FFT Static Parameters - 2023.2 English

Vitis High-Level Synthesis User Guide (UG1399)

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2023.2 English

The static parameters of the FFT define how the FFT is configured and specifies the fixed parameters such as the size of the FFT, whether the size can be changed dynamically, whether the implementation is pipelined or radix_4_burst_io.

The hls_fft.h header file defines a struct hls::ip_fft::params_t which can be used to set default values for the static parameters. If the default values are to be used, the parameterization struct can be used directly with the FFT function.

hls::fft<hls::ip_fft::params_t >  
    (xn1, xk1, &fft_status1, &fft_config1);

A more typical use is to change some of the parameters to non-default values. This is performed by creating a new user-defined parameterization struct based on the default parameterization struct and changing some of the default values.

In the following example, a new user struct my_fft_config is defined with a new value for the output ordering (changed to natural_order). All other static parameters to the FFT use the default values.

struct my_fft_config : hls::ip_fft::params_t {
    static const unsigned ordering_opt = hls::ip_fft::natural_order;

hls::fft<my_fft_config >  
     (xn1, xk1, &fft_status1, &fft_config1);

The parameters used for the FFT struct hls::ip_fft::params_t are explained below. The default values for the parameters and a list of possible values are provided.

Tip: The FFT LogiCORE IP uses the data_format parameter to specify the values as fixed_point or floating_point, with the default being fixed_point. However, with the hls_fft.h library the input variables passed to the FFT function determine the data_format rather than a parameter.
Table 1. FFT Struct Parameters
Parameter Description C Type Default Value Valid Values
input_width Data input port width. unsigned 16 8-34
output_width Data output port width. unsigned 16 input_width to (input_width + max_nfft + 1)
status_width Output status port width. unsigned 8 Depends on FFT configuration
config_width Input configuration port width. unsigned 16 Depends on FFT configuration
max_nfft The size of the FFT data set is specified as 1 << max_nfft. unsigned 10 3-16
has_nfft Determines if the size of the FFT can be runtime configurable. bool false True, False
channels Number of channels. unsigned 1 1-12
arch_opt The implementation architecture. unsigned pipelined_streaming_io automatically_select pipelined_streaming_io radix_4_burst_io radix_2_burst_io radix_2_lite_burst_io
phase_factor_width Configure the internal phase factor precision. unsigned 16 8-34
ordering_opt The output ordering mode. unsigned bit_reversed_order bit_reversed_order natural_order
ovflo Enable overflow mode. bool true true false
scaling_opt Define the scaling options. unsigned scaled scaled unscaled block_floating_point
rounding_opt Define the rounding modes. unsigned truncation truncation convergent_rounding
mem_data Specify using block or distributed RAM for data memory. unsigned block_ram block_ram distributed_ram
mem_phase_factors Specify using block or distributed RAM for phase factors memory. unsigned block_ram block_ram distributed_ram
mem_reorder Specify using block or distributed RAM for output reorder memory. unsigned block_ram block_ram distributed_ram
stages_block_ram Defines the number of block RAM stages used in the implementation. unsigned (max_nfft < 10) ? 0 : (max_nfft - 9) 0-11
mem_hybrid When block RAMs are specified for data, phase factor, or reorder buffer, mem_hybrid specifies where or not to use a hybrid of block and distributed RAMs to reduce block RAM count in certain configurations. bool false true false
complex_mult_type Defines the types of multiplier to use for complex multiplications. unsigned use_mults_resources use_luts use_mults_resources use_mults_performance
butterfly_type Defines the implementation used for the FFT butterfly. unsigned use_luts use_luts use_xtremedsp_slices
Important: When specifying parameter values which are not integer or boolean, the HLS FFT namespace should be used. For example, the possible values for parameter butterfly_type in the following table are use_luts and use_xtremedsp_slices. The values used in the C program should be butterfly_type = hls::ip_fft::use_luts and butterfly_type = hls::ip_fft::use_xtremedsp_slices.