PDSP16256
DUAL INDEPENDENT FILTER OPTIONS
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
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 independent filter mode is selected by setting control
register bits 15 and 4 to a zero. The required filter 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 se-
lected using control register bit 12.
CR
14 13 12
0
0
0
0
1
1
1
0
0
1
1
0
0
1
0
1
0
1
0
1
0
Input
Rate
SCLK
SCLK
SCLK/2
SCLK/2
SCLK/4
SCLK/4
SCLK/8
Output
Rate
SCLK
SCLK/2
SCLK/2
SCLK/4
SCLK/4
SCLK/8
SCLK/8
Filter
Length
8 Taps
16 Taps
16 Taps
32 Taps
32 Taps
64 Taps
64 Taps
Setup
Latency
Ind
16
17
16
18
20
24
24
Cas
27
-
28
-
36
-
40
DUAL CASCADED FILTER OPTIONS
When operating as two cascaded filters the device accepts
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.
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
middle 16 bits from the first networks accumulated result. For
successful operation the first filter network must have unity
gain. See the section on filter accuracy for more details.
The cascade option is used to increase the stop band
rejection in a practical filter application. Theoretically, increas-
ing the number of taps in an FIR filter will increase the stop band
rejection, but this assumes floating point calculations with no
accuracy limitations. In practice, with fixed point arithmetic,
better performance is achieved with two smaller filters in series.
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
Fig. 8 Dual independent filter bus utilisation
Fig. 9 Dual cascaded filter bus utilisation
8
MITEL [ MITEL NETWORKS CORPORATION ]
