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RFM95 Datasheet(PDF) 73 Page - HOPE Microelectronics CO., Ltd.

Part No. RFM95
Description  Low Power Long Range Transceiver Module
Download  121 Pages
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Maker  HOPE [HOPE Microelectronics CO., Ltd.]
Homepage  https://www.hoperf.com/
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 73 page
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Page 73
RFM95/96/97/98(W)
Tel: + 86-755-82973805
Fax: + 86-755-82973550
E-mail: sales@hoperf.com http:/ / www.hoperf.com
WIRELESS & SENSING
PRELIMINARY
DATASHEET
4.2.13.7. DC-Free Data Mechanisms
The payload to be transmitted may contain long sequences of 1's and 0's, which introduces a DC bias in the transmitted
signal. The radio signal thus produced has a non uniform power distribution over the occupied channel bandwidth. It also
introduces data dependencies in the normal operation of the demodulator. Thus it is useful if the transmitted data is random
and DC free.
For such purposes, two techniques are made available in the packet handler: Manchester encoding and data whitening.
Note
Only one of the two methods can be enabled at a time.
Manchester Encoding
Manchester encoding/decoding is enabled if DcFree = 01 and can only be used in Packet mode.
The NRZ data is converted to Manchester code by coding '1' as “10” and '0' as “01”.
In this case, the maximum chip rate is the maximum bit rate given in the specifications section and the actual bit rate is half
the chip rate.
Manchester encoding and decoding is only applied to the payload and CRC checksum while preamble and Sync word are
kept NRZ. However, the chip rate from preamble to CRC is the same and defined by BitRate in RegBitRate (Chip Rate =
Bit Rate NRZ = 2 x Bit Rate Manchester).
Manchester encoding/decoding is thus made transparent for the user, who still provides/retrieves NRZ data to/from the
FIFO.
1/BR ...Sync
1/BR
Payload...
RF chips @ BR
...
1
1
1
0
1
0
0
1
0
0
1
0
1
1
0
1
0
...
User/NRZ bits
t
Manchester OFF
...
1
1
1
0
1
0
0
1
0
0
1
0
1
1
0
1
0
...
User/NRZ bits
Manchester ON
...
1
1
1
0
1
0
0
1
0
0
1
1
...
Data Whitening
Figure 36. Manchester Encoding/Decoding
Another technique called whitening or scrambling is widely used for randomizing the user data before radio transmission.
The data is whitened using a random sequence on the Tx side and de-whitened on the Rx side using the same sequence.
Comparing to Manchester technique it has the advantage of keeping NRZ data rate i.e. actual bit rate is not halved.
The whitening/de-whitening process is enabled if DcFree = 10. A 9-bit LFSR is used to generate a random sequence. The
payload and 2-byte CRC checksum is then XORed with this random sequence as shown below. The data is de-whitened
on the receiver side by XORing with the same random sequence.
Payload whitening/de-whitening is thus made transparent for the user, who still provides/retrieves NRZ data to/from the
FIFO.




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