PIC18F258-ESLASHSOSQTP datasheet(232/402 Pages) MICROCHIP

232 / 402 page

PIC18FXX8

DS41159E-page 230

19.5

Message Reception

19.5.1

RECEIVE MESSAGE BUFFERING

The PIC18FXX8 includes two full receive buffers with

multiple acceptance filters for each. There is also a

separate Message Assembly Buffer (MAB) which acts

as a third receive buffer (see Figure 19-4).

19.5.2

RECEIVE BUFFERS

Of the three receive buffers, the MAB is always commit-

ted to receiving the next message from the bus. The

remaining two receive buffers are called RXB0 and

RXB1 and can receive a complete message from the

protocol engine. The MCU can access one buffer while

the other buffer is available for message reception or

holding a previously received message.

The MAB assembles all messages received. These

messages will be transferred to the RXBn buffers only

if the acceptance filter criteria are met.

When a message is moved into either of the receive

buffers, the appropriate RXBnIF bit is set. This bit must

be cleared by the MCU when it has completed process-

ing the message in the buffer in order to allow a new

message to be received into the buffer. This bit

provides a positive lockout to ensure that the MCU has

finished with the message before the PIC18FXX8

attempts to load a new message into the receive buffer.

If the RXBnIE bit is set, an interrupt will be generated to

indicate that a valid message has been received.

19.5.3

RECEIVE PRIORITY

RXB0 is the higher priority buffer and has two message

acceptance filters associated with it. RXB1 is the lower

priority buffer and has four acceptance filters associ-

ated with it. The lower number of acceptance filters

makes the match on RXB0 more restrictive and implies

a higher priority for that buffer. Additionally, the

RXB0CON register can be configured such if RXB0

contains a valid message and another valid message is

received, an overflow error will not occur and the new

message will be moved into RXB1 regardless of the

acceptance criteria of RXB1. There are also two

programmable acceptance filter masks available, one

for each receive buffer (see Section 19.6 “Message

Acceptance Filters and Masks”).

When a message is received, bits <3:0> of the

RXBnCON register will indicate the acceptance filter

number that enabled reception and whether the

received message is a remote transfer request.

The RXM bits set special Receive modes. Normally,

these bits are set to ‘00’ to enable reception of all valid

messages as determined by the appropriate accep-

tance filters. In this case, the determination of whether

or not to receive standard or extended messages is

determined by the EXIDE bit in the Acceptance Filter

receiver will accept only messages with standard or

extended identifiers, respectively. If an acceptance

filter has the EXIDE bit set, such that it does not corre-

spond with the RXM mode, that acceptance filter is

rendered useless. These two modes of RXM bits can

be used in systems where it is known that only standard

or extended messages will be on the bus. If the RXM

bits are set to ‘11’, the buffer will receive all messages

regardless of the values of the acceptance filters. Also,

if a message has an error before the end of frame, that

portion of the message assembled in the MAB before

the error frame will be loaded into the buffer. This mode

has some value in debugging a CAN system and would

not be used in an actual system environment.

19.5.4

TIME-STAMPING

The CAN module can be programmed to generate a

time-stamp for every message that is received. When

enabled, the module generates a capture signal for

CCP1 which in turns captures the value of either

Timer1 or Timer3. This value can be used as the

message time-stamp.

To use the time-stamp capability, the CANCAP bit

(CIOCAN<4>) must be set. This replaces the capture

input for CCP1 with the signal generated from the CAN

module. In addition, CCP1CON<3:0> must be set to

‘0011’ to enable the CCP special event trigger for CAN

events.

FIGURE 19-4:

RECEIVE BUFFER BLOCK

DIAGRAM

Note:

The entire contents of the MAB are moved

into the receive buffer once a message is

accepted. This means that regardless of

the

type

identifier

(standard

extended) and the number of data bytes

received, the entire receive buffer is over-

written with the MAB contents. Therefore,

the contents of all registers in the buffer

must be assumed to have been modified

when any message is received.

Acceptance Mask

RXM0

Acceptance Filter

RXF0

Acceptance Filter

RXF1

Message Assembly Buffer

Acceptance Filter

RXM2

Acceptance Filter

RXF3

Acceptance Filter

RXF4

Acceptance Filter

RXF5

Acceptance Mask

RXM1

RXB0

RXB1

Data and

Identifier

Data and

Identifier

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