Rainbow-electronics ATmega128RFA1 Bedienungsanleitung PDF herunterladen (Seite 66)

8266A-MCU Wireless-12/09

ATmega128RFA1

subfield b3 is set to one (see section

"Frame Compatibility between IEEE 802.15.4-

2003 and IEEE 802.15.4-2006" on page 65). For details of its structure see

IEEE 802.15.4-2006, 7.6.2 Auxiliary security header.

9.5.1.2.7 MAC Service Data Unit (MSDU)

This is the actual MAC payload. It is usually structured according to the individual frame

type. A description can be found in IEEE 802.15.4-2006, chapter 5.5.3.2.

9.5.1.2.8 MAC Footer (MFR) Fields

The MAC footer consists of a two-octet Frame Checksum (FCS). For details refer to the

following section "Frame Check Sequence (FCS)" below.

9.5.2 Frame Check Sequence (FCS)

The Frame Check Sequence (FCS) is characterized by:

• Indicate bit errors based on a cyclic redundancy check (CRC) of 16 bit length;

• Uses International Telecommunication Union (ITU) CRC polynomial;

• Automatically evaluated during reception;

• Can be automatically generated during transmission.

9.5.2.1 Overview

The FCS is intended for use at the MAC layer to detect corrupted frames at a first level

of filtering. It is computed by applying an ITU CRC polynomial to all transferred bytes

following the length field (MHR and MSDU fields). The frame check sequence has a

length of 16 bit and is located in the last two bytes of a frame (MAC footer, see Figure

9-15 on page 62).

The radio transceiver applies an FCS check on each received frame. The result of the

FCS check is stored in bit RX_CRC_VALID of register PHY_RSSI.

On transmit the radio transceiver generates and appends the FCS bytes during the

frame transmission. This behavior can be disabled by setting the bit

TX_AUTO_CRC_ON = 0 in register TRX_CTRL_1.

9.5.2.2 CRC calculation

The CRC polynomial used in IEEE 802.15.4 networks is defined by

1)(

51216

+++= xxxxG

The FCS shall be calculated for transmission using the following algorithm:

Let

)(

−−

++++=

bxbxbxbxM K

be the polynomial representing the sequence of bits for which the checksum is to be

computed. Multiply M(x) by x

giving the polynomial

)()( xxMxN ⋅=

Divide

)(xN

modulo 2 by the generator polynomial G

(x) to obtain the remainder

polynomial

1514

...)( rxrxrxrxR ++++=

The FCS field is given by the coefficients of the remainder polynomial, R(x).

1 2 ... 61 62 63 64 65 66 67 68 69 70 71 ... 523 524

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