PDSP16515A
generated internally, and applied to the incoming real or
complex data with no time penalty. No external ROM is
needed to support these windows. The Blackman-Harris
window gives improved dynamic range over the Hamming
window when two closely spaced frequencies are to be
detected, and one is of smaller magnitude than the other. It
does, however, reduce the actual frequency resolution, and
the Hamming window may then be preferable.
Data in and out of the device is represented by 16 bit real and
imaginary components, with 16 bit sine and cosine values
contained in an internal ROM. Conditional scaling, coupled
with word growth through the butterfly data path, gives
increased dynamic range. Transforms can be computed with
sample sizes of either 256 or 1024 data points. The 256 point
option can alternatively be used to simultaneously execute
either four 64 point transforms, or sixteen 16 point transforms.
The 16 point mode can only be used with a rectangular
window, and no overlapping of data blocks is possible.
The device can be configured, either, to perform continuous
transforms in a real time application, or as slave processor to
a more general purpose signal processing system. In the
continuous mode, with transform sizes of 256 points or less,
it contains three internal control units which simultaneously
allow new data to be loaded, present data to be transformed,
and previous results to be dumped. Additional, external, input/
output buffering is not needed. The internal input buffer also
allows data blocks to be overlapped by either 50% or 75%,
apart from the mode with no overlaps.
When 1024 point transforms are to be calculated, without loss
of incoming data during the transform time, it is necessary to
use an input buffer. This requirementcan be satisfied by an
external buffer memory.
In any of the real or complex modes it is possible to obtain
higher performance by connecting devices in parallel. It is then
possible to increase the sampling rate to that of the system
clock used for internal operations.
The mode of operation of the device is controlled by 16 bits in
a control register. These are loaded through the AUX15:0 port
when a control signal DEF is active low. This port is also used
to provide the imaginary component of complex input data,
and, if complex transforms are to be performed, an external
tristate buffer will be needed to isolate the control information.
This should only be enabled when DEF is active. DEF is also
used to initialise the internal circuitry, and can be a simple
power on reset if control parameters need not be
subsequently changed.
INPUT
SELECT
RAM
SIN / COS
ROM
Shift left until largest point
has one sign bit.
16
18
MULTIPLIER
S
S
29 - 14 13 - 0
"1"
18
18
FIRST ADDER
19Bit Result
18 - 1
0
REGISTER FILE
SECOND ADDER
19Bit Result
18 - 1
0
REGISTER FILE
THIRD ADDER
19Bit Result
18 - 1
17 - 0
CR
BIT3
SELECT
Figure. 3 One of Four Data Paths
of arithmetic precision, and has a detrimental effect on the
dynamic range achievable.
In practice these large word growths only occur when bipolar
complex square waves are transformed, and even then will
not occur on every pass. The PDSP16515A compromises by
allowing a 2 bit word growth during the butterfly calculation in
the first pass. This is equivalent to ignoring the most significant
bit of the 19 bit final result, which is assumed to be an extra sign
bit, and then selecting the next 18 bits for storage. In
Data Precision
During each pass of a radix-4 fast Fourier transform it is
possible for either component of a particular result to grow by
a factor of up to four in the first pass, and 5.242 in subsequent
passes. This is between two and three bits in each pass and
the data path must allow for this word growth to avoid any
possibility of overflow. At the end of the data path the word is
preserved at 18 bits and stored in the internal RAM. Any un-
necessary word growth to prevent overflow thus results in loss
5
MITEL [ MITEL NETWORKS CORPORATION ]
