MRF39RA Datasheet, PDF (34/72 Pages) Microchip Technology – Low-Power, Integrated UHF Receiver

MRF39RA
4.5.2 PACKET FORMAT
4.5.2.1 Fixed Length Packet Format
Fixed length packet format is selected when bit
PacketFormat is set to ‘0’ and PayloadLength is set to
any value greater than ‘0’.
In applications where the packet length is fixed in
advance, this mode of operation may be of interest to
minimize RF overhead (no length byte field is required).
All nodes must be programmed with the same packet
length value.
The length of the payload is limited to 255 bytes if AES
is not enabled; otherwise, the message is limited to 64
bytes (i.e., maximum 65 bytes payload if Address byte
is enabled).
The length programmed in PayloadLength relates only
to the payload, which includes the message and the
optional address byte. In this mode, the payload must
contain at least one byte (i.e., address or message
byte).
An illustration of a fixed length packet is shown in
Figure 4-9, which contains the following fields:
• Preamble (1010...)
• Sync Word (Network ID)
• Optional Address Byte (Node ID)
• Message Data
• Optional 2-Byte CRC Checksum
FIGURE 4-9:
FIXED LENGTH PACKET FORMAT
DC fr ee Data decoding
CRC checksum calculation
AES Decryption
Pr eambl e
0 to 65535
bytes
Sync Word
0 to 8 bytes
Addres s
byte
Mess age
Up to 255 bytes
Fields processed and removed in Rx
Pay load
(min 1 Byte)
Optional User provided fields which are part of the payload
Message part of the payload
CRC
2- bytes
DS40001778B-page 34
 2015 Microchip Technology Inc.