create new tag
, view all tags
The RFM22B 'Hope' transceiver module is used to enable the SPR to transmit and receive data over the wireless communications medium. At this point in time, the transceiver operates in FIFO mode, which buffers the data to be sent on the transceiver module before the command to transmit is given. It should be noted that the maximum size of the FIFO buffer is 64 bytes, which places a soft limit on the size of packets which can be transmitted. Once the transmit instruction is received, the transceiver goes into transmit mode and uses its own packet handler to format the packet (see Fig. 1) using the data in the FIFO buffer as the payload.


Fig 1 - Packet Format (We need to update the packet format figure to avoid confusion)

The board adds six bytes to the ones in the payload, two for preamble, one for the sync word, one for the destination address, one for the packet length, and one for a cyclic redundancy check, or CRC. While the preamble can be of variable length, it was found that two bytes was the minimum length required to consistently transmit an error-free packet and in FIFO mode the transceiver requires a sync word of at least one byte. The destination address is the source address of the desired board, or 0xFF for a broadcast message, while the packet length is the length of the payload. It should be noted that the destination address byte is checked against the receiving board's source address upon arrival, and allows the board to filter out any messages for which it is not the intended recipient. The addition of the CRC byte at the end of the packet ensures that the packet is error-free and means that it is not necessary to include a CRC in the transmitted packet, thus freeing up extra bytes to be used for data.

Topic revision: r3 - 2014-04-30 - AhmetSekercioglu
This site is powered by the TWiki collaboration platformCopyright © 2008-2020 by the contributing authors. All material on this collaboration platform is the property of the contributing authors.
Ideas, requests, problems regarding TWiki? Send feedback