Altera FFT MegaCore Function Manual do Utilizador descarregar pdf (Página 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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Altera FFT MegaCore Function Manual do Utilizador Página 32