RFM42 Datasheet, PDF (20/110 Pages) –

RFM42/43
3.6.2. Easy Frequency Programming for FHSS
While Registers 73h–77h may be used to program the carrier frequency of the RFM42/43, it is often easier to think in
terms of ―channels‖ or ―channel numbers‖ rather than an absolute frequency value in Hz. Also, there may be some
timing-critical applications (such as for Frequency Hopping Systems) in which it is desirable to change
frequency by programming a single register. Once the channel step size is set, the frequency may be changed by a
single register corresponding to the channel number. A nominal frequency is first set using Registers 73h–77h, as
described above. Registers 79h and 7Ah are then used to set a channel step size and channel number, relative to the
nominal setting. The Frequency Hopping Step Size (fhs[7:0]) is set in increments of 10 kHz with a maximum channel
step size of 2.56 MHz. The Frequency Hopping Channel Select Register then selects channels based on multiples of
the step size.
Fcarrier= Fnom + fhs[7 : 0] X ( fhch[7 : 0] X 10kHz)
For example: if the nominal frequency is set to 900 MHz using Registers 73h–77h and the channel step size is set
to 1 MHz using "Register 7Ah. Frequency Hopping Step Size". For example, if the "Register 79h. Frequency
Hopping Channel Select" is set to 5d, the resulting carrier frequency would be 905 MHz. Once the nominal
frequency and channel step size are programmed in the registers, it is only necessary to program the fhch[7:0]
register in order to change the frequency.
Function/Descript
POR
Add R/W
D7
D6
D5
D4
D3
D2
D1
D0
ion
Def.
Frequency Hopping
79 R/W
fhch[7] fhch[6] fhch[5] fhch[4] fhch[3] fhch [2] fhch [1] fhch [0] 00h
Channel Select
Frequency Hopping
7A R/W
fhs[7] fhs[6] fhs[5] fhs[4] fhs[3] fhs[2] fhs[1] fhs[0] 00h
Step Size
3.6.3. Automatic Frequency Change
If registers 79h or 7Ah are changed in TX mode, the state machine will automatically transition the module back to
tune and change the frequency. This feature is useful to reduce the number of SPI commands required in a Frequency
Hopping System. This in turn reduces microcontroller activity, reducing current consumption.
3.6.4. Frequency Deviation
The peak frequency deviation is configurable from ±1 to ±320 kHz. The Frequency Deviation (Δf) is controlled by the
Frequency Deviation Register (fd), address 71 and 72h, and is independent of the carrier frequency setting. When
enabled, regardless of the setting of the hbsel bit (high band or low band), the resolution of the frequency deviation will
remain in increments of 625 Hz. When using frequency modulation the carrier frequency will deviatefrom the nominal
center channel carrier frequency by ±Δf:
△ f = fd [8: 0] X 625Hz
fd [8: 0] =
△f
625Hz
△f
=
peak deviation
20
Tel: [phone] Fax: [phone] E-mail: [e-mail] http://www.hoperf.com