MRF39RA datasheet(34/72 Pages) MICROCHIP

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MRF39RA

DS40001778B-page 34

 2015 Microchip Technology Inc.

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

Message

Up to 255 bytes

Address

byte

CRC

2-bytes

Sync Word

0 to 8 bytes

Preamble

0 to 65535

bytes

Payload

(min 1 Byte)

CRC checksum calculation

DC free Data decoding

Fields processed and removed in Rx

Optional User provided fields which are part of the payload

Message part of the payload

AES Decryption

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