MCP2510-SLASHST datasheet(29/80 Pages) MICROCHIP

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DS21291F-page 29

MCP2510

4.5

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 (see Figure 4-3). Once a valid message has been

received into the MAB, the identifier fields of the mes-

sage are compared to the filter values. If there is a

match, that message will be loaded into the appropriate

receive buffer. The filter masks (see Register 4-10

through Register 4-17) are used to determine which

bits in the identifier are examined with the filters. A truth

table is shown below in Table 4-1 that indicates how

each bit in the identifier is compared to the masks and

filters to determine if a the 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 4-1:

FILTER/MASK TRUTH TABLE

As shown in the Receive Buffers Block Diagram

(Figure 4-1), 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 RXBNCTRL register FILHIT bit(s). For RXB1

the RXB1CTRL 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)

RXB0CTRL contains two copies of the BUKT bit and

the FILHIT<0> bit.

The coding of the BUKT bit enables these three bits to

be used similarly to the RXB1CTRL.FILHIT bits and to

distinguish a hit on filter RXF0 and RXF1 in either

RXB0 or after a roll over into RXB1.

- 111 = Acceptance Filter 1 (RXF1)

- 110 = Acceptance Filter 0 (RXF0)

- 001 = Acceptance Filter 1 (RXF1)

- 000 = Acceptance Filter 0

If the BUKT bit is clear, there are six codes correspond-

ing to the six filters. If the BUKT bit is set, there are six

codes corresponding to the six filters plus two addi-

tional codes corresponding to RXF0 and RXF1 filters

that roll over into RXB1.

If more than one acceptance filter matches, the FILHIT

bits will encode the binary value of the lowest num-

bered 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 accep-

tance filters with a lower number filter having higher pri-

ority. Messages are compared to filters in ascending

order of filter number.

The mask and filter registers can only be modified

when the MCP2510 is in configuration mode (see

Section 9.0).

Mask Bit

Filter Bit

Message

Identifier bit

n001

Accept or

reject bit n

Reject

Note:

X = don’t care

Note:

000

and 001 can only occur if the BUKT bit

(see Table 4-1) is set in the RXB0CTRL

over into RXB1.

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