6
FN2486.10
October 10, 2008
DDF Control Registers
F_Register (A1 = 0, A0 = 0)
FIGURE 4.
F_OAD
F_BYP
F_ESYM
F_DRATE
F_TAPS
FA0
FB0
ES0
D3
D2
D1
D0
T8T7
T6
T5
T4
T3T2T1T0
F_TAPS
Bits T0-T8 are used to specify the number of FIR filter taps. The number
entered is one less than the number of taps required. For example, to
specify a 511 tap filter F_TAPS would be programmed to 510. The
minimum number of FIR taps = 3 (F_TAPS = 2).
F_DRATE
Bits D0-D3 are used to specify the amount of FIR decimation. The
number entered is one less than the decimation required. For example,
to specify decimation of 16, F_DRATE would be programmed to 15. For
no FIR decimation, F_DRATE would be set equal to 0. FDRATE +1 is
defined as FDEC.
F_ESYM
Bit ES0 is used to select the FIR symmetry. F_ESYM is set equal to one
to select even symmetry and set equal to zero to select odd symmetry.
When F_ESYM is one, data is added in the pre-adder; when it is zero,
data is subtracted. Normally set to one.
F_BYP
FB0 is used to select FIR bypass mode. FIR bypass mode is selected by
setting F_BYP = 1. When FIR bypass mode is selected, the FIR is
internally set up for a 3 tap even symmetric filter, no decimation
(F_DRATE = 0) and F_OAD is set equal to one to zero one side of the
preadder. In FIR bypass mode all FIR filter parameters, except F_CLA,
are ignored, including the contents of the FIR coefficient RAM. In FIR
bypass mode the output data is brought output on the lower 16 bits of the
output bus DATA_OUT 0-15. To disable FIR bypass mode, F_BYP is set
equal to zero. When F_BYP is returned to zero, the coefficients must be
reloaded.
F_OAD
Bit FA0 is used to select the zero the preadder mode. This mode zeros
one of the inputs to the pre-adder. Zero preadder mode is selected by
setting F_OAD equal to one. This feature is useful when implementing
arbitrary phase filters or can be used to verify the filter coefficients. To
disable the Zero Preadder mode F_OAD is set equal to zero.
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
HSP43220