PIC18F258-ESLASHLSQTP datasheet(229/402 Pages) MICROCHIP

229 / 402 page

DS41159E-page 227

PIC18FXX8

19.3.5

LOOPBACK MODE

This mode will allow internal transmission of messages

from the transmit buffers to the receive buffers without

actually transmitting messages on the CAN bus. This

mode can be used in system development and testing.

In this mode, the ACK bit is ignored and the device will

allow incoming messages from itself, just as if they

were coming from another node. The Loopback mode

is a silent mode, meaning no messages will be trans-

mitted while in this state, including error flags or

Acknowledge signals. The TXCAN pin will revert to port

I/O while the device is in this mode. The filters and

masks can be used to allow only particular messages

to be loaded into the receive registers. The masks can

be set to all zeros to provide a mode that accepts all

messages. The Loopback mode is activated by setting

the mode request bits in the CANCON register.

19.3.6

ERROR RECOGNITION MODE

The module can be set to ignore all errors and receive

all message. The Error Recognition mode is activated

by setting the RXM<1:0> bits in the RXBnCON

registers to ‘11’. In this mode, all messages, valid or

invalid, are received and copied to the receive buffer.

19.4

CAN Message Transmission

19.4.1

TRANSMIT BUFFERS

The PIC18FXX8 implements three transmit buffers

(Figure 19-2). Each of these buffers occupies 14 bytes

of SRAM and are mapped into the device memory

map.

For the MCU to have write access to the message

buffer, the TXREQ bit must be clear, indicating that the

message buffer is clear of any pending message to be

transmitted. At a minimum, the TXBnSIDH, TXBnSIDL

and TXBnDLC registers must be loaded. If data bytes

are present in the message, the TXBnDm registers

must also be loaded. If the message is to use extended

identifiers, the TXBnEIDm registers must also be

loaded and the EXIDE bit set.

Prior to sending the message, the MCU must initialize

the TXInE bit to enable or disable the generation of an

interrupt when the message is sent. The MCU must

also initialize the TXP priority bits (see Section 19.4.2

“Transmit Priority”).

19.4.2

TRANSMIT PRIORITY

Transmit

priority

prioritization

within

the

PIC18FXX8 of the pending transmittable messages.

This is independent from and not related to any prioriti-

zation implicit in the message arbitration scheme built

into the CAN protocol. Prior to sending the SOF, the

priority of all buffers that are queued for transmission is

compared. The transmit buffer with the highest priority

will be sent first. If two buffers have the same priority

setting, the buffer with the highest buffer number will be

sent first. There are four levels of transmit priority. If

TXP bits for a particular message buffer are set to ‘11’,

that buffer has the highest possible priority. If TXP bits

for a particular message buffer are ‘00’, that buffer has

the lowest possible priority.

FIGURE 19-2:

TRANSMIT BUFFER

BLOCK DIAGRAM

Message

Queue

Control

Transmit Byte Sequencer

TXREQ

TXB0

TXABT

TXLARB

TXERR

TXBUFF

MESSAGE

TXREQ

TXB1

TXABT

TXLARB

TXERR

TXBUFF

MESSAGE

TXREQ

TXB2

TXABT

TXLARB

TXERR

TXBUFF

MESSAGE

Message

Request

PIC18F258-E/LSQTP Datasheet(PDF) 229 Page - Microchip Technology

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