VP16256
CASCADING DEVICES
When the filter requirements are beyond the capabilities
of a single device, it is possible to connect several devices in
cascade increasing the number of taps available at the required
sample rate. Within each device all filter length, decimate, and
bank swap options are still possible, but each device in the
chain must be similarly programmed and configured as a single
filter.
The number of devices which can be cascaded is only
limited by the possibility of overflow in the 32-bit intermediate
accumulations. If more than sixteen devices are cascaded in
auto EPROM load mode, then an additional EPROM will be
needed.
In modes where the data sample rate does not equal the
clock rate. Then the cascade arrangement shown in Fig. 11 is
used. Delayed data is passed from device to device in one
direction, while intermediate results flow in the opposite
direction. The interface device both accepts the input data and
produces the final result. It is not necessary for each device to
know its exact position in the chain, but the device which
receives the input data and produces the final result must be
identified, as must the device which terminates the chain. The
former is known as the Interface device and the latter as the
Termination device, all others are Intermediate devices.
Control Register bits CR11:10 are used to define these
positions as shown in Table 6.
The control logic in each of the devices must be
synchronised with respect to the Interface device. This is
achieved by connecting the Delayed Filter Enable output
(DFEN) to the Filter Enable input (FEN) of the next device in the
chain. The Interface device, itself, needs a FEN signal
produced by the system, unless in EPROM mode, where FRUN
may be pulled high. Even when the latter is true, the FEN
connection must be made between the remaining devices in
the chain. By effectively extending the filter length, the cascade
latency is therefore the same as for the single device in the
same mode. Once the pipeline is initially flushed the latency is
as given in Table 3.
When devices are cascaded such that the data sample rate
equals the clock rate, (Control register bits 14:13 = 00), then a
different cascade configuration must be used. This is shown in
Fig. 12. The number of devices that can be cascaded is, again,
only limited by the 32-bit accumulators.
In this mode the delayed data is passed from device to
device in the same direction as the intermediate results. The
device which accepts the input data is now at the opposite end
of the chain to the device which produces the final result. The
control logic in each of the devices must be synchronised this
is achieved by connecting all the device FEN inputs to the
global FEN. The cascade latency for the complete filter is built
up from the 12 delays from the termination device, 8 delays
from the interface device and additional intermediate devices
each adding 4 delays.
AVAILABLE OPTIONS
No more than 128 coefficients can be stored internally. This
limits the filter length / decimate / bank swap options to those
which do not require more than that number of coefficients.
Thus when a filter with 128 taps is to be implemented in a single
device, it is not possible to decimate or bank swap. When a filter
with 64 taps is implemented, decimate or bank swap are
possible, but not both. With all other filter lengths, all decimate
and bank swap configurations are possible.
DATA IN
FEN
RESULTS
OUT
RESULTS
OUT
FEN
DA15:0
FEN
F31:0
DB15:0
FEN
F31:0
INTERFACE
DEVICE
DB15:0
DFEN
X31:0
INTERFACE
DEVICE
DA15:0
DFEN
X31:0
DA15:0
FEN
F31:0
DB15:0
FEN
F31:0
INTERMEDIATE
DEVICE
DB15:0
DFEN
X31:0
INTERMEDIATE
DEVICE
DA15:0
DFEN
X31:0
DA15:0
FEN
F31:0
DB15:0
FEN
F31:0
TERMINATION
DEVICE
DB15:0
DFEN
X31:0
TERMINATION
DEVICE
DA15:0
DFEN
X31:0
DATA IN
Fig. 11 Three-device cascaded system
Fig. 12 Full speed cascaded system
9
MITEL [ MITEL NETWORKS CORPORATION ]
