Rainbow Electronics ATmega128RFA1 - Manual (Page 62)

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8266A-MCU Wireless-12/09
ATmega128RFA1
On receive the PHR is returned as the first octet during Frame Buffer read access.
Even though the standard only defines frame lengths ≤127 bytes, the radio transceiver
is able to transmit and receive frame length values >127. For IEEE 802.15.4 compliant
operation bit 8 has to be masked by software. The reception of a valid PHR is signaled
by a TRX24_RX_START interrupt.
On transmit the PHR has to be written first to the Frame Buffer.
9.5.1.1.3 PHY Payload (PHY Service Data Unit, PSDU)
The PSDU has a variable length between 0 and aMaxPHYPacketSize (127, maximum
PSDU size in octets) whereas the last two octets are used for the Frame Check
Sequence (FCS). The length of the PSDU is signaled by the frame length field (PHR)
as described in
Table 9-15 below . The PSDU contains the MAC Protocol Layer Data
Unit (MPDU).
Received frames with a frame length field set to 0x00 (invalid PHR) are not by an
interrupt.
Table 9-15 below summarizes the type of payload versus the frame length value.
Table 9-15. Frame Length Field - PHR
Frame Length Value Payload
0 - 4 Reserved
5 MPDU (Acknowledgement)
6 – 8 Reserved
9 - aMaxPHYPacketSize MPDU
9.5.1.2 MAC Protocol Layer Data Unit (MPDU)
Figure 9-15 below shows the frame structure of the MAC layer.
Figure 9-15. IEEE 802.15.4 Frame Format - MAC-Layer Frame Structure (MPDU)
9.5.1.2.1 MAC Header (MHR) Fields
The MAC header consists of the Frame Control Field (FCF), a sequence number, and
the addressing fields (which are of variable length and can even be empty in certain
situations).