RFM31B Datasheet, PDF (32/63 Pages) List of Unclassifed Manufacturers – ISM RECEIVER

RFM31B
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 16. 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 15 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 16 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 15. 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 16. Manchester Coding Example
6.5. Preamble Detector
The RFM31B has integrated automatic preamble detection. The preamble length is configurable from 1–256 bytes
using the prealen[7:0] field in "Register 33h. Header Control 2" and "Register 34h. Preamble Length," as described
in “6.2. Packet Configuration.” The preamble detection threshold, preath[4:0] as set in "Register 35h. Preamble
Detection Control 1", is in units of 4 bits. The preamble detector searches for a preamble pattern with a length of
preath[4:0].
If a false preamble detect occurs, the receiver will continuing searching for the preamble when no sync word is
detected. When a false preamble detect occurs, the receiver will continuing searching for the preamble when no
sync word is detected. Once preamble is detected (false or real) then the part will then start searching for sync. If
no sync occurs then a timeout will occur and the device will initiate search for preamble again. The timeout period
is defined as the sync word length plus four bits and will start after a non-preamble pattern is recognized after a
valid preamble detection. The preamble detector output may be programmed onto one of the GPIO or read in the
interrupt status registers.
Tel: [phone] Fax: [phone] E-mail: [e-mail] http://www.hoperf.com
32