spi/spi_slave.v

/* (c) Peter McGoron 2022 v0.4
 *
 * This code is disjunctively dual-licensed under the MPL v2.0, or the
 * CERN-OHL-W v2.
 */

module spi_slave #(
	parameter ENABLE_MISO = 1, // Enable MISO and to_master port
	parameter ENABLE_MOSI = 1, // Enable MOSI and from_master port
	parameter WID = 24, // Width of bits per transaction.
	parameter WID_LEN = 5, // Length in bits required to store WID
	parameter POLARITY = 0,
	parameter PHASE = 0 // 0 = rising-read falling-write, 1 = rising-write falling-read.
) (
	input clk,
	input rst_L,
	input sck,
	input ss_L,

	output reg [WID-1:0] from_master,
	input mosi,

	input [WID-1:0] to_master,
	output reg miso,

	output reg finished,
	input rdy,
	output reg err
);

/* MOSI is almost always an external wire, so buffer it. */
reg mosi_hot = 0;
reg read_mosi = 0;

always @ (posedge clk) if (ENABLE_MOSI == 1) begin
	read_mosi <= mosi_hot;
	mosi_hot <= mosi;
end

wire ss = !ss_L;
reg sck_delay = 0;
reg [WID_LEN-1:0] bit_counter = 0;
reg ss_delay = 0;
reg ready_at_start = 0;

`ifndef SPI_SLAVE_NO_WRITE
reg [WID-1:0] send_buf = 0;
`endif

task read_data();
	if (ENABLE_MOSI == 1) begin
		from_master <= from_master << 1;
		from_master[0] <= read_mosi;
	end
endtask

task write_data();
	if (ENABLE_MISO == 1) begin
		send_buf <= send_buf << 1;
		miso <= send_buf[WID-1];
	end
endtask

task setup_bits();
	if (ENABLE_MISO == 1) begin
		/* at Mode 00, the transmission starts with
		 * a rising edge, and at mode 11, it starts with a falling
		 * edge. For both modes, these are READs.
		 *
		 * For mode 01 and mode 10, the first action is a WRITE.
		 */
		if (POLARITY == PHASE) begin
			miso <= to_master[WID-1];
			send_buf <= to_master << 1;
		end else begin
			send_buf <= to_master;
		end
	end
endtask

task check_counter();
	if (bit_counter == WID[WID_LEN-1:0]) begin
		err <= ready_at_start;
	end else begin
		bit_counter <= bit_counter + 1;
	end
endtask

always @ (posedge clk) begin
	if (!rst_L) begin
		sck_delay <= 0;
		bit_counter <= 0;
		ss_delay <= 0;
		ready_at_start <= 0;

		if (ENABLE_MOSI == 1) from_master <= 0;

		if (ENABLE_MISO == 1) begin
			miso <= 0;
			send_buf <= 0;
		end

		finished <= 0;
		err <= 0;
	end else begin
		sck_delay <= sck;
		ss_delay <= ss;

		case ({ss_delay, ss})
		2'b01: begin // rising edge of SS
			bit_counter <= 0;
			finished <= 0;
			err <= 0;
			ready_at_start <= rdy;

			setup_bits();
		end
		2'b10: begin // falling edge
			finished <= ready_at_start;
		end
		2'b11: begin
			case ({sck_delay, sck})
			2'b01: begin // rising edge
				if (PHASE == 1) begin
					write_data();
				end else begin
					read_data();
				end

				if (POLARITY == 0) begin
					check_counter();
				end
			end
			2'b10: begin // falling edge
				if (PHASE == 1) begin
					read_data();
				end else begin
					write_data();
				end

				if (POLARITY == 1) begin
					check_counter();
				end
			end
			default: ;
			endcase
		end
		2'b00: if (!rdy) begin
			finished <= 0;
			err <= 0;
		end
		endcase
	end
end

endmodule