MRF89XA datasheet(85/140 Pages) MICROCHIP

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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

X15

CRC Polynomial =X16+ X 12 + X 5+ 1

* * *

data input

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