Altera FFT MegaCore Function - Manual (Page 31)

fftpts Transform Size
00001000000 64
Changing Direction
To change direction on a block-by-block basis:
1. Assert or deassert
inverse
(appropriately) simultaneously with the application of the
sink_sop
pulse
(concurrent with the first input data sample of the block).
When the FFT completes the transform of the input block, it asserts
source_valid
and outputs the
complex transform domain data block. The FFT function asserts the
source_sop
to indicate the first
output sample. The order of the output data depends on the output order that you select in IP
Toolbench. The output of the FFT may be in natural order or bit-reversed order. Figure 3-6 shows the
output flow control when the output order is bit-reversed. If the output order is natural order, data
flow control remains the same, but the order of samples at the output is in sequential order 1..N.
Figure 3-6: Output Flow Control—Bit Reversed Order
clock
source_sop
source_eop
source_valid
source_ready
source_real
source_imag
x0 x512 x256 x768 x128 x640 x384 x896
x0 x512 x256 x768 x128 x640 x384 x896
x1023
x1023
I/O Order
The input order allows you to select the order in which you feed the samples to the FFT.
Table 3-3: Input Order
Order Description
Natural
order
The FFT requires the order of the input samples to be sequential (1,
2 …, n – 1, n) where n is the size of the current transform.
Bit
reverse
order
The FFT requires the input samples to be in bit-reversed order.
Digit
Reverse
Order
The FFT requires the input samples to be in digit-reversed order.
–N/2 to
N/2
The FFT requires the input samples to be in the order –N/2 to (N/
2) – 1 (also known as DC-centered order)
Similarly the output order specifies the order in which the FFT generates the output. Whether you can
select Bit Reverse Order or Digit Reverse Order depends on your Data Representation (Fixed Point or
Floating Point). If you select Fixed Point, the FFT variation implements the radix-22 algorithm and the
3-8 Changing Direction
UG-FFT
2014.12.15
Altera Corporation FFT IP Core Functional Description
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