RFM22
6.5. 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 64 kbps. 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 22. Operation of Data Whitening, Manchester Encoding, and CRC
6.6. Preamble Detector
The RFM22 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].
When a false preamble detect occurs, the receiver will continuing searching for the preamble when no sync word is
detected.
The Preamble Detector output may be programmed onto one of the GPIOs or read in the Interrupt Status registers.
6.7. Preamble Length
The required preamble length threshold will depend on when the receive mode is entered in relation to the
transmitted packet. When the receiver is enabled long before the arrival of the packet, then a short preamble
detection threshold might result in false detects on the received noise before the actual preamble arrives. In this
case, it is recommended to program a 20 bit preamble detection threshold. A shorter Preamble Detection
Threshold might be chosen when occasional false detects are tolerable. When antenna diversity is enabled, it is
advised to use a 20 bit preamble detection threshold. When the receiver is synchronously enabled just before the
start of the packet, then a shorter preamble detection threshold might be chosen (e.g., 8 bit).
The required preamble length is determined from the sum of the receiver settling time and the preamble detection
threshold. The receiver settling time is listed in Table 15.
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