RF31
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 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 16. Operation of Data Whitening, Manchester Encoding, and CRC
6.5. Preamble Detector
The RF31 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.
6.6. 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 14.
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