RFM95 datasheet(48/121 Pages) HOPERF

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Page 48

RFM95/96/97/98(W)

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PreambleDetectorSize

# of Bytes

2 (recommended)

reserved

WIRELESS & SENSING

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, FRF. 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:

Clear the former AFC correction value, if AfcAutoClearOn = 1. Allowing the next frequency correction to be performed

from the initial centre frequency.

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

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.

RFM95 Datasheet(PDF) 48 Page - Shenzhen Hope Microelectronics Co., Ltd.