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MCP2510-IST Datasheet(PDF) 29 Page - Microchip Technology |
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MCP2510-IST Datasheet(HTML) 29 Page - Microchip Technology |
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29 / 76 page  2000 Microchip Technology Inc. Preliminary DS21291C-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 4-10 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 4-10: 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 n Filter Bit n Message Identifier bit n001 Accept or reject bit n 0X X Accept 10 0 Accept 10 1 Reject 11 0 Reject 11 1 Accept 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 register allowing RXB0 messages to roll over into RXB1. |
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