PDSP16256/A
Dual Indipendant Filter Options
length is selected using control register bits 14 and 13 as
summarised in Table 4, which also shows the resulting
latency. As in single filter mode normal or decimate-by-
two operation can be selected using control register bit
12.
When operating as two independent filters the device
accepts 16 bit data on both the DA and DB buses at the
selected sample rate, see Fig. 8. Results are available
from both the F and X buses. The F bus may be tristated
using the OEN input. Signal OEN is registered onto the
device and does not therefore take effect until the first
SCLK rising edge
Dual Cascaded Filter Options
When operating as two cascaded filters the device ac-
cepts 16 bit data on the DA bus at the selected sample
rate. Results are presented on the 32-bit X bus, see Fig.
9. Each filter must be configured in the same manner.
Multiple device expansion is not possible in this mode.
Each filter must be configured in the same manner, and
multiple device expansion is not possible due to the pin
re-organization. The latter requirement can, of course,
still be satisfied by several devices configured as single
filters.
Dual cascaded filter mode is selected by setting control
register bit 15 to a zero and bit 4 to a one. The required
filter length is selected using control register bits 14 and
13 as summarised in Table 4, which also shows the
resulting latency. The decimate-by-two option is not
available in this mode.
The data for the second filter network is extracted as the
middle16bitsfromthefirstnetworksaccumulatedresult.
Forsuccessfuloperationthefirstfilternetworkmusthave
unity gain. See the section on filter accuracy for more
details.
Dual independent filter mode is selected by setting
control register bits 15 and 4 to a zero. The required filter
CR
Input
Output
Rate
Filter
Length
Setup
Latency
1413 12 Rate
Ind Cas
0 0 0 SCLK
0 0 1 SCLK
0 1 0 SCLK/2
0 1 1 SCLK/2
1 0 0 SCLK/4
1 0 1 SCLK/4
1 1 0 SCLK/8
SCLK
8 Taps
16 Taps
16 Taps
32 Taps
32 Taps
64 Taps
64 Taps
16
17
16
18
20
24
24
27
-
28
-
SCLK/2
SCLK/2
SCLK/4
SCLK/4
SCLK/8
SCLK/8
36
The cascade option is used to increase the stop band
rejection in a practical filter application. Theoretically,
increasingthenumberoftapsinanFIRfilterwillincrease
the stop band rejection, but this assumes floating point
calculationswithnoaccuracylimitations.Inpractice,with
fixed point arithmetic, better performance is achieved
with two smaller filters in series.
-
40
Table 4. Dual Filter options
DA15:0
F31:0
OEN
DA15:0
F31:0
OEN
NETWORK
A
NETWORK
A
DUAL
MODE
DUAL
MODE
MUX
MUX
NETWORK
B
NETWORK
B
SINGLE
MODE
SINGLE
MODE
DB15:0
X31:0
DB15:0
X31:0
Figure. 8 Dual independent filter bus utilisation
Figure. 9 Dual cascaded filter bus utilisation
9
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