/**
 * Specialized SPI slave.
 * Reads width bits at a time, and sets the ready flag
 * whenever a full 32 bits has been read. Also, recognizes
 * 0x00 as a pattern, and when full 0s are read, sets the done flag.
 * 0x00 is a special value in the CPU programming process that indicates
 * end-of-program.
 *
 * master_clk, ss, and mosi are all SPI-specific inputs.
 * data should only be read when ready is high.
 */
module spi_slave #(width=32)
				  (input logic clk, reset,
				   input logic master_clk, ss, mosi,
				   output logic ready,
				   output logic done,
				   output logic [width-1:0] data);
	logic [width-1:0] storage;
	logic unsigned [$clog2(width)-1:0] counter;
	logic old_clk;

	always_ff@(posedge clk)
		if(reset) begin
			counter <= 0;
			old_clk <= 0;
			storage <= 0;
			done <= 0;
			ready <= 0;
		end else begin
			if (~ss & master_clk & ~old_clk) begin
				storage <= storage << 1 | mosi;
				if (counter == width - 1) begin
					ready <= 1;
					done <= ~(|storage);
					counter <= 0;
				end else begin
					done <= 0;
					ready <= 0;
					counter <= counter + 1;
				end
			end
			old_clk <= master_clk;
		end

	assign data = storage;

endmodule