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MCP2515

DS21801F-page 32

2010 Microchip Technology Inc.

4.5

Message Acceptance Filters and

Masks

The message acceptance filters and masks are used to

determine if a message in the message assembly buf-

fer should be loaded into either of the receive buffers

(see Figure 4-5). 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.

4.5.1

DATA BYTE FILTERING

When receiving standard data frames (11-bit identifier),

the MCP2515 automatically applies 16 bits of masks

and

filters

normally

associated

with

extended

identifiers to the first 16 bits of the data field (data bytes

0 and 1). Figure 4-4 illustrates how masks and filters

apply to extended and standard data frames.

Data byte filtering reduces the load on the MCU when

implementing Higher Layer Protocols (HLPs) that filter

on the first data byte (e.g., DeviceNet).

4.5.2

FILTER MATCHING

The filter masks (see Register 4-14 through Register 4-

17) are used to determine which bits in the identifier are

examined with the filters. A truth table is shown in

Table 4-2 that indicates how each bit in the identifier is

compared to the masks and filters to determine if 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,

that bit will automatically be accepted, regardless of the

filter bit.

TABLE 4-2:

FILTER/MASK TRUTH TABLE

As shown in the receive buffers block diagram

(Figure 4-2), acceptance filters RXF0 and RXF1 (and

filter mask RXM0) are associated with RXB0. Filters

RXF2, RXF3, RXF4, RXF5 and mask RXM1 are

associated with RXB1.

FIGURE 4-4:

MASKS AND FILTERS APPLY TO CAN FRAMES

Mask Bit n

Filter Bit n

Message

Identifier

bit

Accept or

Reject bit n

Reject

Note:

= don’t care

Extended Frame

Standard Data Frame

ID10

ID0 EID17

EID0

Masks and Filters apply to the entire 29-bit ID field

ID10

ID0

Data Byte 0

Data Byte 1

11-bit ID Standard frame

16-bit data filtering *

* The two MSb (EID17 and EID16) mask and filter bits are not used.

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