RFM95 Datasheet, PDF (28/121 Pages) List of Unclassifed Manufacturers – Low Power Long Range Transceiver Module

WIRELESS & SENSING
RFM95/96/97/98(W)
PRELIMINARY
DATASHEET
Principle of Operation
The principle behind the FHSS scheme is that a portion of each LoRa^TM packet is transmitted on each hopping channel
from a look up table of frequencies managed by the host microcontroller. After a predetermined hopping period the
transmitter and receiver change to the next channel in a predefined list of hopping frequencies to continue transmission
and reception of the next portion of the packet. The time which the transmission will dwell in any given channel is
determined by HoppingPeriod which is an integer multiple of symbol periods:
HoppingPeriod = Ts × FreqHoppingPeriod
The frequency hopping transmission and reception process starts at channel 0. The preamble and header are transmitted
first on channel 0. At the beginning of each transmission the interrupt the channel counter FhssPresentChannel is
incremented and the interrupt signal FhssChangeChannel is generated. The new frequency must then be programmed
within the hopping period to ensure it is taken into account for the next hop, the interrupt FhssChangeChannel is then to be
cleared by writing a logical ‘1’.
FHSS Reception always starts on channel 0. The receiver waits for a valid preamble detection before starting the
frequency hopping process as described above. Note that in the eventuality of header CRC corruption, the receiver will
automatically request channel 0 and recommence the valid preamble detection process.
Timing of Channel Updates
The interrupt requesting the channel change, FhssChannelChange, is generated upon transition to the new frequency. The
frequency hopping process is recapitulated in the diagram below:
Figure 6. Interrupts generated in the case of successful frequency hopping communication.
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