nRF903 Nordic VLSI, nRF903 Datasheet - Page 13

• Manufacturer Part Number: nRF903
• Description: Multichannel RF Transceiver 433/868/915MHz
• Manufacturer: Nordic VLSI

PRODUCT SPECIFICATION
A UART is recommended for handling baseband data.
When transmitting wireless data, information should be divided into separate data
packages. Each package usually contains preamble, address, data-payload and a
checksum. Please refer to Application note nAN400-07,
for details.
Always aim to obtain zero DC-level (equal number of logic ’0’s and ’1’s) in the
transmitted data packages. Long sequences without transitions should be avoided.
If the user allows efficient scrambling or biphase (Manchester) encoding of data,
startup- and switching times may be lowered at the expense of increased data
redundancy. Generally, a system with short packages and low transmission duty-cycle
will benefit from a lower overall energy consumption per package even if biphase
encoding is used. Note that chosen data encoding protocol influence on the choice of
PLL loop filter (refer to the
chapter p. 18 and the
chapter p. 14).
The preamble is actually a training sequence for the demodulator in the receiver. The
purpose of the preamble is to retrieve the correct DC level of the modulation signal
before the address and data bits are demodulated. In addition, the UART must also be
synchronised to the recieved datastream. To obtain synchronisation of the UART, a
sequence that locates the start and stop bits uniquely must be transmitted before the
address and data bits.
The preamble sequence that gives the most rapid settling in the nRF903 demodulator
is a repeated ’0011’ sequence at 76.8kbit/s with a total length of 40 bits.
However, if a microcontroller with a UART is used, the receiving UART will not be
able to locate what is the start and stop bits in this sequence. The preamble data
should therefore be modified to the following sequence: ’0xCC 0xCC 0xCC 0xCC
0xF0’ i.e. four bytes of ’11001100’ followed by ’11110000’.
The F0-byte makes it possible for the UART to locate the correct position of the start
and stop bits (see Figure 8). Please note that the data bits in the figure have been
reversed compared to what is specified above. This is due to the fact that UARTs
normally reverse data before adding startbit (0) and stopbit (1) to each byte.
Data direction
0
0 0 1 1
0 0 1 1
1
0
0 0 1 1
0 0 1 1
1
0
0 0 1 1
0 0 1 1
1
0
0 0 1 1
0 0 1 1
1
0
0 0 0 0
1 1 1 1
1
Figure 8. Recommended nRF903 preamble
Nordic VLSI ASA
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Vestre Rosten 81, N-7075 Tiller, Norway
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Revision: 3.1
December 2002
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