MRF89XAM8A-I Datasheet, PDF (86/140 Pages) Microchip Technology – Ultra Low-Power, Integrated ISM Band Sub-GHz Transceiver

MRF89XA
3.11.4 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, which causes a non-uniform
power distribution spectrum over the occupied channel
bandwidth. These sequences also degrade the
performance of the demodulation and data, and clock
recovery functions in the receiver, which basically
introduces data dependencies in the normal operation
of the demodulator. System performance can be
enhanced if the payload bits are randomized to reduce
DC biases and increase the number of bit transitions.
Therefore, it is useful if the transmitted data is random
and DC-free.
To handle such instances, two techniques are available
in the packet handler: Manchester encoding and Data
Whitening. However, only one of the two methods
should be enabled at a time.
3.11.4.1 Manchester Data Encoding
Manchester encoding/decoding is enabled by setting
the MCHSTREN bit (PLOADREG<7>) and can be
used in Packet mode only. The NRZ data is converted
to Manchester code by coding ‘1’ as ‘10’ and ‘0’ as ‘01’.
Figure 3-29 illustrates Manchester data encoding. NRZ
data is converted to Manchester by encoding 1 bits as
10 chip sequences, and 0 bits as 01 chip sequences.
Manchester encoding guarantees DC-balance and
frequent data transitions in the encoded data. The
maximum Manchester chip rate corresponds to the
maximum bit rate given in the Transmitter Electrical
specifications in Table 5-6.
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 the BRVAL<6:0> bits
(BRSREG<6:0>) (Chip Rate = Bit Rate NRZ = 2 x Bit
Rate Manchester).
Therefore Manchester encoding/decoding is made
transparent for the user, who still provides/retrieves
NRZ data to/from the FIFO. See the Manchester
encoding/decoding bit pattern in Figure 3-30.
3.11.4.2 Data Whitening
Another technique called data 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 dewhitened on
the RX side using the same sequence. Compared to
Manchester technique it has the advantage of retaining
the NRZ data rate (that is, actual bit rate is not halved).
The whitening/dewhitening process is enabled by
setting the WHITEN1 bit (PKTCREG<4>). A 9-bit
Linear Feedback Shift Register (LFSR) is used to
generate a random sequence. The payload and 2-byte
CRC checksum is then XORed with this random
sequence as illustrated in Figure 3-31. The data is
dewhitened on the receiver side by XORing with the
same random sequence.
Payload whitening/dewhitening is made transparent for
the user, who still provides/retrieves NRZ data to/from
the FIFO.
FIGURE 3-29:
MANCHESTER DATA ENCODING
FIGURE 3-30:
MANCHESTER ENCODING/DECODING
RF chips @ BR
User/NRZ bits
Manchester OFF
User/NRZ bits
Manchester ON
1/BR 1_./_..Sync
... 1 1 1 0 1 0 0 1
... 1 1 1 0 1 0 0 1
BR
00 1
00 1
... 1 1 1 0 1 0 0
10
Payload...
0 1 10 1
0 1 10 1
0 ...
0 ...
t
01
1
...
DS70622C-page 86
Preliminary
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