RFM22 Datasheet, PDF (97/150 Pages) –

RFM22
Register 1Fh. Clock Recovery Gearshift Override
Bit
D7
D6
D5
D4
D3
Name
Reserved
rxready
crfast[2:0]
Type
R/W
R/W
R/W
Reset value = 00000011
D2
D1
D0
crslow[2:0]
R/W
Bit
Name
7
Reserved Reserved.
Function
Improves Receiver Noise Immunity when in Direct Mode.
It is recommended to set this bit after preamble is detected. When in FIFO
6
rxready
mode this bit should be set to ―0‖ since noise immunity is controlled
automatically.
5:3
crfast[2:0] Clock Recovery Fast Gearshift Value.
2:0
crslow[2:0] Clock Recovery Slow Gearshift Value.
The oversampling rate can be calculated as rxosr = 500 kHz/(2^ndec_exp x RX_DR). The ndec_exp and the
dwn3_bypass values found at Address: 1Ch – IF Filter Bandwidth register together with the receive data rate (Rb)
are the parameters needed to calculate rxosr:
500×(1+2×dwn3_bypass)
rxosr = _{2ndec_exp-3 ×Rb×（1+enmanch）}
The Rb unit used in this equation is in kbps. The enmanch is the Manchester Coding parameter (see Reg. 70h,
enmach is 1 when Manchester coding is enabled, enmanch is 0 when disabled). The number found in the equation
should be rounded to an integer. The integer can be translated to a hexadecimal.
For optimal modem performance it is recommended to set the rxosr to at least 8. A higher rxosr can be obtained by
choosing a lower value for ndec_exp or enable dwn3_bypass. A correction in filset might be needed to correct the
channel select bandwidth to the desired value. Note that when ndec_exp or dwn3_bypass are changed the related
parameters (rxosr, ncoff and crgain) need to be updated.
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