PDSP16510A Datasheet, PDF (19/23 Pages) Mitel Networks Corporation – Stand Alone FFT Processor

PDSP16510A
USER NOTES - STOPPING DOS
(1) GENERAL DESCRIPTION
The transform is calculated internally fully synchronous to
SCLK. However, as all outputs are referenced to DOS, a
transfer has to be made between the two clocks. In addition,
some dummy DOS strobes are needed to operate the internal
control logic, and to advance data from the internal RAMs to
the output pins.
The most simple configuration for the device is to have
DOS running continuously and for DEN to be permanently
active. When this happens the user will just be aware of data
appearing on the output pins on the same DOS cycle when
DAV goes active. However, there are many situations where
either DOS is not continuously running, or DEN is not
permanently active. To help explain how to operate the device
in these situations, the internal operation of the output circuits
must be described. For those who are not going to be
interrupting DOS, the remainder of this section can be
ignored.
(2) INTERNAL RAM - GENERAL DESCRIPTION.
For single device operation of transforms less than 1024
points, the internal RAM is shared between three separate
operations which enable the device to output old transformed
results, calculate the current transform, and input new data
ready for the next transform. All these operations, along with
the internal control logic, are controlled by a 12-cycle state
machine. The RAM operations are:
(a) 2 cycles in every 12 are dedicated to reading new
information in the input buffer and writing it to the RAM.
(b) 2 cycles in every 12 are dedicated to reading the
contents of the RAM and advancing that data to the
output buffer.
(c) 8 cycles in every 12 are dedicated to the read and write
operations of the transform currently being calculated.
(3) SEQUENCE OF EVENTS
The sequence of events relating to the output control and
data flow is as follows :
(3.1) An SCLK rising edge :
(a) An internal flag is raised to indicate that the
transform has finished and data is available to be
dumped. Data will be present in the internal RAM,
and the output address generator will be at the
correct address. Access to the RAM at this
moment, however, has not been made.
(b) If at this moment the device is programmed to be a
single device, and DEN is inactive, then DAV will be
made active - ie without the presence of DOS. If
DEN is active at this point, or the device is
programmed in any multiple device mode, then
DAV will remain inactive.
(3.2) Accessing the RAM at this point
At this moment, when DAV has been made active
before data appears on the output pins, data is not yet
in the output buffer. Internally the precise SCLK cycle
at which the RAMs are read and written to the output
buffers now has to be waited for. This cycle, as
described above occurrs 2 in every 12 SCLK cycles, so
at worst case 6 SCLK cycles have to elapse until data
is guaranteed to be in the output buffer.
If the DOS rate is similar to the SCLK rate, and the user
has been immediately applying DOS pulses (on
seeing DAV go active) hoping to get data off the chip,
then this will not actually happen.
The next internal flag raised is the one which indicates
that the output data has been successfully read from
the RAMs and is now in the output buffer.
(3.3) The next DOS rising edge (regardless of DEN status)
The flag indicating that the RAMs have been read is
transferred to circuitry operating on DOS. The output
enable signal, DEN, does not have to be present at this
point.
(3.4) The next DEN-Enabled DOS rising edge (ie the 1st one
of this sequence)
The output state machine receives it's first edge.
(3.5) The next DEN-Enabled DSO rising edge (ie the 2nd)
Internal output address generators start to count
(ready for fetching the next set of output data).
(3.6) The next DEN-Enabled DOS rising edge (ie the 3rd)
An enable signal is raised for the final data latch in the
output buffer.
(3.7) The next DEN-Enabled DOS rising edge (ie the 4th)
(a) The final data in the output buffer latch clocks-
through new data and presents it to the output
pads.
(b) The output pads come output of high impedance
.
(c) If DAV was previously inactive, it is now made
active.
19