most of the way there

Verilator_Tests/spi/spi_master.v

@@ -0,0 +1,81 @@
+module spi_master (
+    input  wire        clk,
+
+    output reg         spi_clk = 0,
+
+    input  wire        start, // Pulse high to begin transfer
+    input reg [7:0]    data_out = 8'd0,
+    output reg         spi_mosi = 0,
+
+    output  wire [7:0]  data_in,
+    input  wire        spi_miso,
+
+    output reg         busy = 0,
+    output reg         spi_cs = 1 // Active low
+);
+// SPI Mode 0: CPOL = 0, CPHA = 0 (drive on falling, sample on rising)
+reg [3:0] bit_cnt = 0;
+reg [7:0] shift_reg_in  = 0;
+reg [7:0] shift_reg_out = 0;
+
+typedef enum logic [1:0] {
+    IDLE,
+    CHIP_SEL,
+    BUSY,
+    DONE
+} state_t;
+
+state_t state = IDLE;
+
+always @(posedge clk) begin
+    case (state)
+        IDLE: begin
+            spi_cs  <= 1;
+            spi_clk <= 0;
+            busy    <= 0;
+
+            if (start) begin
+                state         <= CHIP_SEL;
+                shift_reg_out <= data_out;
+                bit_cnt       <= 0;
+            end
+        end
+
+        // data is one cycle after CS is brought low
+        CHIP_SEL: begin
+            spi_cs  <= 0;
+            busy    <= 1;
+            state   <= BUSY;
+        end
+
+        BUSY: begin
+            if (clk) begin
+                spi_clk <= ~spi_clk;
+
+                // Falling, drive data
+                if (!spi_clk) begin
+                    spi_mosi <= shift_reg_out[7];
+                    shift_reg_out <= {shift_reg_out[6:0], 1'b0};
+
+                end else begin
+                    // Rising, sample data
+                    shift_reg_in <= {shift_reg_in[6:0], spi_miso};
+                    bit_cnt <= bit_cnt + 1;
+
+                    if (bit_cnt == 7) begin
+                        state <= DONE;
+                    end
+                end
+            end
+        end
+
+        DONE: begin
+            spi_cs   <= 1;
+            data_in  <= {shift_reg_in[6:0], spi_miso};
+            state    <= IDLE;
+        end
+    endcase
+end
+
+endmodule
+

Verilator_Tests/spi/top.v

@@ -1,62 +1,36 @@
 module top (
-    input wire clk_25mhz,
+    input wire  clk_25mhz,
 
     output wire spi_clk,
     output wire spi_mosi,
-    input wire spi_miso,
+    input wire  spi_miso,
     output wire spi_cs
 );
 
-reg[7:0] miso;
-reg[7:0] mosi = 8'b01101011;
+// we would prefer fifo
+wire[7:0] miso;
+wire[7:0] mosi = 8'b01101011;
+
+reg busy = 0;
+reg start = 0;
+
+always @(posedge clk_25mhz) begin
+        start <= 0;
+    if (!busy && !start) begin
+        start <= 1;
+    end
+end
 
 spi_master spimaster0(
-    .clk_25mhz(clk_25mhz),
+    .clk(clk_25mhz),
     .spi_clk(spi_clk),
-    .data_in(mosi),
-    .spi_miso(spi_miso),
+    .start(start),
     .data_out(mosi),
     .spi_mosi(spi_mosi),
+    .data_in(miso),
+    .spi_miso(spi_miso),
+    .busy(busy),
     .spi_cs(spi_cs)
 );
 
 endmodule
-
-
-module spi_master (
-    input wire clk_25mhz,
-
-    output reg spi_clk = 0,
-
-    output reg[7:0] data_in,
-    output reg spi_miso = 0,
-
-    input reg[7:0] data_out,
-    input reg spi_mosi = 0,
-
-    output wire spi_cs
-);
-
-
-assign spi_cs = 0;  // Always selected (for test)
-
-reg [7:0] mosi_shift = 8'b01101011;  // Example byte
-reg [3:0] bit_counter = 0;
-reg spi_clk_en = 1;
-
-localparam TRANSFERRING = 0, IDLE = 1;
-wire spi_state = IDLE;
-
-always_ff @(posedge clk_25mhz) begin
-    spi_clk <= ~spi_clk;
-
-    if (spi_clk == 0) begin
-        // Falling edge: shift data
-        spi_mosi <= mosi_shift[7];
-        mosi_shift <= {mosi_shift[6:0], 1'b0};
-        bit_counter <= bit_counter + 1;
-    end
-end
-
-endmodule
-