Rainbow Electronics AT86RF231 - Manual (Page 79)

79
8111A–AVR–05/08
AT86RF231
8. Functional Description
8.1 Introduction - IEEE 802.15.4 - 2006 Frame Format
Figure 8-1 on page 79 provides an overview of the physical layer (PHY) frame structure as
defined by IEEE 802.15.4. Figure 8-2 on page 80 shows the frame structure of the medium
access control (MAC) layer.
Figure 8-1. IEEE 802.15.4 Frame Format - PHY-Layer Frame Structure (PPDU)
8.1.1 PHY Protocol Layer Data Unit (PPDU)
8.1.1.1 8.1.1.1 Synchronization Header (SHR)
The SHR consists of a four-octet preamble field (all zero), followed by a single byte start-of-
frame delimiter (SFD) which has the predefined value 0xA7. During transmit, the SHR is auto-
matically generated by the AT86RF231, thus the Frame Buffer shall contain PHR and PSDU
only.
The transmission of the SHR requires 160 µs (10 symbols). As the SPI data rate is normally
higher than the over-air data rate, this allows the microcontroller to initiate a transmission without
having transferred the full frame data already. Instead it is possible to subsequently write the
frame content.
During frame reception, the SHR is used for synchronization purposes. The matching SFD
determines the beginning of the PHR and the following PSDU payload data.
8.1.1.2 PHY Header (PHR)
The PHY header is a single octet following the SHR. The least significant 7 bits denote the frame
length of the following PSDU, while the most significant bit of that octet is reserved, and shall be
set to 0 for IEEE 802.15.4 compliant frames.
On receive the PHR is returned as the first octet during Frame Buffer read access, the most sig-
nificant bit always set to 0. The reception of a valid PHR is signaled by an interrupt
IRQ_2 (RX_START).
On transmit the PHR is to be supplied by the microcontroller during Frame Buffer write access
as the first octet.
8.1.1.3 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), refer to
Table 8-1 on page 80 .
The PSDU contains the MAC Protocol Layer Data Unit (MPDU).
max. 127 octets
PHY Service Data Unit (PSDU)
1 octet
(PHR)
5 octets
Synchronization Header (SHR)
PHY Protocol Data Unit (PPDU)
Preamble Sequence SFD Frame Length PHY Payload
MAC Protocol Data Unit (MPDU)