PIC18F258-ESLASHLSQTP datasheet(234/402 Pages) MICROCHIP

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PIC18FXX8

DS41159E-page 232

19.6

Message Acceptance Filters

and Masks

The message acceptance filters and masks are used to

determine if a message in the message assembly

buffer should be loaded into either of the receive buff-

ers. Once a valid message has been received into the

MAB, the identifier fields of the message are compared

to the filter values. If there is a match, that message will

be loaded into the appropriate receive buffer. The filter

masks are used to determine which bits in the identifier

are examined with the filters. A truth table is shown

below in Table 19-2 that indicates how each bit in the

identifier is compared to the masks and filters to deter-

mine if a message should be loaded into a receive

buffer. The mask essentially determines which bits to

apply the acceptance filters to. If any mask bit is set to

a zero, then that bit will automatically be accepted

regardless of the filter bit.

TABLE 19-2:

FILTER/MASK TRUTH TABLE

As shown in the receive buffer block diagram

(Figure 19-4), acceptance filters RXF0 and RXF1 and

filter mask RXM0 are associated with RXB0. Filters

RXF2, RXF3, RXF4 and RXF5 and mask RXM1 are

associated with RXB1. When a filter matches and a

message is loaded into the receive buffer, the filter

number that enabled the message reception is loaded

into the FILHIT bit(s).

For RXB1, the RXB1CON register contains the

FILHIT<2:0> bits. They are coded as follows:

• 101 = Acceptance Filter 5 (RXF5)

• 100 = Acceptance Filter 4 (RXF4)

• 011 = Acceptance Filter 3 (RXF3)

• 010 = Acceptance Filter 2 (RXF2)

• 001 = Acceptance Filter 1 (RXF1)

• 000 = Acceptance Filter 0 (RXF0)

The coding of the RXB0DBEN bit enables these three

bits to be used similarly to the FILHIT bits and to

distinguish a hit on filter RXF0 and RXF1, in either

RXB0, or after a rollover into RXB1.

• 111 = Acceptance Filter 1 (RXF1)

• 110 = Acceptance Filter 0 (RXF0)

• 001 = Acceptance Filter 1 (RXF1)

• 000 = Acceptance Filter 0

If the RXB0DBEN bit is clear, there are six codes

corresponding to the six filters. If the RXB0DBEN bit is

set, there are six codes corresponding to the six filters

plus two additional codes corresponding to RXF0 and

RXF1 filters that rollover into RXB1.

If more than one acceptance filter matches, the FILHIT

bits will encode the binary value of the lowest

numbered filter that matched. In other words, if filter

RXF2 and filter RXF4 match, FILHIT will be loaded with

the value for RXF2. This essentially prioritizes the

acceptance filters with a lower number filter having

higher priority. Messages are compared to filters in

ascending order of filter number.

The mask and filter registers can only be modified

when the PIC18FXX8 is in Configuration mode. The

mask and filter registers cannot be read outside of

Configuration mode. When outside of Configuration

mode, all mask and filter registers will be read as ‘0’.

FIGURE 19-6:

MESSAGE ACCEPTANCE MASK AND FILTER OPERATION

Mask

bit n

Filter bit n

Message

Identifier

bit n001

Accept or

Reject

bit n

Reject

Legend:

= don’t care

Note:

‘000’ and ‘001’ can only occur if the

RXB0DBEN bit is set in the RXB0CON

rollover into RXB1.

Acceptance Filter Register

Acceptance Mask Register

RxRqst

Message Assembly Buffer

RXFn0

RXFn1

RXFnn

RXMn0

RXMn1

RXMnn

Identifier

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