Using the FIR Function - 2021.2 English

Vitis High-Level Synthesis User Guide (UG1399)

Document ID
English (United States)
Release Date
2021.2 English

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

// Create an instance of the FIR 
static hls::FIR<STATIC_PARAM> fir1;
// Execute the FIR instance fir1, OUTPUT_DATA_ARRAY);

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

Both the input and output data are supplied to the function as arrays (INPUT_DATA_ARRAY and OUTPUT_DATA_ARRAY). In the final implementation, these ports on the FIR IP will be implemented as AXI4-Stream ports. Xilinx recommends always using the FIR function in a region using the 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 FIR cannot be used in a region which is pipelined. If high-performance operation is required, pipeline the loops or functions before and after the FIR then use dataflow optimization on all loops and functions in the region.

The multichannel functionality of the FIR is supported through interleaving the data in a single input and single output array.

  • The size of the input array should be large enough to accommodate all samples: num_channels * input_length.
  • The output array size should be specified to contain all output samples: num_channels * output_length.

The following code example demonstrates, for two channels, how the data is interleaved. In this example, the top-level function has two channels of input data (din_i, din_q) and two channels of output data (dout_i, dout_q). Two functions, at the front-end (fe) and back-end (be) are used to correctly order the data in the FIR input array and extract it from the FIR output array.

void dummy_fe(din_t din_i[LENGTH], din_t din_q[LENGTH], din_t out[FIR_LENGTH]) {
    for (unsigned i = 0; i < LENGTH; ++i) {
        out[2*i] = din_i[i];
        out[2*i + 1] = din_q[i];
void dummy_be(dout_t in[FIR_LENGTH], dout_t dout_i[LENGTH], dout_t dout_q[LENGTH]) {   
    for(unsigned i = 0; i < LENGTH; ++i) {
        dout_i[i] = in[2*i];
        dout_q[i] = in[2*i+1];
void fir_top(din_t din_i[LENGTH], din_t din_q[LENGTH],
             dout_t dout_i[LENGTH], dout_t dout_q[LENGTH]) {   

 din_t fir_in[FIR_LENGTH];
    dout_t fir_out[FIR_LENGTH];
    static hls::FIR<myconfig> fir1;

    dummy_fe(din_i, din_q, fir_in);, fir_out);
    dummy_be(fir_out, dout_i, dout_q);