SPIREG is an SPI slave controller that can be used for register level access for user logic

• Easy interoperating with Arduino's SPI
• Supports register widths of N * 8 bits and 1 to 64 registers
• The status byte can be obtained when the first byte is read
• FastCmd can send command code when the first byte is written
• Very low resources overhead (~100LUT, ~200FF)

MCU <--SPI--> SPIREG <--regaccess--> User logic

• SPI: MOSI MISO SCLK NSS
• regaccess: reg_addr, reg_data_i, reg_data_o, reg_data_o_vld
• regaccess(additional): status, fastcmd, fastcmd_vld

• Databits = 8, MSB First, Mode = 0 (CPOL=Low, CPHA=1edge)
• sclk_freq <= clk / 2

• Read data cmd = addr, addr = 0 ~ 63

    MOSI: |addr   |0xff        |0xff         |0xff          |0xff           | ...
    MISO: |status |d[addr][7:0]|d[addr][15:8]|d[addr+1][7:0]|d[addr+1][15:8]| ...

• Write data cmd = 0x80+addr, addr = 0 ~ 63

    MOSI: |cmd    |d[addr][7:0]|d[addr][15:8]|d[addr+1][7:0]|d[addr+1][15:8]| ...
    MISO: |status |0x00        |0x00         |0x00          |0x00           | ...

• Fast command cmd = 0xc0+fastcmd, fastcmd = 0 ~ 63

    MOSI: |cmd    |
    MISO: |status |

• Status query

    MOSI: |0x00   |
    MISO: |status |

TBD

wire [3:0] reg_addr;
wire [15:0] reg_data_i, reg_data_o;
wire reg_data_o_vld;
wire [7:0] status;
wire [5:0] fastcmd;
wire fastcmd_vld;
reg [15:0] mem [0:15];

spireg #(
    .REG_W(REG_WIDTH)
) spireg_inst(
    .clk(clk),
    .nrst(nrst),
    .mosi(mosi),
    .miso(miso),
    .sclk(sclk),
    .nss(nss),
    .reg_addr(reg_addr),
    .reg_data_i(reg_data_i),
    .reg_data_o(reg_data_o),
    .reg_data_o_vld(reg_wr),
    .status(status),
    .fastcmd(),
    .fastcmd_vld()
);

//status signals
assign status = mem[0][7:0];

//register read access
assign reg_data_i = mem[reg_addr];

integer i;
always @(posedge clk or negedge nrst)
if(!nrst) begin
    for(i = 0; i < 2**ADDR_WIDTH; i = i+1)
        mem[i] <= 0;
end else begin
    if(reg_wr) begin
    //register write access
        mem[reg_addr] <= reg_data_o;
    end
end

Check out spireg_example.v for "fastcmd" feature

Tested Device: Xilinx XC7S6 User registers: 16*16bits LUT used: 112 of 3750 FF used: 199 of 7500