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5.3 Wireless Protocol
.... ....
0
Pre-Amble
1 0 1
1 1 1
Packet Start Bit
Word0 Synch Bit Word1 Synch Bit
Word0 Word1
Communication Protocol
0
Bit Width = 26 S
....
Software Overview
The wireless data communication protocol is digital FSK at 38,400 bits per second with NRZ coding. The
communication protocol used for the demonstration board includes preamble, packet start bit, and
word-sync bit as illustrated in
Figure 10 .
Figure 10. Communication Protocol
Preamble
The function of the preamble is to train the receive hardware to set the proper reference voltage threshold
to extract 1 and 0 data. The preamble consists of a train of alternating ones and zeros. A longer preamble
may give better performance at a cost of more time to deliver an equivalent data packet. The current
design has a preamble 100 bits long, or 2.6 ms in duration. Shorter lengths may be used for more
time-critical applications.
5.3.1 Packet Start and Word-Sync Bits
The packet start bit and the word sync bit are used by the receive routine to adjust microprocessor timing
for sampling the incoming data stream. The packet start bit occurs once at the beginning of the data
packet to indicate the end of the preamble and the start of data. The sync bit is used to resynchronize the
bit stream and the data sampling timer. Over time the receive timing could shift in phase relative to
transmit timing due to the small variation in component tolerances and performance. This shows up as an
error in sampled data. The problem gets worse as the packet size increases. Resynchronization minimizes
this drift over time.
Although resynchronization can be done at byte boundaries, word boundary resynchronization is slightly
more efficient.
The data clock recovery feature of the TRF4903 and the TRF6903 provides a convenient way to
synchronize data.
5.3.2 Wireless Data Stream
The wireless data stream for the game application is implemented as follows:
•
User initiates an RF transmission on Board A (TRF4903 demo board) by pushing four buttons in any
sequence.
•
User sequence is sent to Board B (TRF6903 demo board) using the transmission data packet
described in
Table 4 , embedded in the communication protocol described in Figure 10 . The
transmission packet includes a unique header, four bytes with the button IDs and a checksum.
•
Board A then goes to the IDLE_STATE and waits for further button pushes.
•
Board B receives the 6 data bytes, calculates the checksum and compares it with the received
checksum. If checksums match, it lights up the LED’s on the receive board in the same order in which
the transmit buttons were pressed. The Board B then goes into the receive mode and listens for
transmissions.
SWRU010A – October 2004 – Revised May 2005 17 TRF4903 With MSP430 Demonstration and Development Evaluation Kit