RFM22B/23B
3.5.2. Easy Frequency Programming for FHSS
While Registers 73h–77h may be used to program the carrier frequency of the RFM22B/23B, 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]( fhch[7 :0]10kHz)
For example, if the nominal frequency is set to 900 MHz using Registers 73h–77h, the channel step size is set to
1 MHz using "Register 7Ah. Frequency Hopping Step Size," and "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.
D7
D6
D5
D4
D3
D2
D1
D0
POR Def.
Add R/W
Function/Description
79 R/W Frequency Hopping Channel fhch[7] fhch[6] fhch[5] fhch[4] fhch[3] fhch[2] fhch[1] fhch[0]
Select
00h
7A R/W
Frequency Hopping Step
Size
fhs[7]
fhs[6]
fhs[5]
fhs[4]
fhs[3]
fhs[2]
fhs[1]
fhs[0]
00h
3.5.3. Automatic State Transition for Frequency Change
If registers 79h or 7Ah are changed in either TX or mode, the state machine will automatically transition the chip
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. The exception to this is during TX FIFO mode. If a frequency change is
initiated during a TX packet, then the part will complete the current TX packet and will only change the frequency
for subsequent packets.
3.5.4. Frequency Deviation
The peak frequency deviation is configurable from ±0.625 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]625Hz
f
fd[8 : 0]
f = peak deviation
625Hz
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