RXB1 is the lower priority buffer and

has four acceptance filters: RXF2, RXF3, RXF4, and RXF5. Two programmable

acceptance filter masks, RXM0 and RXM1, are also available, one for each receive

buffer.

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