Add CRC generation for sd card

Works great according to the testbench

design/sd/crc_gen.sv

@@ -0,0 +1,41 @@
+// parameterizable sequential crc generator
+// This probably could be combinational logic too, though it would be way slow
+module crc_gen #(
+    parameter CRCBITS=7,
+    parameter COMMANDLEN=40,
+    parameter POLYNOMIAL='h89
+) (
+    input clk,
+    input reset,
+    input start,
+    input [COMMANDLEN-1:0] num,
+    output logic ready,
+    output logic [CRCBITS-1:0] crc
+);
+
+logic [$clog2(COMMANDLEN):0] counter;
+logic [COMMANDLEN+CRCBITS-1:0] div_reg;
+
+always_ff @(posedge clk) begin
+    if (reset) begin
+        counter <= 0;
+        crc <= 0;
+    end
+    else if (start) begin
+        counter <= COMMANDLEN;
+        div_reg <= {num, {CRCBITS{1'b0}}};
+        ready <= 0;
+    end
+    else if (counter != 0) begin
+        if (div_reg[counter+CRCBITS-1] == 1) begin
+            div_reg <= div_reg ^ (POLYNOMIAL << (counter-1));
+        end
+        counter <= counter - 1;
+    end
+    else begin
+        ready <= 1;
+        crc <= div_reg[CRCBITS-1:0];
+    end
+end
+
+endmodule

design/sd/send_command.sv

@@ -0,0 +1,14 @@
+module send_command(
+    input clk, 
+    input reset,
+    input start,
+    input [5:0] command,
+    input [31:0] arguments
+);
+
+
+// Some commands have hardcoded crcs, and as such they can be found in a LUT,
+// otherwise we need to shell out to the crc module and wait a while for it to
+// run
+
+endmodule