Floating 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/float/

2- Include vt_fft.hpp

3- Use namespace xf::dsp::fft

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

5- call fft<params_float>(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_float:ssr_fft_default_params
{
   static const int N = 1024;
   static const int R = 4;
   static const fft_output_order_enum output_data_order = SSR_FFT_NATURAL;
   static const transform_direction_enum transform_direction = FORWARD_TRANSFORM;
};

4. Call 1-D SSR FFT as follows:

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

where inD and outD are 1-dimensional hls::stream 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 into consideration scaling mode based bit-growth and input bit-widths not relevant for type float).

typedef std::complex< float > I_TYPE;
typedef xf::dsp::fft::ssr_fft_output_type<ssr_fft_params,I_TYPE>::t_ssr_fft_out O_TYPE;
I_TYPE inD[SSR_FFT_R][SSR_FFT_L/SSR_FFT_R];
O_TYPE outD [R][L/R];

Here SSR_FFT_R defines SSR factor and SSR_FFT_L defines the size of the FFT transform.

Float/Double Type: First define the double/float input type, then using type traits calculate output type based on ssr_fft_params struct. For float types the output type calculation will return the same type as input.

typedef std::complex< float/double > I_TYPE;
typedef hls::ssr_fft::ssr_fft_output_type<ssr_fft_params,I_TYPE>::t_ssr_fft_out O_TYPE;
I_TYPE inD[SSR_FFT_R][SSR_FFT_L/SSR_FFT_R];
O_TYPE outD[SSR_FFT_R][SSR_FFT_L/SSR_FFT_R];