Altera FFT MegaCore Function - Manual (Page 30)

When the FFT completes the transform of the input block, it asserts source_valid and outputs the
complex transform domain data block in natural order. The FFT function asserts
source_sop
to
indicate the first output sample.
Figure 3-5: FFT Streaming Data Flow Output Flow Control
clk
source_real
source_imag
exponent_out
source_ready
source_valid
source_sop
source_eop
Xr[0] Xr[1] Xr[2] Xr[3] Xr[5] Xr[6] Xr[7] Xr[8] Xr[10] Xr[11] Xr[12] Xr[9]
Xi[0] Xi[1] Xi[2] Xi[3] Xi[5] Xi[6] Xi[7] Xi[8] Xi[11] Xi[12]
EXP0
Xi[4] Xi[9]
Xr[4]
Xi[10]
After N data transfers, the FFT asserts
source_eop
to indicate the end of the output data block
Enabling the Streaming FFT
1. You must assert the
sink_valid
signal for the FFT to assert
source_valid
(and a valid data output).
2. To extract the final frames of data from the FFT, you need to provide several frames where the
sink_valid
signal is asserted and apply the
sink_sop
and
sink_eop
signals in accordance with the
Avalon-ST specification.
Variable Streaming
The variable streaming FFT allows continuous streaming of input data and produces a continuous stream
of output data similar to the streaming FFT. With the variable streaming FFT, the transform length
represents the maximum transform length. You can perform all transforms of length 2m where 6 < m <
log2(transform length) at runtime.
Changing Block Size
To change the size of the FFT on a block-by-block basis, change the value of the
fftpts
simultaneously
with the application of the
sink_sop
pulse (concurrent with the first input data sample of the block).
fftpts
uses a binary representation of the size of the transform, therefore for a block with maximum
transfer size of 1,024. Table 3-2 shows the value of the
fftpts
signal and the equivalent transform size.
Table 3-2: fftpts and Transform Size
fftpts Transform Size
10000000000 1,024
01000000000 512
00100000000 256
00010000000 128
UG-FFT
2014.12.15
Enabling the Streaming FFT 3-7
FFT IP Core Functional Description Altera Corporation
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