module axil_adapter_rd #
(
    // Width of address bus in bits
    parameter ADDR_WIDTH = 32,
    // Width of input (slave) interface data bus in bits
    parameter S_DATA_WIDTH = 32,
    // Width of input (slave) interface wstrb (width of data bus in words)
    parameter S_STRB_WIDTH = (S_DATA_WIDTH/8),
    // Width of output (master) interface data bus in bits
    parameter M_DATA_WIDTH = 32,
    // Width of output (master) interface wstrb (width of data bus in words)
    parameter M_STRB_WIDTH = (M_DATA_WIDTH/8)
)
(
    input  wire                     clk,
    input  wire                     rst,

    /*
     * AXI lite slave interface
     */
    input  wire [ADDR_WIDTH-1:0]    s_axil_araddr,
    input  wire [2:0]               s_axil_arprot,
    input  wire                     s_axil_arvalid,
    output wire                     s_axil_arready,
    output wire [S_DATA_WIDTH-1:0]  s_axil_rdata,
    output wire [1:0]               s_axil_rresp,
    output wire                     s_axil_rvalid,
    input  wire                     s_axil_rready,

    /*
     * AXI lite master interface
     */
    output wire [ADDR_WIDTH-1:0]    m_axil_araddr,
    output wire [2:0]               m_axil_arprot,
    output wire                     m_axil_arvalid,
    input  wire                     m_axil_arready,
    input  wire [M_DATA_WIDTH-1:0]  m_axil_rdata,
    input  wire [1:0]               m_axil_rresp,
    input  wire                     m_axil_rvalid,
    output wire                     m_axil_rready
);

parameter S_ADDR_BIT_OFFSET = $clog2(S_STRB_WIDTH);
parameter M_ADDR_BIT_OFFSET = $clog2(M_STRB_WIDTH);
parameter S_WORD_WIDTH = S_STRB_WIDTH;
parameter M_WORD_WIDTH = M_STRB_WIDTH;
parameter S_WORD_SIZE = S_DATA_WIDTH/S_WORD_WIDTH;
parameter M_WORD_SIZE = M_DATA_WIDTH/M_WORD_WIDTH;
parameter S_ADDR_MASK = {ADDR_WIDTH{1'b1}} << S_ADDR_BIT_OFFSET;
parameter M_ADDR_MASK = {ADDR_WIDTH{1'b1}} << M_ADDR_BIT_OFFSET;

// output bus is wider
parameter EXPAND = M_STRB_WIDTH > S_STRB_WIDTH;
parameter DATA_WIDTH = EXPAND ? M_DATA_WIDTH : S_DATA_WIDTH;
parameter STRB_WIDTH = EXPAND ? M_STRB_WIDTH : S_STRB_WIDTH;
// required number of segments in wider bus
parameter SEGMENT_COUNT = EXPAND ? (M_STRB_WIDTH / S_STRB_WIDTH) : (S_STRB_WIDTH / M_STRB_WIDTH);
parameter SEGMENT_COUNT_WIDTH = SEGMENT_COUNT == 1 ? 1 : $clog2(SEGMENT_COUNT);
// data width and keep width per segment
parameter SEGMENT_DATA_WIDTH = DATA_WIDTH / SEGMENT_COUNT;
parameter SEGMENT_STRB_WIDTH = STRB_WIDTH / SEGMENT_COUNT;

// bus width assertions
initial begin
    if (S_WORD_SIZE * S_STRB_WIDTH != S_DATA_WIDTH) begin
        $error("Error: AXI slave interface data width not evenly divisble (instance %m)");
        $finish;
    end

    if (M_WORD_SIZE * M_STRB_WIDTH != M_DATA_WIDTH) begin
        $error("Error: AXI master interface data width not evenly divisble (instance %m)");
        $finish;
    end

    if (S_WORD_SIZE != M_WORD_SIZE) begin
        $error("Error: word size mismatch (instance %m)");
        $finish;
    end

    if (2**$clog2(S_WORD_WIDTH) != S_WORD_WIDTH) begin
        $error("Error: AXI slave interface word width must be even power of two (instance %m)");
        $finish;
    end

    if (2**$clog2(M_WORD_WIDTH) != M_WORD_WIDTH) begin
        $error("Error: AXI master interface word width must be even power of two (instance %m)");
        $finish;
    end
end

localparam [0:0]
    STATE_IDLE = 1'd0,
    STATE_DATA = 1'd1;

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

reg [SEGMENT_COUNT_WIDTH-1:0] current_segment_reg = 0, current_segment_next;

reg s_axil_arready_reg = 1'b0, s_axil_arready_next;
reg [S_DATA_WIDTH-1:0] s_axil_rdata_reg = {S_DATA_WIDTH{1'b0}}, s_axil_rdata_next;
reg [1:0] s_axil_rresp_reg = 2'd0, s_axil_rresp_next;
reg s_axil_rvalid_reg = 1'b0, s_axil_rvalid_next;

reg [ADDR_WIDTH-1:0] m_axil_araddr_reg = {ADDR_WIDTH{1'b0}}, m_axil_araddr_next;
reg [2:0] m_axil_arprot_reg = 3'd0, m_axil_arprot_next;
reg m_axil_arvalid_reg = 1'b0, m_axil_arvalid_next;
reg m_axil_rready_reg = 1'b0, m_axil_rready_next;

assign s_axil_arready = s_axil_arready_reg;
assign s_axil_rdata = s_axil_rdata_reg;
assign s_axil_rresp = s_axil_rresp_reg;
assign s_axil_rvalid = s_axil_rvalid_reg;

assign m_axil_araddr = m_axil_araddr_reg;
assign m_axil_arprot = m_axil_arprot_reg;
assign m_axil_arvalid = m_axil_arvalid_reg;
assign m_axil_rready = m_axil_rready_reg;

always @* begin
    state_next = STATE_IDLE;

    current_segment_next = current_segment_reg;

    s_axil_arready_next = 1'b0;
    s_axil_rdata_next = s_axil_rdata_reg;
    s_axil_rresp_next = s_axil_rresp_reg;
    s_axil_rvalid_next = s_axil_rvalid_reg && !s_axil_rready;
    m_axil_araddr_next = m_axil_araddr_reg;
    m_axil_arprot_next = m_axil_arprot_reg;
    m_axil_arvalid_next = m_axil_arvalid_reg && !m_axil_arready;
    m_axil_rready_next = 1'b0;

    if (SEGMENT_COUNT == 1 || EXPAND) begin
        // master output is same width or wider; single cycle direct transfer
        case (state_reg)
            STATE_IDLE: begin
                s_axil_arready_next = !m_axil_arvalid;

                if (s_axil_arready && s_axil_arvalid) begin
                    s_axil_arready_next = 1'b0;
                    m_axil_araddr_next = s_axil_araddr;
                    m_axil_arprot_next = s_axil_arprot;
                    m_axil_arvalid_next = 1'b1;
                    m_axil_rready_next = !m_axil_rvalid;
                    state_next = STATE_DATA;
                end else begin
                    state_next = STATE_IDLE;
                end
            end
            STATE_DATA: begin
                m_axil_rready_next = !s_axil_rvalid;

                if (m_axil_rready && m_axil_rvalid) begin
                    m_axil_rready_next = 1'b0;
                    if (M_WORD_WIDTH == S_WORD_WIDTH) begin
                        s_axil_rdata_next = m_axil_rdata;
                    end else begin
                        s_axil_rdata_next = m_axil_rdata >> (m_axil_araddr_reg[M_ADDR_BIT_OFFSET - 1:S_ADDR_BIT_OFFSET] * S_DATA_WIDTH);
                    end
                    s_axil_rresp_next = m_axil_rresp;
                    s_axil_rvalid_next = 1'b1;
                    s_axil_arready_next = !m_axil_arvalid;
                    state_next = STATE_IDLE;
                end else begin
                    state_next = STATE_DATA;
                end
            end
        endcase
    end else begin
        // master output is narrower; may need several cycles
        case (state_reg)
            STATE_IDLE: begin
                s_axil_arready_next = !m_axil_arvalid;

                current_segment_next = s_axil_araddr >> M_ADDR_BIT_OFFSET;
                s_axil_rresp_next = 2'd0;

                if (s_axil_arready && s_axil_arvalid) begin
                    s_axil_arready_next = 1'b0;
                    m_axil_araddr_next = s_axil_araddr;
                    m_axil_arprot_next = s_axil_arprot;
                    m_axil_arvalid_next = 1'b1;
                    m_axil_rready_next = !m_axil_rvalid;
                    state_next = STATE_DATA;
                end else begin
                    state_next = STATE_IDLE;
                end
            end
            STATE_DATA: begin
                m_axil_rready_next = !s_axil_rvalid;

                if (m_axil_rready && m_axil_rvalid) begin
                    m_axil_rready_next = 1'b0;
                    m_axil_araddr_next = (m_axil_araddr_reg & M_ADDR_MASK) + SEGMENT_STRB_WIDTH;
                    s_axil_rdata_next[current_segment_reg*SEGMENT_DATA_WIDTH +: SEGMENT_DATA_WIDTH] = m_axil_rdata;
                    current_segment_next = current_segment_reg + 1;
                    if (m_axil_rresp) begin
                        s_axil_rresp_next = m_axil_rresp;
                    end
                    if (current_segment_reg == SEGMENT_COUNT-1) begin
                        s_axil_rvalid_next = 1'b1;
                        s_axil_arready_next = !m_axil_arvalid;
                        state_next = STATE_IDLE;
                    end else begin
                        m_axil_arvalid_next = 1'b1;
                        state_next = STATE_DATA;
                    end
                end else begin
                    state_next = STATE_DATA;
                end
            end
        endcase
    end
end

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

    current_segment_reg <= current_segment_next;

    s_axil_arready_reg <= s_axil_arready_next;
    s_axil_rdata_reg <= s_axil_rdata_next;
    s_axil_rresp_reg <= s_axil_rresp_next;
    s_axil_rvalid_reg <= s_axil_rvalid_next;

    m_axil_araddr_reg <= m_axil_araddr_next;
    m_axil_arprot_reg <= m_axil_arprot_next;
    m_axil_arvalid_reg <= m_axil_arvalid_next;
    m_axil_rready_reg <= m_axil_rready_next;

    if (rst) begin
        state_reg <= STATE_IDLE;

        s_axil_arready_reg <= 1'b0;
        s_axil_rvalid_reg <= 1'b0;

        m_axil_arvalid_reg <= 1'b0;
        m_axil_rready_reg <= 1'b0;
    end
end

endmodule