PDSP16515A Datasheet, PDF (10/25 Pages) Mitel Networks Corporation – Stand Alone FFT Processor

PDSP16515A
When continuously transforming data such that new outputs
are internally available before the previous block has been
completely dumped, then DAV would normally stay active and
give no indication that one block dump had been finished and
another block started. Additional internal circuitry is, however,
provided to ensure that DAV goes inactive for one DOS high
time, thus supplying an inter block marker.
Asynchronous Dav Mode
If DEN is not active in a single device when the transform is
complete, then the device will wait for DEN to go active before
any data is dumped. This mode is suitable for applications in
which output processing is under the control of a remote host,
such as a general purpose digital signal processor. The DAV
output will then go active as soon as the output buffer is full,
and will not be synchronised to the DOS edge. In such
systems the DOS strobe may not necessarily be present at
this time. Table 3 gives the relevant timing information.
In this host controlled dump mode the PDSP16515A waits for
the host to activate the DEN input after DAV has gone active.
DEN then functions as an enable for the host produced data
strobes on the DOS pin. DEN may either stay active for the
complete transfer, or may be used to enable each DOS input.
When DEN and DOS are both active an internal read
operation occurs, and an address generator is incremented.
DAV goes in-active in response to the DOS edge needed to
read the last output, unless Bit 15 in the Control Register is set.
In this case DAV goes in-active when the next INEN edge is
received for reasons given later.
In host controlled systems the time to dump data could be
longer than the transform time. The dump time in such a
system will dictate the maximum sampling rate that can be
used without the loss of incoming data. In the 1024 point
mode, when the loss of data is not important, the
PDSP16515A is designed to not accept new data until the
previous results have been dumped. Such a system needs no
input buffer, and INEN can be permanently tied low if the edge
activated mode is not in use. If the loss of data is to be avoided
an input buffer is needed and the host must have received all
the results before a new block of data has been loaded into the
buffer.
For 256 point transforms, with host controlled dumping, it is
still possible to overlap load and dump operations. The
maximum dump times, however, must be less than the load
times to avoid data corruption. Previously converted outputs
will be actually corrupted, rather than inputs simply not being
used.
If the loss of incoming data is not important, the device can be
forced to do separate load, transform, and then dump
operations. The corruption of results will then never occur, no
matter what dump time is taken. This can be achieved by
ensuring that INEN is not active between loading a block of
data and completing the dump of the results from that data.
The same ends can be achieved if the INEN edge activated
mode ( Bit 12 reset ) is used, and the inverted DAV edge is
used to drive the INEN input. This then initializes a new load
operation only when the previous dump has been completed.
Results are transferred from the device with the rising edge of
the DOS strobe when DEN is active. This is consistent with
using the device in a data flow architecture, as is commonly
employed in data processing systems. In a typical
microprocessor based system, however, data is normally
expected to become valid before the end of the data strobe
produced by the processor. It is thus necessary for the user
to provide a ‘dummy’ data strobe in order to transfer data to
the outputs which can then be read by the host during the next
data strobe. In addition further ' dummy ' strobes are needed
each time DAV goes active in order to prime the output
circuitry. The actual output sequence is given in Table 3 for a
single device systemand is described more fully in "user notes
- stopping DOS".
General Dump Considerations
The tri-state drivers on the output buses are only enabled
when both DAV and DEN are active. When DEN is tied
permanently low the output bus will start to become valid from
the DOS edge which also generates the DAV output. The next
DOS edge can then be used to transfer the first output to the
next device. When DEN is driven low in response to the DAV
output, the outputs start to become valid when DEN goes low.
The Scale Tag outputs become valid at the same time as data,
and when enabled will continue to indicate the correct value
until all frequency bins have been dumped. If at any time
during the dump operation DEN goes in- active, then both the
data and scale tag outputs will go high impedance after the
delay shown in Table 3.
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