RFM23
3.6.2. Easy Frequency Programming for FHSS
While Registers 73h–77h may be used to program the carrier frequency of the RFM23, 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.
POR
Def.
Function/Descript
ion
D7
D6
D5
D4
D3
D2
D1
D0
Add R/W
Frequency Hopping
Channel Select
Frequency Hopping
Step Size
R/W
00h
00h
fhch[7] fhch[6] fhch[5] fhch[4] fhch[3] fhch [2] fhch [1] fhch [0]
79
R/W
fhs[7]
fhs[6]
fhs[5]
fhs[4]
fhs[3]
fhs[2]
fhs[1]
fhs[0]
7A
3.6.3. Automatic Frequency Change
If registers 79h or 7Ah are changed in either TX or mode, the state machine will automatically transition the module
back to tune, change the frequency, and automatically go back to either TX or RX. 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 deviate
from the nominal center channel carrier frequency by ±Δf:
△ f = fd [8: 0] X 625Hz
△f
fd [8: 0] =
△f = peak deviation
625Hz
24
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