Optional FIR Runtime Configuration - 2021.2 English

Vitis High-Level Synthesis User Guide (UG1399)

Document ID
Release Date
2021.2 English

In some modes of operation, the FIR requires an additional input to configure how the coefficients are used. For a complete description of which modes require this input configuration, refer to the FIR Compiler LogiCORE IP Product Guide (PG149).

This input configuration can be performed in the C code using a standard ap_int.h 8-bit data type. In this example, the header file fir_top.h specifies the use of the FIR and ap_fixed libraries, defines a number of the design parameter values and then defines some fixed-point types based on these:

#include "ap_fixed.h"
#include "hls_fir.h"

const unsigned FIR_LENGTH   = 21;
const unsigned INPUT_WIDTH = 16;
const unsigned INPUT_FRACTIONAL_BITS = 0;
const unsigned OUTPUT_WIDTH = 24;
const unsigned OUTPUT_FRACTIONAL_BITS = 0;
const unsigned COEFF_WIDTH = 16;
const unsigned COEFF_FRACTIONAL_BITS = 0;
const unsigned COEFF_NUM = 7;
const unsigned COEFF_SETS = 3;
const unsigned INPUT_LENGTH = FIR_LENGTH;
const unsigned OUTPUT_LENGTH = FIR_LENGTH;
const unsigned CHAN_NUM = 1;
typedef ap_uint<8> config_t;

In the top-level code, the information in the header file is included, the static parameterization struct is created using the same constant values used to specify the bit-widths, ensuring the C code and FIR configuration match, and the coefficients are specified. At the top-level, an input configuration, defined in the header file as 8-bit data, is passed into the FIR.

#include "fir_top.h"

struct param1 : hls::ip_fir::params_t {
    static const double coeff_vec[total_num_coeff];
    static const unsigned input_length = INPUT_LENGTH;
    static const unsigned output_length = OUTPUT_LENGTH;
    static const unsigned num_coeffs = COEFF_NUM;
    static const unsigned coeff_sets = COEFF_SETS;
const double param1::coeff_vec[total_num_coeff] = 

void dummy_fe(s_data_t in[INPUT_LENGTH], s_data_t out[INPUT_LENGTH], 
                config_t* config_in, config_t* config_out)
    *config_out = *config_in;
    for(unsigned i = 0; i < INPUT_LENGTH; ++i)
        out[i] = in[i];

void dummy_be(m_data_t in[OUTPUT_LENGTH], m_data_t out[OUTPUT_LENGTH])
    for(unsigned i = 0; i < OUTPUT_LENGTH; ++i)
        out[i] = in[i];

// DUT
void fir_top(s_data_t in[INPUT_LENGTH],
             m_data_t out[OUTPUT_LENGTH],
             config_t* config)

    s_data_t fir_in[INPUT_LENGTH];
    m_data_t fir_out[OUTPUT_LENGTH];
    config_t fir_config;
    // Create struct for config
    static hls::FIR<param1> fir1;
// Dataflow process
    dummy_fe(in, fir_in, config, &fir_config);
    fir1.run(fir_in, fir_out, &fir_config);
    dummy_be(fir_out, out);

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