Using the FFT Function - 2021.2 English

Vitis High-Level Synthesis User Guide (UG1399)

Document ID
UG1399
ft:locale
English (United States)
Release Date
2021-12-15
Version
2021.2 English

The FFT function is defined in the HLS namespace and can be called as follows:

hls::fft<STATIC_PARAM> (
INPUT_DATA_ARRAY,
OUTPUT_DATA_ARRAY, 
OUTPUT_STATUS, 
INPUT_RUN_TIME_CONFIGURATION);

The STATIC_PARAM is the static parameterization struct that defines the static parameters for the FFT.

Both the input and output data are supplied to the function as arrays (INPUT_DATA_ARRAY and OUTPUT_DATA_ARRAY). In the final implementation, the ports on the FFT RTL block will be implemented as AXI4-Stream ports. Xilinx recommends always using the FFT function in a region using dataflow optimization (set_directive_dataflow), because this ensures the arrays are implemented as streaming arrays. An alternative is to specify both arrays as streaming using the set_directive_stream command.

Important: The FFT cannot be used in a region which is pipelined. If high-performance operation is required, pipeline the loops or functions before and after the FFT then use dataflow optimization on all loops and functions in the region.

The data types for the arrays can be float or ap_fixed.

typedef float data_t;
complex<data_t> xn[FFT_LENGTH];
complex<data_t> xk[FFT_LENGTH];

To use fixed-point data types, the Vitis HLS arbitrary precision type ap_fixed should be used.

#include "ap_fixed.h"
typedef ap_fixed<FFT_INPUT_WIDTH,1> data_in_t;
typedef ap_fixed<FFT_OUTPUT_WIDTH,FFT_OUTPUT_WIDTH-FFT_INPUT_WIDTH+1> data_out_t;
#include <complex>
typedef hls::x_complex<data_in_t> cmpxData;
typedef hls::x_complex<data_out_t> cmpxDataOut;

In both cases, the FFT should be parameterized with the same correct data sizes. In the case of floating point data, the data widths will always be 32-bit and any other specified size will be considered invalid.

Important: The input and output width of the FFT can be configured to any arbitrary value within the supported range. The variables which connect to the input and output parameters must be defined in increments of 8-bit. For example, if the output width is configured as 33-bit, the output variable must be defined as a 40-bit variable.

The multichannel functionality of the FFT can be used by using two-dimensional arrays for the input and output data. In this case, the array data should be configured with the first dimension representing each channel and the second dimension representing the FFT data.

typedef float data_t;
static complex<data_t> xn[CHANNEL][FFT_LENGTH];
static complex<data_t> xk[CHANELL][FFT_LENGTH];

The FFT core consumes and produces data as interleaved channels (for example, ch0-data0, ch1-data0, ch2-data0, etc, ch0-data1, ch1-data1, ch2-data2, etc.). Therefore, to stream the input or output arrays of the FFT using the same sequential order that the data was read or written, you must fill or empty the two-dimensional arrays for multiple channels by iterating through the channel index first, as shown in the following example:

cmpxData   in_fft[FFT_CHANNELS][FFT_LENGTH];
cmpxData  out_fft[FFT_CHANNELS][FFT_LENGTH];
 
// Write to FFT Input Array
for (unsigned i = 0; i < FFT_LENGTH; i++) {
 for (unsigned j = 0; j < FFT_CHANNELS; ++j) {
 in_fft[j][i] = in.read().data;
 }
}
   
// Read from FFT Output Array
for (unsigned i = 0; i < FFT_LENGTH; i++) {
 for (unsigned j = 0; j < FFT_CHANNELS; ++j) {
 out.data = out_fft[j][i];
 
 }
}

Design examples using the FFT C library are provided in the Vitis HLS examples and can be accessed using menu option Help > Welcome > Open Example Project > Design Examples > FFT.