Texas Instruments 3138 155 232931 - Manual (Page 38)

CC2420
SWRS041B Page 38 of 89
There is no hardware support for the data
sequence number, this field must be
inserted and verified by software. 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 12-13 14-15
Frame
Type Security
Enabled Frame
Pending Acknowledge
request Intra
PAN Reserved Destination
addressing
mode
Reserved Source
addressing
mode
Figure 19. Format of the Frame Control Field (FCF) [1]
16.4 Frame check sequence
A 2-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]:
x 16 + x 12 + x 5 + 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.
r0 r1 r2 r3 r4 r5 r6 r7 r8 r9 r10 r11 r12 r13 r14 r15
Data
input
(LSB
first)
Figure 20.
CC2420
Frame Check Sequence (FCS) hardware implementation [1]