MRF39RA Datasheet, PDF (40/72 Pages) Microchip Technology – Low-Power, Integrated UHF Receiver

MRF39RA
4.5.7 DC-FREE DATA MECHANISMS
The received payload can be de-whitened or
automatically Manchester-decoded in the MRF39RA
packet handler.
Note: Only one of the two methods must be
enabled at a time.
4.5.7.1 Manchester Decoding
Manchester decoding is enabled if DcFree = 01 and it
can only be used in Packet mode.
The Manchester data is decoded to NRZ code by
decoding ‘10' as '1' and ‘01‘ as '0'.
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 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 decoding is thus made transparent for the
user, who still retrieves NRZ data from the FIFO.
FIGURE 4-13:
MANCHESTER DECODING
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
Manchester OFF
... 1 1 1 0 1 0 0 1 0
0101
1 0 1 0 ...
t
User/NRZ bits
Manchester ON
... 1 1 1 0 1 0 0
1
0
0
1
1
...
4.5.7.2 Data De-Whitening
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. Compared to the
Manchester technique it has the advantage of keeping
the NRZ data rate (i.e., the actual bit rate is not halved).
FIGURE 4-14:
DATA DE-WHITENING
The de-whitening process is enabled if DcFree = 10.
The data, including payload and 2-byte CRC
checksum, is de-whitened by XORing it with a random
sequence generated in a 9-bit LFSR as shown in
Figure 4-14.
Payload de-whitening is thus made transparent for the
user, who still retrieves NRZ data from the FIFO.
DS40001778B-page 40
 2015 Microchip Technology Inc.