PIC18C658T-ESLASHCL datasheet(211/366 Pages) MICROCHIP

211 / 366 page

 2000 Microchip Technology Inc.

Advanced Information

DS30475A-page 211

PIC18CXX8

17.3.5

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 transmitted

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 Loop-

back mode is activated by setting the mode request bits

in the CANCON register.

17.3.6

ERROR RECOGNITION MODE

The module can be set to ignore all errors and receive

any message. The Error Recognition mode is activated

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

ters to 11. In this mode, the data which is in the mes-

sage assembly buffer until the error time, is copied in

the receive buffer and can be read via the CPU inter-

face. In addition, the data which was on the internal

sampling of the CAN bus at the error time and the state

vector of the protocol state machine and the bit counter

CntCan, are stored in registers and can be read.

17.4

CAN Message Transmission

17.4.1

TRANSMIT BUFFERS

The PIC18CXX8 implements three Transmit Buffers.

Each of these buffers occupies 14 bytes of SRAM and

are mapped into the device memory maps.

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 17.4.2).

17.4.2

TRANSMIT PRIORITY

Transmit priority is a prioritization, within the PIC18CXX8,

of the pending transmittable messages. This is indepen-

dent from, and not related to, any prioritization implicit in

the message arbitration scheme built into the CAN proto-

col. 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 buff-

ers have the same priority setting, the buffer with the

highest buffer number will be sent first. There are four lev-

els 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.

17.4.3

INITIATING TRANSMISSION

To initiate message transmission, the TXREQ bit must be

set for each buffer to be transmitted.

When TXREQ is set, the TXABT, TXLARB and TXERR

bits will be cleared.

Setting the TXREQ bit does not initiate a message

transmission, it merely flags a message buffer as ready

for transmission. Transmission will start when the

device detects that the bus is available. The device will

then begin transmission of the highest priority message

that is ready.

When the transmission has completed successfully, the

TXREQ bit will be cleared, the TXBnIF bit will be set, and

an interrupt will be generated if the TXBnIE bit is set.

If the message transmission fails, the TXREQ will remain

set indicating that the message is still pending for trans-

mission and one of the following condition flags will be set.

If the message started to transmit but encountered an

error condition, the TXERR and the IRXIF bits will be set

and an interrupt will be generated. If the message lost

arbitration, the TXLARB bit will be set.

17.4.4

ABORTING TRANSMISSION

The MCU can request to abort a message by clearing

the TXBnCON.TXREQ bit associated with the corre-

sponding message buffer. Setting CANCON.ABAT bit

will request an abort of all pending messages. If the

message has not yet started transmission, or if the

message started but is interrupted by loss of arbitration

or an error, the abort will be processed. The abort is

indicated when the module sets TXBnCON.ABTF bits.

If the message has started to transmit, it will attempt to

transmit the current message fully. If the current mes-

sage is transmitted fully and is not lost to arbitration or

an error, the ABTF bit will not be set, because the mes-

sage was transmitted successfully. Likewise, if a mes-

sage is being transmitted during an abort request and

the message is lost to arbitration or an error, the mes-

sage will not be re-transmitted and the ABTF bit will be

set, indicating that the message was successfully

aborted.

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