A place to keep my synthesizable verilog examples.

Table of Contents

• OVERVIEW
• BASIC CODE
  • COMBINATIONAL LOGIC
  • SEQUENTIAL LOGIC
• COMBINATIONAL LOGIC
  • ALUs
  • DATA OPERATORS
  • DECODERS & ENCODERS
  • MULTIPLEXERS & DEMULTIPLEXERS
• FPGA DEVELOPMENT BOARDS
  • BUTTONS
• SEQUENTIAL LOGIC
  • ARBITERS
  • COUNTERS
  • FINITE SATE MACHINES
  • MEMORY
  • REGISTERS
  • SHIFTERS
• SYSTEMS
  • MICROPROCESSORS
  • PIPELINES

Documentation and Reference

• verilog
• iverilog is a free tool for simulation and synthesis
• GTKWave is a free HDL waveform viewer
• xilinx vivado is a complete IDE for synthesis and analysis of HDL designs
• digilent ARTY-S7 is a FPGA development board
• My Master's Thesis has an explanation of HDLs and how they fit into frameworks
• My Master's Thesis also has my design of a programable_8_bit_microprocessor
• This repos github webpage built with concourse

Each example uses iverilog to simulate and GTKWave to view the waveform. I also used Xilinx Vivado to synthesize and program these verilog examples on a Digilent ARTY-S7 FPGA development board.

I declare my ports as follows because that's what the synthesis tools want. Who am I to argue.

    module NAME (
        input             a,     // Input A
        input       [7:0] b,     // Input B
        output reg  [3:0] y);    // Output Y

Also, I would stay away from asynchronous design. It can have problems when you synthesize to an FPGA.

    // DO THIS
    always @(posedge clk) begin
        if (~reset) begin
            ...

    // NOT THIS
    always @(posedge clk or negedge reset) begin

Each example has the following 4 files,

• *.v - The verilog code files(s)
• *.vh - A header file listing the included verilog files
• *_tb.v - The verilog testbench code
• *_tb.tv - Test vectors used with the testbench

The artifacts created are,

• *_tb.vvp - The verilog compiled code to be used by the simulator
• *_tb.vcd - The dump of the waveform data
• *_tb.gtkw - The GTKWave saved waveform

Where the testbench structure is,

• COMBINATIONAL LOGIC

  • and2

    2-input AND gate used in my programable_8_bit_microprocessor.

  • nand4

    4-input NAND gate used in my programable_8_bit_microprocessor.

  • nor2

    2-input NOR gate used in my programable_8_bit_microprocessor.

  • not1

    NOT gate used in my programable_8_bit_microprocessor.

  • or2

    2-input OR gate used in my programable_8_bit_microprocessor.

  • xor2

    2-input XOR gate used in my programable_8_bit_microprocessor.

• SEQUENTIAL LOGIC

  • sr_latch

    A sr (set ready) latch which is level-triggered that can be set and reset. The latch forms the basic building block of other types of latches and flip-flops.

  • sr_flip_flop

    A sr (set ready) flip-flop which is pulse-triggered can be set and reset.

  • jk_flip_flop

    A jk flip-flop which is pulse-triggered can be set, reset and toggled. This has a race condition when clock is high.

  • jk_flip_flop_pulse_triggered

    A pulse-triggered jk flip-flop (cascading) can be set, reset and toggled.

  • jk_flip_flop_pos_edge_sync_clear

    A posedge-triggered jk flip-flop with synchronous clear used in my jeff_74x161.

  • t_flip_flop

    A t flip-flop which is pulse-triggered can be toggled. This has a race condition when clock is high.

  • d_flip_flop

    A d flip-flop which is pulse-triggered can save input data on output.

  • d_flip_flop_pulse_triggered

    A pulse-triggered d flip-flop (cascading) can save input data on output.

  • d_flip_flop_pos_edge_sync_en

    A posedge-triggered d flip-flop with synchronous enable used in my jeff_74x377.

• ALUs

  • jeff_74x181

    4-bit alu (arithmetic logic unit) and function generator. Provides 16 binary logic operations and 16 arithmetic operations on two 4-bit words. Based on the 7400-series integrated circuits used in my programable_8_bit_microprocessor.

• DATA OPERATORS

  • full_adder

    A 2-bit full-adder.

  • half_adder

    A 2-bit half-adder.

• DECODERS & ENCODERS

  • encoder_8_3

    Encoder - Eights inputs (1 hot) encodes to output.

  • decoder_3_8

    Decoder - Three inputs decodes to 1 of 8 outputs (hot).

  • encoder_to_decoder

    Combining the encoder_8_3 to the decoder_3_8 to prove the input will equal the output.

• MULTIPLEXERS & DEMULTIPLEXERS

  • demux_1x4

    Demultiplexer - One input, four outputs.

  • jeff_74x151

    8-line to 1-line data selector/multiplexer. Based on the 7400-series integrated circuits used in my programable_8_bit_microprocessor.

  • jeff_74x157

    Quad 2-line to 1-line data selector/multiplexer, non-inverting outputs. Based on the 7400-series integrated circuits used in my programable_8_bit_microprocessor.

  • mux_4x1

    Multiplexer - Four inputs, one output.

  • mux_to_demux

    Combining the mux_4x1 to the demux_1x4 to prove the input will equal the output (for the selected output).

• BUTTONS

  • buttons

    A few different ways to use buttons on a FPGA development board.

• ARBITERS

  • priority_arbiter

    A three level priority arbiter with asynchronous reset.

• COUNTERS

  • jeff_74x161

    Synchronous presettable 4-bit binary counter, asynchronous clear. Based on the 7400-series integrated circuits used in my programable_8_bit_microprocessor.

• FINITE STATE MACHINES

  • mealy_state_machine

    Recognize the pattern 00110 in a serial stream. Output depends on current state and current inputs.

  • moore_state_machine

    Recognize the pattern 00110 in a serial stream. Output depends on current state only.

• MEMORY

  • single_port_ram_synchronous

    Single-port synchronous RAM.

  • dual_port_ram_synchronous

    Dual-port synchronous RAM.

  • dual_port_ram_asynchronous

    Dual-port asynchronous RAM using two different clocks.

  • fifo_synchronous

    A synchronous fifo using dual-port synchronous RAM.

  • fifo_asynchronous

    An asynchronous fifo using dual-port asynchronous RAM.

  • lifo_synchronous

    A synchronous lifo using dual-port synchronous RAM.

• REGISTERS

  • jeff_74x377

    8-bit register, clock enable. Based on the 7400-series integrated circuits used in my programable_8_bit_microprocessor.

  • simple_8_bit_register

    A simple 8-bit register with synchronous load and clear.

• SHIFTERS

  • left_shift_register

    A 4-bit left shift register.

• MICROPROCESSORS

  • programable_8_bit_microprocessor

    A programable 8-bit microprocessor. Originally designed in VHDL for part of my master's thesis.

• PIPELINES

  • simple_pipeline

    A simple pipeline.