Texas Instruments 3138 155 232931 - Manual (Page 36)

CC2420
SWRS041B Page 36 of 89
Digital
IF Channel
Filtering
ADC
Digital
Data
Filtering
Frequency
Offset
Compensation
Symbol
Correlators and
Synchronisation
RSSI
Generator
I / Q Analog
IF signal
Data
Symbol
Output
RSSI
Average
Correlation
Value (may be
used for LQI)
Figure 16. Demodulator Simplified Block Diagram
16 Frame Format
CC2420 has hardware support for parts of
the IEEE 802.15.4 frame format. This
section gives a brief summary to the IEEE
802.15.4 frame format, and describes how
CC2420 is set up to comply with this.
Figure 17 [1] shows a schematic view of
the IEEE 802.15.4 frame format. Similar
figures describing specific frame formats
(data frames, beacon frames,
acknowledgment frames and MAC
command frames) are included in [1].
Figure 17. Schematic view of the IEEE 802.15.4 Frame Format [1]
16.1 Synchronisation header
The synchronisation header (SHR)
consists of the preamble sequence
followed by the start of frame delimiter
(SFD). In [1], the preamble sequence is
defined to be 4 bytes of 0x00. The SFD is
one byte, set to 0xA7.
In CC2420 , the preamble length and SFD is
configurable. The default values are
compliant with [1]. Changing these values
will make the system non-compliant to
IEEE 802.15.4.
A synchronisation header is always
transmitted first in all transmit modes.
The preamble sequence length can be set
by MDMCTRL0.PREAMBLE_LENGTH, while
the SFD is programmed in the SYNCWORD
register. SYNCWORD is 2 bytes long, which
gives the user some extra flexibility as
described below. Figure 18 shows how the
CC2420 synchronisation header relates to
the IEEE 802.15.4 specification.
The programmable preamble length only
applies to transmission, it does not affect
receive mode. The preamble length
should not be set shorter than the default
value. Note that 2 of the 8 zero-symbols in
the preamble sequence required by [1] are
included in the SYNCWORD register so that
the
CC2420 preamble sequence is only 6
symbols long for compliance with [1]. Two
PHY
Layer
Frame
Control Field
(FCF)
Data
Sequence
Number
2 1 Bytes:
Address
Information
0 to 20
Frame payload
n
Frame Check
Sequence
(FCS)
2
MAC Header (MHR) MAC Payload MAC Footer
(MFR)
Frame
Length
MAC Protocol
Data Unit
(MPDU)
Start of frame
Delimiter
(SFD)
Bytes: 1 1 5 + (0 to 20) + n
Preamble
Sequence
4
Synchronisation Header
(SHR)
PHY Header
(PHR)
PHY Service Data Unit
(PSDU)
PHY Protocol Data Unit
(PPDU)
11 + (0 to 20) + n
MAC
Layer