2025-07-23 Verilog I2C Master Example

Source: https://github.com/alexforencich/verilog-i2c/blob/master/rtl/i2c_master.v

/*

Copyright (c) 2015-2017 Alex Forencich

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
THE SOFTWARE.

*/

// Language: Verilog 2001

`timescale 1ns / 1ps

/*
 * I2C master
 */
module i2c_master (
    input  wire        clk,
    input  wire        rst,

    /*
     * Host interface
     */
    input  wire [6:0]  s_axis_cmd_address,
    input  wire        s_axis_cmd_start,
    input  wire        s_axis_cmd_read,
    input  wire        s_axis_cmd_write,
    input  wire        s_axis_cmd_write_multiple,
    input  wire        s_axis_cmd_stop,
    input  wire        s_axis_cmd_valid,
    output wire        s_axis_cmd_ready,

    input  wire [7:0]  s_axis_data_tdata,
    input  wire        s_axis_data_tvalid,
    output wire        s_axis_data_tready,
    input  wire        s_axis_data_tlast,

    output wire [7:0]  m_axis_data_tdata,
    output wire        m_axis_data_tvalid,
    input  wire        m_axis_data_tready,
    output wire        m_axis_data_tlast,

    /*
     * I2C interface
     */
    input  wire        scl_i,
    output wire        scl_o,
    output wire        scl_t,
    input  wire        sda_i,
    output wire        sda_o,
    output wire        sda_t,

    /*
     * Status
     */
    output wire        busy,
    output wire        bus_control,
    output wire        bus_active,
    output wire        missed_ack,

    /*
     * Configuration
     */
    input  wire [15:0] prescale,
    input  wire        stop_on_idle
);

/*

I2C

Read
    __    ___ ___ ___ ___ ___ ___ ___         ___ ___ ___ ___ ___ ___ ___ ___     ___ ___ ___ ___ ___ ___ ___ ___        __
sda   \__/_6_X_5_X_4_X_3_X_2_X_1_X_0_\_R___A_/_7_X_6_X_5_X_4_X_3_X_2_X_1_X_0_\_A_/_7_X_6_X_5_X_4_X_3_X_2_X_1_X_0_\_A____/
    ____   _   _   _   _   _   _   _   _   _   _   _   _   _   _   _   _   _   _   _   _   _   _   _   _   _   _   _   ____
scl  ST \_/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ SP

Write
    __    ___ ___ ___ ___ ___ ___ ___ ___     ___ ___ ___ ___ ___ ___ ___ ___     ___ ___ ___ ___ ___ ___ ___ ___ ___    __
sda   \__/_6_X_5_X_4_X_3_X_2_X_1_X_0_/ W \_A_/_7_X_6_X_5_X_4_X_3_X_2_X_1_X_0_\_A_/_7_X_6_X_5_X_4_X_3_X_2_X_1_X_0_/ N \__/
    ____   _   _   _   _   _   _   _   _   _   _   _   _   _   _   _   _   _   _   _   _   _   _   _   _   _   _   _   ____
scl  ST \_/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ SP

Commands:

read
    read data byte
    set start to force generation of a start condition
    start is implied when bus is inactive or active with write or different address
    set stop to issue a stop condition after reading current byte
    if stop is set with read command, then m_axis_data_tlast will be set

write
    write data byte
    set start to force generation of a start condition
    start is implied when bus is inactive or active with read or different address
    set stop to issue a stop condition after writing current byte

write multiple
    write multiple data bytes (until s_axis_data_tlast)
    set start to force generation of a start condition
    start is implied when bus is inactive or active with read or different address
    set stop to issue a stop condition after writing block

stop
    issue stop condition if bus is active

Status:

busy
    module is communicating over the bus

bus_control
    module has control of bus in active state

bus_active
    bus is active, not necessarily controlled by this module

missed_ack
    strobed when a slave ack is missed

Parameters:

prescale
    set prescale to 1/4 of the minimum clock period in units
    of input clk cycles (prescale = Fclk / (FI2Cclk * 4))

stop_on_idle
    automatically issue stop when command input is not valid

Example of interfacing with tristate pins:
(this will work for any tristate bus)

assign scl_i = scl_pin;
assign scl_pin = scl_t ? 1'bz : scl_o;
assign sda_i = sda_pin;
assign sda_pin = sda_t ? 1'bz : sda_o;

Equivalent code that does not use *_t connections:
(we can get away with this because I2C is open-drain)

assign scl_i = scl_pin;
assign scl_pin = scl_o ? 1'bz : 1'b0;
assign sda_i = sda_pin;
assign sda_pin = sda_o ? 1'bz : 1'b0;

Example of two interconnected I2C devices:

assign scl_1_i = scl_1_o & scl_2_o;
assign scl_2_i = scl_1_o & scl_2_o;
assign sda_1_i = sda_1_o & sda_2_o;
assign sda_2_i = sda_1_o & sda_2_o;

Example of two I2C devices sharing the same pins:

assign scl_1_i = scl_pin;
assign scl_2_i = scl_pin;
assign scl_pin = (scl_1_o & scl_2_o) ? 1'bz : 1'b0;
assign sda_1_i = sda_pin;
assign sda_2_i = sda_pin;
assign sda_pin = (sda_1_o & sda_2_o) ? 1'bz : 1'b0;

Notes:

scl_o should not be connected directly to scl_i, only via AND logic or a tristate
I/O pin.  This would prevent devices from stretching the clock period.

*/

localparam [4:0]
    STATE_IDLE = 4'd0,
    STATE_ACTIVE_WRITE = 4'd1,
    STATE_ACTIVE_READ = 4'd2,
    STATE_START_WAIT = 4'd3,
    STATE_START = 4'd4,
    STATE_ADDRESS_1 = 4'd5,
    STATE_ADDRESS_2 = 4'd6,
    STATE_WRITE_1 = 4'd7,
    STATE_WRITE_2 = 4'd8,
    STATE_WRITE_3 = 4'd9,
    STATE_READ = 4'd10,
    STATE_STOP = 4'd11;

reg [4:0] state_reg = STATE_IDLE, state_next;

localparam [4:0]
    PHY_STATE_IDLE = 5'd0,
    PHY_STATE_ACTIVE = 5'd1,
    PHY_STATE_REPEATED_START_1 = 5'd2,
    PHY_STATE_REPEATED_START_2 = 5'd3,
    PHY_STATE_START_1 = 5'd4,
    PHY_STATE_START_2 = 5'd5,
    PHY_STATE_WRITE_BIT_1 = 5'd6,
    PHY_STATE_WRITE_BIT_2 = 5'd7,
    PHY_STATE_WRITE_BIT_3 = 5'd8,
    PHY_STATE_READ_BIT_1 = 5'd9,
    PHY_STATE_READ_BIT_2 = 5'd10,
    PHY_STATE_READ_BIT_3 = 5'd11,
    PHY_STATE_READ_BIT_4 = 5'd12,
    PHY_STATE_STOP_1 = 5'd13,
    PHY_STATE_STOP_2 = 5'd14,
    PHY_STATE_STOP_3 = 5'd15;

reg [4:0] phy_state_reg = STATE_IDLE, phy_state_next;

reg phy_start_bit;
reg phy_stop_bit;
reg phy_write_bit;
reg phy_read_bit;
reg phy_release_bus;

reg phy_tx_data;

reg phy_rx_data_reg = 1'b0, phy_rx_data_next;

reg [6:0] addr_reg = 7'd0, addr_next;
reg [7:0] data_reg = 8'd0, data_next;
reg last_reg = 1'b0, last_next;

reg mode_read_reg = 1'b0, mode_read_next;
reg mode_write_multiple_reg = 1'b0, mode_write_multiple_next;
reg mode_stop_reg = 1'b0, mode_stop_next;

reg [16:0] delay_reg = 16'd0, delay_next;
reg delay_scl_reg = 1'b0, delay_scl_next;
reg delay_sda_reg = 1'b0, delay_sda_next;

reg [3:0] bit_count_reg = 4'd0, bit_count_next;

reg s_axis_cmd_ready_reg = 1'b0, s_axis_cmd_ready_next;

reg s_axis_data_tready_reg = 1'b0, s_axis_data_tready_next;

reg [7:0] m_axis_data_tdata_reg = 8'd0, m_axis_data_tdata_next;
reg m_axis_data_tvalid_reg = 1'b0, m_axis_data_tvalid_next;
reg m_axis_data_tlast_reg = 1'b0, m_axis_data_tlast_next;

reg scl_i_reg = 1'b1;
reg sda_i_reg = 1'b1;

reg scl_o_reg = 1'b1, scl_o_next;
reg sda_o_reg = 1'b1, sda_o_next;

reg last_scl_i_reg = 1'b1;
reg last_sda_i_reg = 1'b1;

reg busy_reg = 1'b0;
reg bus_active_reg = 1'b0;
reg bus_control_reg = 1'b0, bus_control_next;
reg missed_ack_reg = 1'b0, missed_ack_next;

assign s_axis_cmd_ready = s_axis_cmd_ready_reg;

assign s_axis_data_tready = s_axis_data_tready_reg;

assign m_axis_data_tdata = m_axis_data_tdata_reg;
assign m_axis_data_tvalid = m_axis_data_tvalid_reg;
assign m_axis_data_tlast = m_axis_data_tlast_reg;

assign scl_o = scl_o_reg;
assign scl_t = scl_o_reg;
assign sda_o = sda_o_reg;
assign sda_t = sda_o_reg;

assign busy = busy_reg;
assign bus_active = bus_active_reg;
assign bus_control = bus_control_reg;
assign missed_ack = missed_ack_reg;

wire scl_posedge = scl_i_reg & ~last_scl_i_reg;
wire scl_negedge = ~scl_i_reg & last_scl_i_reg;
wire sda_posedge = sda_i_reg & ~last_sda_i_reg;
wire sda_negedge = ~sda_i_reg & last_sda_i_reg;

wire start_bit = sda_negedge & scl_i_reg;
wire stop_bit = sda_posedge & scl_i_reg;

always @* begin
    state_next = STATE_IDLE;

    phy_start_bit = 1'b0;
    phy_stop_bit = 1'b0;
    phy_write_bit = 1'b0;
    phy_read_bit = 1'b0;
    phy_tx_data = 1'b0;
    phy_release_bus = 1'b0;

    addr_next = addr_reg;
    data_next = data_reg;
    last_next = last_reg;

    mode_read_next = mode_read_reg;
    mode_write_multiple_next = mode_write_multiple_reg;
    mode_stop_next = mode_stop_reg;

    bit_count_next = bit_count_reg;

    s_axis_cmd_ready_next = 1'b0;

    s_axis_data_tready_next = 1'b0;

    m_axis_data_tdata_next = m_axis_data_tdata_reg;
    m_axis_data_tvalid_next = m_axis_data_tvalid_reg & ~m_axis_data_tready;
    m_axis_data_tlast_next = m_axis_data_tlast_reg;

    missed_ack_next = 1'b0;

    // generate delays
    if (phy_state_reg != PHY_STATE_IDLE && phy_state_reg != PHY_STATE_ACTIVE) begin
        // wait for phy operation
        state_next = state_reg;
    end else begin
        // process states
        case (state_reg)
            STATE_IDLE: begin
                // line idle
                s_axis_cmd_ready_next = 1'b1;

                if (s_axis_cmd_ready & s_axis_cmd_valid) begin
                    // command valid
                    if (s_axis_cmd_read ^ (s_axis_cmd_write | s_axis_cmd_write_multiple)) begin
                        // read or write command
                        addr_next = s_axis_cmd_address;
                        mode_read_next = s_axis_cmd_read;
                        mode_write_multiple_next = s_axis_cmd_write_multiple;
                        mode_stop_next = s_axis_cmd_stop;

                        s_axis_cmd_ready_next = 1'b0;

                        // start bit
                        if (bus_active) begin
                            state_next = STATE_START_WAIT;
                        end else begin
                            phy_start_bit = 1'b1;
                            bit_count_next = 4'd8;
                            state_next = STATE_ADDRESS_1;
                        end
                    end else begin
                        // invalid or unspecified - ignore
                        state_next = STATE_IDLE;
                    end
                end else begin
                    state_next = STATE_IDLE;
                end
            end
            STATE_ACTIVE_WRITE: begin
                // line active with current address and read/write mode
                s_axis_cmd_ready_next = 1'b1;

                if (s_axis_cmd_ready & s_axis_cmd_valid) begin
                    // command valid
                    if (s_axis_cmd_read ^ (s_axis_cmd_write | s_axis_cmd_write_multiple)) begin
                        // read or write command
                        addr_next = s_axis_cmd_address;
                        mode_read_next = s_axis_cmd_read;
                        mode_write_multiple_next = s_axis_cmd_write_multiple;
                        mode_stop_next = s_axis_cmd_stop;

                        s_axis_cmd_ready_next = 1'b0;
                        
                        if (s_axis_cmd_start || s_axis_cmd_address != addr_reg || s_axis_cmd_read) begin
                            // address or mode mismatch or forced start - repeated start

                            // repeated start bit
                            phy_start_bit = 1'b1;
                            bit_count_next = 4'd8;
                            state_next = STATE_ADDRESS_1;
                        end else begin
                            // address and mode match

                            // start write
                            s_axis_data_tready_next = 1'b1;
                            state_next = STATE_WRITE_1;
                        end
                    end else if (s_axis_cmd_stop && !(s_axis_cmd_read || s_axis_cmd_write || s_axis_cmd_write_multiple)) begin
                        // stop command
                        phy_stop_bit = 1'b1;
                        state_next = STATE_IDLE;
                    end else begin
                        // invalid or unspecified - ignore
                        state_next = STATE_ACTIVE_WRITE;
                    end
                end else begin
                    if (stop_on_idle & s_axis_cmd_ready & ~s_axis_cmd_valid) begin
                        // no waiting command and stop_on_idle selected, issue stop condition
                        phy_stop_bit = 1'b1;
                        state_next = STATE_IDLE;
                    end else begin
                        state_next = STATE_ACTIVE_WRITE;
                    end
                end
            end
            STATE_ACTIVE_READ: begin
                // line active to current address
                s_axis_cmd_ready_next = ~m_axis_data_tvalid;

                if (s_axis_cmd_ready & s_axis_cmd_valid) begin
                    // command valid
                    if (s_axis_cmd_read ^ (s_axis_cmd_write | s_axis_cmd_write_multiple)) begin
                        // read or write command
                        addr_next = s_axis_cmd_address;
                        mode_read_next = s_axis_cmd_read;
                        mode_write_multiple_next = s_axis_cmd_write_multiple;
                        mode_stop_next = s_axis_cmd_stop;

                        s_axis_cmd_ready_next = 1'b0;
                        
                        if (s_axis_cmd_start || s_axis_cmd_address != addr_reg || s_axis_cmd_write) begin
                            // address or mode mismatch or forced start - repeated start

                            // write nack for previous read
                            phy_write_bit = 1'b1;
                            phy_tx_data = 1'b1;
                            // repeated start bit
                            state_next = STATE_START;
                        end else begin
                            // address and mode match

                            // write ack for previous read
                            phy_write_bit = 1'b1;
                            phy_tx_data = 1'b0;
                            // start next read
                            bit_count_next = 4'd8;
                            data_next = 8'd0;
                            state_next = STATE_READ;
                        end
                    end else if (s_axis_cmd_stop && !(s_axis_cmd_read || s_axis_cmd_write || s_axis_cmd_write_multiple)) begin
                        // stop command
                        // write nack for previous read
                        phy_write_bit = 1'b1;
                        phy_tx_data = 1'b1;
                        // send stop bit
                        state_next = STATE_STOP;
                    end else begin
                        // invalid or unspecified - ignore
                        state_next = STATE_ACTIVE_READ;
                    end
                end else begin
                    if (stop_on_idle & s_axis_cmd_ready & ~s_axis_cmd_valid) begin
                        // no waiting command and stop_on_idle selected, issue stop condition
                        // write ack for previous read
                        phy_write_bit = 1'b1;
                        phy_tx_data = 1'b1;
                        // send stop bit
                        state_next = STATE_STOP;
                    end else begin
                        state_next = STATE_ACTIVE_READ;
                    end
                end
            end
            STATE_START_WAIT: begin
                // wait for bus idle

                if (bus_active) begin
                    state_next = STATE_START_WAIT;
                end else begin
                    // bus is idle, take control
                    phy_start_bit = 1'b1;
                    bit_count_next = 4'd8;
                    state_next = STATE_ADDRESS_1;
                end
            end
            STATE_START: begin
                // send start bit

                phy_start_bit = 1'b1;
                bit_count_next = 4'd8;
                state_next = STATE_ADDRESS_1;
            end
            STATE_ADDRESS_1: begin
                // send address
                bit_count_next = bit_count_reg - 1;
                if (bit_count_reg > 1) begin
                    // send address
                    phy_write_bit = 1'b1;
                    phy_tx_data = addr_reg[bit_count_reg-2];
                    state_next = STATE_ADDRESS_1;
                end else if (bit_count_reg > 0) begin
                    // send read/write bit
                    phy_write_bit = 1'b1;
                    phy_tx_data = mode_read_reg;
                    state_next = STATE_ADDRESS_1;
                end else begin
                    // read ack bit
                    phy_read_bit = 1'b1;
                    state_next = STATE_ADDRESS_2;
                end
            end
            STATE_ADDRESS_2: begin
                // read ack bit
                missed_ack_next = phy_rx_data_reg;

                if (mode_read_reg) begin
                    // start read
                    bit_count_next = 4'd8;
                    data_next = 1'b0;
                    state_next = STATE_READ;
                end else begin
                    // start write
                    s_axis_data_tready_next = 1'b1;
                    state_next = STATE_WRITE_1;
                end
            end
            STATE_WRITE_1: begin
                s_axis_data_tready_next = 1'b1;

                if (s_axis_data_tready & s_axis_data_tvalid) begin
                    // got data, start write
                    data_next = s_axis_data_tdata;
                    last_next = s_axis_data_tlast;
                    bit_count_next = 4'd8;
                    s_axis_data_tready_next = 1'b0;
                    state_next = STATE_WRITE_2;
                end else begin
                    // wait for data
                    state_next = STATE_WRITE_1;
                end
            end
            STATE_WRITE_2: begin
                // send data
                bit_count_next = bit_count_reg - 1;
                if (bit_count_reg > 0) begin
                    // write data bit
                    phy_write_bit = 1'b1;
                    phy_tx_data = data_reg[bit_count_reg-1];
                    state_next = STATE_WRITE_2;
                end else begin
                    // read ack bit
                    phy_read_bit = 1'b1;
                    state_next = STATE_WRITE_3;
                end
            end
            STATE_WRITE_3: begin
                // read ack bit
                missed_ack_next = phy_rx_data_reg;

                if (mode_write_multiple_reg && !last_reg) begin
                    // more to write
                    state_next = STATE_WRITE_1;
                end else if (mode_stop_reg) begin
                    // last cycle and stop selected
                    phy_stop_bit = 1'b1;
                    state_next = STATE_IDLE;
                end else begin
                    // otherwise, return to bus active state
                    state_next = STATE_ACTIVE_WRITE;
                end
            end
            STATE_READ: begin
                // read data

                bit_count_next = bit_count_reg - 1;
                data_next = {data_reg[6:0], phy_rx_data_reg};
                if (bit_count_reg > 0) begin
                    // read next bit
                    phy_read_bit = 1'b1;
                    state_next = STATE_READ;
                end else begin
                    // output data word
                    m_axis_data_tdata_next = data_next;
                    m_axis_data_tvalid_next = 1'b1;
                    m_axis_data_tlast_next = 1'b0;
                    if (mode_stop_reg) begin
                        // send nack and stop
                        m_axis_data_tlast_next = 1'b1;
                        phy_write_bit = 1'b1;
                        phy_tx_data = 1'b1;
                        state_next = STATE_STOP;
                    end else begin
                        // return to bus active state
                        state_next = STATE_ACTIVE_READ;
                    end
                end
            end
            STATE_STOP: begin
                // send stop bit
                phy_stop_bit = 1'b1;
                state_next = STATE_IDLE;
            end
        endcase
    end
end

always @* begin
    phy_state_next = PHY_STATE_IDLE;

    phy_rx_data_next = phy_rx_data_reg;

    delay_next = delay_reg;
    delay_scl_next = delay_scl_reg;
    delay_sda_next = delay_sda_reg;

    scl_o_next = scl_o_reg;
    sda_o_next = sda_o_reg;

    bus_control_next = bus_control_reg;

    if (phy_release_bus) begin
        // release bus and return to idle state
        sda_o_next = 1'b1;
        scl_o_next = 1'b1;
        delay_scl_next = 1'b0;
        delay_sda_next = 1'b0;
        delay_next = 1'b0;
        phy_state_next = PHY_STATE_IDLE;
    end else if (delay_scl_reg) begin
        // wait for SCL to match command
        delay_scl_next = scl_o_reg & ~scl_i_reg;
        phy_state_next = phy_state_reg;
    end else if (delay_sda_reg) begin
        // wait for SDA to match command
        delay_sda_next = sda_o_reg & ~sda_i_reg;
        phy_state_next = phy_state_reg;
    end else if (delay_reg > 0) begin
        // time delay
        delay_next = delay_reg - 1;
        phy_state_next = phy_state_reg;
    end else begin
        case (phy_state_reg)
            PHY_STATE_IDLE: begin
                // bus idle - wait for start command
                sda_o_next = 1'b1;
                scl_o_next = 1'b1;
                if (phy_start_bit) begin
                    sda_o_next = 1'b0;
                    delay_next = prescale;
                    phy_state_next = PHY_STATE_START_1;
                end else begin
                    phy_state_next = PHY_STATE_IDLE;
                end
            end
            PHY_STATE_ACTIVE: begin
                // bus active
                if (phy_start_bit) begin
                    sda_o_next = 1'b1;
                    delay_next = prescale;
                    phy_state_next = PHY_STATE_REPEATED_START_1;
                end else if (phy_write_bit) begin
                    sda_o_next = phy_tx_data;
                    delay_next = prescale;
                    phy_state_next = PHY_STATE_WRITE_BIT_1;
                end else if (phy_read_bit) begin
                    sda_o_next = 1'b1;
                    delay_next = prescale;
                    phy_state_next = PHY_STATE_READ_BIT_1;
                end else if (phy_stop_bit) begin
                    sda_o_next = 1'b0;
                    delay_next = prescale;
                    phy_state_next = PHY_STATE_STOP_1;
                end else begin
                    phy_state_next = PHY_STATE_ACTIVE;
                end
            end
            PHY_STATE_REPEATED_START_1: begin
                // generate repeated start bit
                //         ______
                // sda XXX/      \_______
                //            _______
                // scl ______/       \___
                //

                scl_o_next = 1'b1;
                delay_scl_next = 1'b1;
                delay_next = prescale;
                phy_state_next = PHY_STATE_REPEATED_START_2;
            end
            PHY_STATE_REPEATED_START_2: begin
                // generate repeated start bit
                //         ______
                // sda XXX/      \_______
                //            _______
                // scl ______/       \___
                //

                sda_o_next = 1'b0;
                delay_next = prescale;
                phy_state_next = PHY_STATE_START_1;
            end
            PHY_STATE_START_1: begin
                // generate start bit
                //     ___
                // sda    \_______
                //     _______
                // scl        \___
                //

                scl_o_next = 1'b0;
                delay_next = prescale;
                phy_state_next = PHY_STATE_START_2;
            end
            PHY_STATE_START_2: begin
                // generate start bit
                //     ___
                // sda    \_______
                //     _______
                // scl        \___
                //

                bus_control_next = 1'b1;
                phy_state_next = PHY_STATE_ACTIVE;
            end
            PHY_STATE_WRITE_BIT_1: begin
                // write bit
                //      ________
                // sda X________X
                //        ____
                // scl __/    \__

                scl_o_next = 1'b1;
                delay_scl_next = 1'b1;
                delay_next = prescale << 1;
                phy_state_next = PHY_STATE_WRITE_BIT_2;
            end
            PHY_STATE_WRITE_BIT_2: begin
                // write bit
                //      ________
                // sda X________X
                //        ____
                // scl __/    \__

                scl_o_next = 1'b0;
                delay_next = prescale;
                phy_state_next = PHY_STATE_WRITE_BIT_3;
            end
            PHY_STATE_WRITE_BIT_3: begin
                // write bit
                //      ________
                // sda X________X
                //        ____
                // scl __/    \__

                phy_state_next = PHY_STATE_ACTIVE;
            end
            PHY_STATE_READ_BIT_1: begin
                // read bit
                //      ________
                // sda X________X
                //        ____
                // scl __/    \__

                scl_o_next = 1'b1;
                delay_scl_next = 1'b1;
                delay_next = prescale;
                phy_state_next = PHY_STATE_READ_BIT_2;
            end
            PHY_STATE_READ_BIT_2: begin
                // read bit
                //      ________
                // sda X________X
                //        ____
                // scl __/    \__

                phy_rx_data_next = sda_i_reg;
                delay_next = prescale;
                phy_state_next = PHY_STATE_READ_BIT_3;
            end
            PHY_STATE_READ_BIT_3: begin
                // read bit
                //      ________
                // sda X________X
                //        ____
                // scl __/    \__

                scl_o_next = 1'b0;
                delay_next = prescale;
                phy_state_next = PHY_STATE_READ_BIT_4;
            end
            PHY_STATE_READ_BIT_4: begin
                // read bit
                //      ________
                // sda X________X
                //        ____
                // scl __/    \__

                phy_state_next = PHY_STATE_ACTIVE;
            end
            PHY_STATE_STOP_1: begin
                // stop bit
                //                 ___
                // sda XXX\_______/
                //             _______
                // scl _______/

                scl_o_next = 1'b1;
                delay_scl_next = 1'b1;
                delay_next = prescale;
                phy_state_next = PHY_STATE_STOP_2;
            end
            PHY_STATE_STOP_2: begin
                // stop bit
                //                 ___
                // sda XXX\_______/
                //             _______
                // scl _______/

                sda_o_next = 1'b1;
                delay_next = prescale;
                phy_state_next = PHY_STATE_STOP_3;
            end
            PHY_STATE_STOP_3: begin
                // stop bit
                //                 ___
                // sda XXX\_______/
                //             _______
                // scl _______/

                bus_control_next = 1'b0;
                phy_state_next = PHY_STATE_IDLE;
            end
        endcase
    end
end

always @(posedge clk) begin
    state_reg <= state_next;
    phy_state_reg <= phy_state_next;

    phy_rx_data_reg <= phy_rx_data_next;

    addr_reg <= addr_next;
    data_reg <= data_next;
    last_reg <= last_next;

    mode_read_reg <= mode_read_next;
    mode_write_multiple_reg <= mode_write_multiple_next;
    mode_stop_reg <= mode_stop_next;

    delay_reg <= delay_next;
    delay_scl_reg <= delay_scl_next;
    delay_sda_reg <= delay_sda_next;

    bit_count_reg <= bit_count_next;

    s_axis_cmd_ready_reg <= s_axis_cmd_ready_next;

    s_axis_data_tready_reg <= s_axis_data_tready_next;

    m_axis_data_tdata_reg <= m_axis_data_tdata_next;
    m_axis_data_tlast_reg <= m_axis_data_tlast_next;
    m_axis_data_tvalid_reg <= m_axis_data_tvalid_next;

    scl_i_reg <= scl_i;
    sda_i_reg <= sda_i;

    scl_o_reg <= scl_o_next;
    sda_o_reg <= sda_o_next;

    last_scl_i_reg <= scl_i_reg;
    last_sda_i_reg <= sda_i_reg;

    busy_reg <= !(state_reg == STATE_IDLE || state_reg == STATE_ACTIVE_WRITE || state_reg == STATE_ACTIVE_READ) || !(phy_state_reg == PHY_STATE_IDLE || phy_state_reg == PHY_STATE_ACTIVE);

    if (start_bit) begin
        bus_active_reg <= 1'b1;
    end else if (stop_bit) begin
        bus_active_reg <= 1'b0;
    end else begin
        bus_active_reg <= bus_active_reg;
    end

    bus_control_reg <= bus_control_next;
    missed_ack_reg <= missed_ack_next;

    if (rst) begin
        state_reg <= STATE_IDLE;
        phy_state_reg <= PHY_STATE_IDLE;
        delay_reg <= 16'd0;
        delay_scl_reg <= 1'b0;
        delay_sda_reg <= 1'b0;
        s_axis_cmd_ready_reg <= 1'b0;
        s_axis_data_tready_reg <= 1'b0;
        m_axis_data_tvalid_reg <= 1'b0;
        scl_o_reg <= 1'b1;
        sda_o_reg <= 1'b1;
        busy_reg <= 1'b0;
        bus_active_reg <= 1'b0;
        bus_control_reg <= 1'b0;
        missed_ack_reg <= 1'b0;
    end
end

endmodule

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