Cirrus Logic CS5378 - Manual (Page 38)

CS5378
DS639F3 38
11. DIGITAL FILTER INITIALIZATION
The CS5378 digital filter consists of three multi-
stage sections: a three stage SINC filter, a two stage
FIR filter, and a two stage IIR filter.
To initialize the digital filter, FIR and IIR coeffi-
cient sets are selected using configuration com-
mands and the FILTCFG register (0x20) is written
to select the output filter stage, the output word
rate, and the number of enabled channels. The dig-
ital filter clock rate is then selected by writing the
CONFIG register (0x00).
11.1 Filter Coefficient Selection
Selection of SINC filter coefficients is not required
as they are selected automatically based on the pro-
grammed output word rate.
Digital filter FIR and IIR coefficients are selected
using the ‘Write FIR Coefficients’ and ‘Write IIR
Coefficients’, or the ‘Write ROM Coefficients’
configuration commands. When writing the FIR
and IIR coefficients from ROM, a data word selects
an on-chip coefficient set for each filter stage. Fig-
ure 22 shows the format of the coefficient selection
word, and the available coefficient sets for each se-
lection.
Characteristics of the on-chip digital filter coeffi-
cients are discussed in the ‘SINC Filter’, ‘FIR Fil-
ter’, and ‘IIR Filter’ sections of this data sheet.
11.2 Filter Configuration Options
Digital filter parameters are selected by bits in the
FILTCFG register (0x20), and the digital filter
clock rate is selected by bits in the CONFIG regis-
ter (0x00).
11.2.1 Output Filter Stage
The digital filter can output data following any
stage in the filter chain. The output filter stage is
selected by the FSEL bits in the FILTCFG register.
Taking data from the SINC or FIR1 filter stages re-
duces the overall decimation of the filter chain and
increases the output rate, as discussed in the next
section. Taking data from FIR2, IIR1, IIR2, or IIR3
results in data at the selected rate.
Figure 21. Digital Filter Stages
SINC Filter
2 - 64000
FIR1
4
FIR2
2
IIR1 IIR2
1st Order 2nd Order
Output to High Speed Serial Data Interface
DC Offset
Correction
Output Rate 4000 SPS ~ 1 SPS
& Gain
Modulator
512 kHz
Input