Fixed Point 1-D SSR FFT Usage - 2023.2 English

Vitis 1-D SSR FFT L1 module can be used in a C++ HLS design by: 1- cloning Vitis DSP Library git repository and add the following path to compiler include path:

REPO_PATH/dsp/L1/include/hw/vitis_fft/fixed/

2- Include vt_fft.hpp

3- Use namespace xf::dsp::fft

4- Define fft parameter structure say call it params_fix by extending ssr_fft_default_params like Defining 1-D SSR FFT Parameter Structure

5- call fft<params_fix>(input_stream,output_stream)

The following section gives usage examples and explains some other interface level details for use in C++ based HLS design. To use the 1-D SSR FFT L1 module:

1. Include the vt_fft.hpp header:

#include "vt_fft.hpp"

2. Use namespace xf::dsp::fft

using namespace xf::dsp::fft;

3. Define a C++ structure that extends ssr_fft_default_params:

struct params_fix:ssr_fft_default_params
{
        static const int N-SSR_FFT_L;
        static const int R=SSR_FFT_R;
        static const scaling_mode_enum
        scaling_mode=SSR_FFT_GROW_TO_MAX_WIDTH;
        static const fft_output_order_enum
        output_data_order=SSR_FFT_NATURAL;
        static const int twiddle_table_word_length=18;
        static const int twiddle_table_intger_part_length=2;
};

4. Call 1-D SSR FFT as follows:

fft<params_fix>(inD,outD);
//OR
fft<params_fix,IID>(inD,outD);
// IID: is a constant giving instance ID

where inD and outD are 1-dimensional hls::stream of complex of ap_fixed, float or double type, synthesis and simulation use is already explained in the previous table. The I/O arrays can be declared as follows:

Fixed Point Type First define input type, then using type traits calculate output type based on ssr_fft_params struct (output type calculation takes in consideration scaling mode based bit-growth and input bit-widths).