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HDL code for a DDS (direct digital synthesizer) with AXI stream interface

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Direct Digital Synthesizer

Overview

This is a DDS core written in system verilog. It uses a quarter-wave lut plus optional taylor series approximation. The code is optimized and tested for XILINX Series 7 FPGAs with Vivado 2020.2 but should also work on other products. In that case some register widths might be not optimal, most register widths are currently optimized to fit in a DSP48E1 unit.

The sin-cos lut uses a lookup table that needs to be precalculated using tools/generate_sine_lut.py script. The generated hex file needs to be placed in a location where the $readmemh command can find it. This might vary depending on the synthesizer used.

PARAMETERS

  • PHASE_DW selects the number of bits for the phase input
  • OUT_DW selects the number of bits for the output
  • USE_TAYLOR enables taylor series correction if set to 1
  • LUT_DW not used if USE_TAYLOR = 0. In this case LUT_DW is set to PHASE_DW - 2 so that the entire waveform is created by the lut. If USE_TAYLOR = 1, this value can be used to set the degree of interpolation by taylor series correction.
  • SIN_COS output additional cosine if set to one
  • NEGATIVE_SINE inverts sine output if set to 1
  • NEGATIVE_COSINE inverts cosine output if set to 1

PORTS

  • CLK clock
  • reset_n active low reset
  • s_axis_phase AXI Stream interface for phase input
  • m_axis_out AXI Stream interface for combined sin and cos output, width is 2*OUT_DW
  • m_axis_out_sin AXI Stream interface for sin output
  • m_axis_out_cos AXI Stream interface for cos output

Verification

To run the unit tests install

  • python >3.8
  • iverilog >1.4
  • python modules: cocotb, cocotb_test, pytest, pytest-parallel, pytest-cov

and run pytest in the repo directory

pytest -v --workers 10

TODO

  • add taylor series correction in negative direction, should improve accuracy slightly
  • put sin-cos lut in separate module
  • benchmark error against Xilinx DDS

References

License

GPL

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HDL code for a DDS (direct digital synthesizer) with AXI stream interface

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