RFM42B Datasheet, PDF (33/51 Pages) List of Unclassifed Manufacturers – ISM TRANSMITTER

RFM42B/43B
6.4. Data Whitening, Manchester Encoding, and CRC
Data whitening can be used to avoid extended sequences of 0s or 1s in the transmitted data stream to achieve a
more uniform spectrum. When enabled, the payload data bits are XORed with a pseudorandom sequence output
from the built-in PN9 generator. The generator is initialized at the beginning of the payload. The receiver recovers
the original data by repeating this operation. Manchester encoding can be used to ensure a dc-free transmission
and good synchronization properties. When Manchester encoding is used, the effective datarate is unchanged but
the actual datarate (preamble length, etc.) is doubled due to the nature of the encoding. The effective datarate
when using Manchester encoding is limited to 128 kbps. The implementation of Manchester encoding is shown in
Figure 15. Data whitening and Manchester encoding can be selected with "Register 70h. Modulation Mode Control
1". The CRC is configured via "Register 30h. Data Access Control". Figure 14 demonstrates the portions of the
packet which have Manchester encoding, data whitening, and CRC applied. CRC can be applied to only the data
portion of the packet or to the data, packet length and header fields. Figure 15 provides an example of how the
Manchester encoding is done and also the use of the Manchester invert (enmaniv) function.
Manchester
Whitening
CRC
CRC
(Over data only)
Preamble
Sync
Header/
Address
PK
Length
Data
CRC
Figure 14. Operation of Data Whitening, Manchester Encoding, and CRC
Data before Manchester
1
1
1
1
1
1
1
1
0
0
0
1
0
Preamble = 0xFF
First 4bits of the synch. word = 0x2
Data after Machester ( manppol = 1, enmaninv = 0)
Data after Machester ( manppol = 1, enmaninv = 1)
Data before Manchester
0
0
0
0
0
0
0
0
0
0
0
1
0
Preamble = 0x00
First 4bits of the synch. word = 0x2
Data after Machester ( manppol = 0, enmaninv = 0)
Data after Machester ( manppol = 0, enmaninv = 1)
Figure 15. Manchester Coding Example
6.5. Synchronization Word Configuration
The synchronization word length can be configured in Reg 33h, synclen[1:0]. The expected or transmitted sync
word can be configured from 1 to 4 bytes as defined below:
 synclen[1:0] = 00—Transmitted Synchronization Word (sync word) 3.
 synclen[1:0] = 01—Transmitted Synchronization Word 3 first, followed by sync word 2.
 synclen[1:0] = 10—Transmitted Synchronization Word 3 first, followed by sync word 2, followed by sync word 1.
 synclen[1:0] = 1—Transmitted Synchronization Word 3 first, followed by sync word 2, followed by sync word 1,
followed by sync word 0.
The sync is transmitted in the following sequence: sync 3sync 2sync 1sync 0. The sync word values can be
programmed in Registers 36h–39h.
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