Texas Instruments 3138 155 232931 manual Frame Format, Demodulator Simplified Block Diagram

I / Q Analog

IF signal

CC2420

ADC	Digital	Frequency	Digital	Symbol		Data
	IF Channel	Offset	Data	Correlators and		Symbol
	Filtering	Compensation	Filtering	Synchronisation		Output
					Average
		RSSI	RSSI		Correlation
		Generator			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

802.15.4frame 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].

			Bytes: 2		1	0 to 20	n	2
MAC				Frame	Data	Address		Frame Check
				Control Field	Sequence		Frame payload	Sequence
Layer						Information
				(FCF)	Number			(FCS)
				MAC Header (MHR)			MAC Payload	MAC Footer
								(MFR)
Bytes:	4	1	1			5 + (0 to 20) + n
PHY	Preamble	Start of frame	Frame			MAC Protocol
		Delimiter				Data Unit
Layer	Sequence		Length
		(SFD)				(MPDU)
	Synchronisation	Header	PHY Header			PHY Service Data Unit
	(SHR)		(PHR)			(PSDU)
				11 + (0 to 20) + n
				PHY Protocol Data Unit
					(PPDU)

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

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