MRF89XA datasheet(83/140 Pages) MICROCHIP

83 / 140 page

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Preliminary

DS70000622D-page 83

MRF89XA

3.11.0.3

Variable Length Packet Format

This mode is necessary in applications where the

length of the packet is not known in advance and can

vary over time. It is then necessary for the transmitter

to send the length information together with each

packet in order for the receiver to operate properly.

In this mode, the length of the payload (indicated by the

length byte in Figure 3-27) is given by the first byte of

the FIFO and is limited only by the width of the FIFO

selected. In this mode, the payload must contain at

least 2 bytes (that is, length plus address or message

byte).

A variable length packet frame format is illustrated in

Figure 3-27, which contains the following fields:

• Preamble (1010...)

• Sync Word (Network ID)

• Length byte

• Optional Address byte (Node ID)

• Message data

• Optional 2-bytes CRC checksum

FIGURE 3-27:

VARIABLE LENGTH PACKET FORMAT

Note:

The length byte is not included in the CRC

calculation.

Message

0 to (FIFO size - 1) bytes

Address

byte

Length

byte

CRC

2-bytes

Sync Word

1 to 4 bytes

Preamble

1 to 4 bytes

Payload/FIFO

CRC checksum calculation

Fields added by the packet handler in TX and processed and removed in RX

Optional, user provided field which are part of the payload

Message part of the payload

Optional DC-free data coding

Length

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