MRF39RA Datasheet, PDF (35/72 Pages) Microchip Technology – Low-Power, Integrated UHF Receiver

MRF39RA
4.5.2.2 Variable Length Packet Format
Variable length packet format is selected when bit
PacketFormat is set to ’1’.
This mode is useful in applications where the length of
the packet is not known in advance and can vary over
time. It is necessary for the transmitter to send the
length information together with each packet to enable
the receiver to operate properly.
In this mode, the length of the payload indicated by the
length byte is given by the first byte of the FIFO and is
limited to 255 bytes if AES is not enabled; otherwise,
the message is limited to 64 bytes (i.e., max 66 bytes
payload, if address byte is enabled). Note that the
length byte itself is not included in its calculation. In this
mode, the payload must contain at least two bytes (i.e.,
length + address or message byte).
An illustration of a variable length packet is shown in
Figure 4-10, which contains the following fields:
• Preamble (1010...)
• Sync Word (Network ID)
• Length Byte
• Optional Address Byte (Node ID)
• Message Data
• Optional 2-Byte CRC Checksum
FIGURE 4-10:
VARIABLE LENGTH PACKET FORMAT
Pr eambl e
0 to 65535
bytes
Sync Word
0 to 8 bytes
DC free Data decoding
CRC checksum calculation
AES Decr yption
Length
byte
Address
byte
Message
Up to 255 bytes
Fields processed and removed in Rx
Payload
(min 2 bytes)
Optional User provided fields which are part of the payload
Message part of the payload
CRC
2-bytes
 2015 Microchip Technology Inc.
DS40001778B-page 35