Texas Instruments 3138 155 232931 Format of the Frame Control Field FCF Frame check sequence

There is no hardware support for the data sequence number, this field must be inserted and verified by software.

CC2420

CC2420 includes hardware address recognition, as described in the Address Recognition section on page 41.

Bits: 0-2	3	4	5	6	7-9	10-11
Frame	Security	Frame	Acknowledge	Intra	Reserved	Destination
Type	Enabled	Pending	request	PAN		addressing
						mode

12-13 14-15

Reserved Source addressing mode

Figure 19. Format of the Frame Control Field (FCF) [1]

16.4 Frame check sequence

A2-byte frame check sequence (FCS) follows the last MAC payload byte as shown in Figure 17. The FCS is calculated over the MPDU, i.e. the length field is not part of the FCS. This field is automatically generated and verified by hardware when the MODEMCTRL0.AUTOCRC control bit is set. It is recommended to always have this enabled, except possibly for debug purposes. If cleared, CRC generation and verification must be performed by software.

The FCS polynomial is [1]:

x16 + x12 + x5 + 1

The CC2420 hardware implementation is shown in Figure 20. Please refer to [1] for further details.

In transmit mode the FCS is appended at the correct position defined by the length field. The FCS is not written to the TXFIFO, but stored in a separate 16-bit register.

In receive mode the FCS is verified by hardware. The user is normally only

interested in the correctness of the FCS, not the FCS sequence itself. The FCS sequence itself is therefore not written to the RXFIFO during receive.

Instead, when MODEMCTRL0.AUTOCRC is set the two FCS bytes are replaced by the RSSI value, average correlation value (used for LQI) and CRC OK/not OK. This is illustrated in Figure 21.

The first FCS byte is replaced by the 8-bit RSSI value. This RSSI value is measured over the first 8 symbols following the SFD. See the RSSI section on page 48 for details.

The 7 least significant bits in the last FCS byte are replaced by the average correlation value of the 8 first symbols of the received PHY header (length field) and PHY Service Data Unit (PSDU). This correlation value may be used as a basis for calculating the LQI. See the Link Quality Indication section on page 49 for details.

The most significant bit in the last byte of each frame is set high if the CRC of the received frame is correct and low otherwise.

Data input (LSB first)

r0 r1 r2 r3 r4 r5 r6 r7 r8 r9 r10 r11 r12 r13 r14 r15

Figure 20. CC2420 Frame Check Sequence (FCS) hardware implementation [1]

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