Altera FFT MegaCore Function - Manual (Page 32)

reverse I/O order option is Bit Reverse Order. If you select Floating Point, the FFT variation implements
the mixed radix-4/2 algorithm and the reverse I/O order option is Digit Reverse Order.
For sample digit-reversed order, if n is a power of four, the order is radix-4 digit-reversed order, in which
two-bit digits in the sample number are units in the reverse ordering. For example, if n = 16, sample
number 4 becomes the second sample in the sample stream (by reversal of the digits in 0001, the location
in the sample stream, to 0100). However, in mixed radix-4/2 algorithm, n need not be a power of four. If n
is not a power of four, the two-bit digits are grouped from the least significant bit, and the most significant
bit becomes the least significant bit in the digit-reversed order. For example, if n = 32, the sample number
18 (10010) in the natural ordering becomes sample number 17 (10001) in the digit-reversed order.
Enabling the Variable Streaming FFT
1. Assert
sink_valid
.
2. Transfer valid data to the FFT. The FFT processes data.
Example 3-1: FFT Behavior When sink_valid is Deasserted
Clock
Frame 1 Frame 2
Input Data
The input data stops,
but the output continues
Output Data
sink_valid
source_valid
When the FFT is stopped within
a frame, the output pauses
1. Deassert
sink_valid
during a frame to stall the FFT, which then processes no data until you
assert
sink_valid
. Any previous frames that are still in the FFT also stall.
2. If you deassert
sink_valid
between frames, the FFT processes and transfers the data currently
in the FFT to the output.
3. Disable the FFT by deasserting the
clk_en
signal.
Dynamically Changing the FFT Size
The FFT stalls the incoming data (deasserts the
sink_ready
signal) until all the FFT processes and
transfers all of the previous FFT frames of the previous FFT size to the output.
UG-FFT
2014.12.15
Enabling the Variable Streaming FFT 3-9
FFT IP Core Functional Description Altera Corporation
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