Rf22 – Rainbow Electronics RF22 User Manual
Page 66
RF22
Version: 0.1 Date: 12/23/2008
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crfast
crgain
n
BCRLoopGai
2
=
Once the preamble is detected, internal state machine automatically shift BCR loop gain to the following:
crslow
crgain
n
BCRLoopGai
2
=
crfast = 3’b000 and crslow = 3’b101 are recommended for most applications. The value of “crslow” should be greater
than “crfast”.
Register 1Fh. Clock Recovery Gearshift Override
Bit
D7
D6
D5
D4
D3
D2
D1
D0
Name
Reserved
rxready
crfast[2:0]
crslow[2:0]
Type
R/W
R/W
R/W
R/W
Reset value = 00010011
Bit
Name
Function
7
Reserved
Reserved.
6
rxready
Improves Receiver Noise Immunity when in Direct Mode.
It is recommended to set this bit after preamble is detected. When in FIFO 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:
(
)
(
)
enmanch
Rb
bypass
dwn
rxosr
ndec
+
Ч
Ч
Ч
+
Ч
=
−
1
2
_
3
2
1
500
3
exp
_
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.
Register 20h. Clock Recovery Oversampling Rate
Bit
D7
D6
D5
D4
D3
D2
D1
D0
Name
rxosr[7:0]
Type R/W
Reset value = 01100100
Bit
Name
Function
7:0
rxosr[7:0]
Oversampling Rate.
3 LSBs are the fraction, default = 0110 0100 = 12.5 clock cycles per data bit
The offset can be calculated as follows:
(
)
(
)
bypass
dwn
enmanch
Rb
ncoff
ndec
_
3
2
1
500
2
1
exp
_
20
Ч
+
Ч
Ч
+
Ч
=
+
The default values for register 20h to 23h gives 40 kbps RX_DR with Manchester coding is disenabled.