RFM95 Datasheet, PDF (48/121 Pages) List of Unclassifed Manufacturers – Low Power Long Range Transceiver Module

WIRELESS & SENSING
RFM95/96/97/98(W)
PRELIMINARY
DATASHEET
4.2.3.5. AFC
The AFC is based on the FEI measurement, therefore the same input signal and receiver setting conditions apply. When
the AFC procedure is performed the AfcValue is directly subtracted from the register that defines the frequency of
operation of the chip, F_RF. The AFC is executed each time the receiver is enabled, if AfcAutoOn = 1.
When the AFC is enabled (AfcAutoOn = 1), the user has the option to:
Š Clear the former AFC correction value, if AfcAutoClearOn = 1. Allowing the next frequency correction to be performed
from the initial centre frequency.
Š Start the AFC evaluation from the previously corrected frequency. This may be useful in systems in which the centre
frequency experiences cumulative drift - such as the ageing of a crystal reference.
The RFM95/96/97/98(W) offers an alternate receiver bandwidth setting during the AFC phase allowing the
accommodation of larger frequency errors. The setting RegAfcBw sets the receive bandwidth during the AFC process. In
a typical receiver application the, once the AFC is performed, the radio will revert to the receiver communication or
channel bandwidth (RegRxBw) for the ensuing communication phase.
Note that the FEI measurement is valid only during the reception of preamble. The provision of the PreambleDetect flag
can hence be used to detect this condition and allow a reliable AFC or FEI operation to be triggered. This process can be
performed automatically by using the appropriate options in StartDemodOnPreamble found in the RegRxConfig register.
A detailed description of the receiver setup to enable the AFC is provided in section 4.2.6.
4.2.3.6. Preamble Detector
The Preamble Detector indicates the reception of a carrier modulated with a 0101...sequence. It is insensitive to the
frequency offset, as long as the receiver bandwidth is large enough. The size of detection can be programmed from 1 to 3
bytes with PreambleDetectorSize in RegPreambleDetect as defined in the next table.
Table 65 Preamble Detector Settings
PreambleDetectorSize
00
01
10
11
# of Bytes
1
2 (recommended)
3
reserved
For normal operation, PreambleDetectTol should be set to be set to 10 (0x0A), with a qualifying preamble size of 2 bytes.
The PreambleDetect interrupt (either in RegIrqFlags1 or mapped to a specific DIO) then goes high every time a valid
preamble is detected, assuming PreambleDetectorOn=1.
The preamble detector can also be used as a gate to ensure that AFC and AGC are performed on valid preamble. See
section 4.2.6. for details.
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