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MRF89XA Arkusz danych(PDF) 85 Page - Microchip Technology |
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MRF89XA Arkusz danych(HTML) 85 Page - Microchip Technology |
85 / 140 page 2010-2017 Microchip Technology Inc. Preliminary DS70000622D-page 85 MRF89XA 3.11.3.3 Address-Based Address filtering can be enabled through the ADDFIL<1:0> bits (PKTCREG<2:1>). It adds another level of filtering above the Sync Word, which is typically useful in multi-node networks where a network ID is shared between all nodes (Sync Word) and each node has its own ID (address). Three address-based filtering options are available: • ADDFIL = 01: Received address field is com- pared with the internal register, NADDSREG. If they match, the packet is accepted and processed; otherwise, it is discarded. • ADDFIL = 10: Received address field is com- pared with the internal register, NADDSREG, and the constant 0x00. If either is a match, the received packet is accepted and processed; oth- erwise, it is discarded. This additional check with a constant is useful for implementing broadcast in multi-node networks. • ADDFIL = 11: Received address field is com- pared with the internal register, NADDSREG, and the constants 0x00 and 0xFF. If any of the three matches, the received packet is accepted and processed; otherwise, it is discarded. These addi- tional checks with constants are useful for implementing broadcast commands of all nodes. Here the received address byte, as part of the pay- load, is not stripped off the packet and is made available in the FIFO. In addition, NADDSREG and ADDFIL<1:0> bits from PKTCREG only apply to RX. On TX side, if address filtering is expected, the address byte should be put into the FIFO like any other byte of the payload. 3.11.3.4 CRC-Based The CRC check is enabled by setting the CHKCRCEN bit (PKTCREG<3>). This bit is used for checking the integrity of the message. A 16-bit CRC checksum is calculated on the payload part of the packet and is appended to the end of the transmitted message. The CRC checksum is calculated on the received payload and compared to the transmitted CRC. The result of the comparison is stored in the STSCRCEN bit (PKTCREG<0>), and an interrupt can also be gener- ated on IRQ1. • On the TX side, a 2-byte CRC checksum is calcu- lated on the payload part of the packet and appended to the end of the message. • On the RX side, the checksum is calculated on the received payload and compared with the two checksum bytes received. The result of the com- parison is stored in the STSCRCEN bit from and the CRCOK IRQ source (refer to Register 2-14 for details). By default, if the CRC check fails, the FIFO is cleared and no interrupt is generated. This filtering function can be disabled through the ACFCRC bit (FCRCREG<7>), and, if CRC fails, the FIFO is not cleared and only the PLREADY interrupt goes high (for more information, refer to Register 2-14). In both cases, the two CRC checksum bytes are stripped off by the packet handler and only the payload is made available in the FIFO. The CRC is based on the CCITT polynomial as illus- trated in Figure 3-28. This implementation also detects errors due to leading and trailing zeros. For more infor- mation on CRC polynomial computation in C#, see Appendix B: “CRC Computation in C”. FIGURE 3-28: CRC POLYNOMIAL IMPLEMENTATION X14 X13 X12 X11 X5 X0 X15 CRC Polynomial =X16+ X 12 + X 5+ 1 * * * X4 * * * data input |
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