Add SD Card controller for sending commands

Adds the start of the SD card controller which is capable of sending
commands using the SD protocol.

It is accessed by writing the arguments first and triggered by writing
the command number.

hw/fpga/sd_controller.sv

@@ -0,0 +1,124 @@
+module sd_controller(
+    input clk,
+    input rst,
+
+    input [3:0] addr,
+    input [7:0] data,
+    input cs,
+
+    input i_sd_cmd,
+    output logic o_sd_cmd,
+
+    input i_sd_data,
+    output logic o_sd_dat
+);
+
+logic [31:0] arg;
+logic [5:0] cmd;
+
+logic [47:0] rxcmd_buf;
+
+typedef enum bit [1:0] {IDLE, CRC, TXCMD, RXCMD} macro_t;
+struct packed {
+    macro_t macro;
+    logic [5:0] count;
+} state, next_state;
+
+always_ff @(posedge clk) begin
+    if (rst) begin
+        state.macro <= IDLE;
+        state.count <= '0;
+    end else begin
+        state <= next_state;
+    end
+
+    if (state.macro == IDLE) begin
+        if (cs) begin
+            if (addr < 4'h4) begin
+                arg[8 * addr +: 8] <= data;
+            end else if (addr == 4'h4) begin
+                cmd <= data;
+            end
+        end
+    end else if (state.macro == RXCMD) begin
+        rxcmd_buf[6'd46-state.count] <= i_sd_cmd;   //we probabily missed bit 47
+    end
+end
+
+logic [6:0] crc;
+logic load_crc;
+logic crc_valid;
+logic [39:0] _packet;
+assign _packet = {1'b0, 1'b1, cmd, arg};
+logic [47:0] packet_crc;
+assign packet_crc = {_packet, crc, 1'b1};
+
+crc7 u_crc7(
+    .clk(clk),
+    .rst(rst),
+    .load(load_crc),
+    .data_in(_packet),
+    .crc_out(crc),
+    .valid(crc_valid)
+);
+
+always_comb begin
+    next_state = state;
+
+    case (state.macro)
+        IDLE: begin
+            if (~i_sd_cmd) begin        // receive data if sd pulls cmd low
+                next_state.macro = RXCMD;
+            end
+
+            if (addr == 4'h4 & cs) begin     // transmit if cpu writes to cmd
+                next_state.macro = CRC;
+            end
+        end
+
+        CRC: begin
+            next_state.macro = TXCMD;
+        end
+
+        TXCMD: begin
+            if (state.count < 47) begin
+                next_state.count = state.count + 6'b1;
+            end else begin
+                next_state.macro = IDLE;
+                next_state.count = '0;
+            end
+        end
+
+        RXCMD: begin
+            if (state.count < 47) begin
+                next_state.count = state.count + 6'b1;
+            end else begin
+                next_state.macro = IDLE;
+                next_state.count = '0;
+            end
+        end
+    endcase
+end
+
+always_comb begin
+    o_sd_cmd = '1;  //default to 1
+    o_sd_dat = '1;
+
+    load_crc = '0;
+
+    case (state.macro)
+        IDLE:;
+
+        CRC: begin
+            load_crc = '1;
+        end
+
+        TXCMD: begin
+            o_sd_cmd = packet_crc[6'd47 - state.count];
+        end
+
+        RXCMD:;
+    endcase
+end
+
+endmodule