// ***************************************************************************
// ***************************************************************************
// Copyright 2013(c) Analog Devices, Inc.
//  Author: Lars-Peter Clausen <lars@metafoo.de>
// 
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// ***************************************************************************
// ***************************************************************************

module fifo_address_gray (
	input m_axis_aclk,
	input m_axis_aresetn,
	input m_axis_ready,
	output reg m_axis_valid,
	output [C_ADDRESS_WIDTH-1:0] m_axis_raddr_next,

	input s_axis_aclk,
	input s_axis_aresetn,
	output reg s_axis_ready,
	input s_axis_valid,
	output reg s_axis_empty,
	output [C_ADDRESS_WIDTH-1:0] s_axis_waddr
);

parameter C_ADDRESS_WIDTH = 4;

reg [C_ADDRESS_WIDTH:0] _s_axis_waddr = 'h00;
reg [C_ADDRESS_WIDTH:0] _s_axis_waddr_next;

reg [C_ADDRESS_WIDTH:0] _m_axis_raddr = 'h00;
reg [C_ADDRESS_WIDTH:0] _m_axis_raddr_next;

reg [C_ADDRESS_WIDTH:0] s_axis_waddr_gray = 'h00;
wire [C_ADDRESS_WIDTH:0] s_axis_waddr_gray_next;
wire [C_ADDRESS_WIDTH:0] s_axis_raddr_gray;

reg [C_ADDRESS_WIDTH:0] m_axis_raddr_gray = 'h00;
wire [C_ADDRESS_WIDTH:0] m_axis_raddr_gray_next;
wire [C_ADDRESS_WIDTH:0] m_axis_waddr_gray;

assign s_axis_waddr = _s_axis_waddr[C_ADDRESS_WIDTH-1:0];
assign m_axis_raddr_next = _m_axis_raddr_next[C_ADDRESS_WIDTH-1:0];

always @(*)
begin
	if (s_axis_ready && s_axis_valid)
		_s_axis_waddr_next <= _s_axis_waddr + 1;
	else
		_s_axis_waddr_next <= _s_axis_waddr;
end

assign s_axis_waddr_gray_next = _s_axis_waddr_next ^ _s_axis_waddr_next[C_ADDRESS_WIDTH:1];

always @(posedge s_axis_aclk)
begin
	if (s_axis_aresetn == 1'b0) begin
		_s_axis_waddr <= 'h00;
		s_axis_waddr_gray <= 'h00;
	end else begin
		_s_axis_waddr <= _s_axis_waddr_next;
		s_axis_waddr_gray <= s_axis_waddr_gray_next;
	end
end

always @(*)
begin
	if (m_axis_ready && m_axis_valid)
		_m_axis_raddr_next <= _m_axis_raddr + 1;
	else
		_m_axis_raddr_next <= _m_axis_raddr;
end

assign m_axis_raddr_gray_next = _m_axis_raddr_next ^ _m_axis_raddr_next[C_ADDRESS_WIDTH:1];

always @(posedge m_axis_aclk)
begin
	if (m_axis_aresetn == 1'b0) begin
		_m_axis_raddr <= 'h00;
		m_axis_raddr_gray <= 'h00;
	end else begin
		_m_axis_raddr <= _m_axis_raddr_next;
		m_axis_raddr_gray <= m_axis_raddr_gray_next;
	end
end

sync_bits #(
	.NUM_BITS(C_ADDRESS_WIDTH + 1)
) i_waddr_sync (
	.out_clk(m_axis_aclk),
	.out_resetn(m_axis_aresetn),
	.in(s_axis_waddr_gray),
	.out(m_axis_waddr_gray)
);

sync_bits #(
	.NUM_BITS(C_ADDRESS_WIDTH + 1)
) i_raddr_sync (
	.out_clk(s_axis_aclk),
	.out_resetn(s_axis_aresetn),
	.in(m_axis_raddr_gray),
	.out(s_axis_raddr_gray)
);

always @(posedge s_axis_aclk)
begin
	if (s_axis_aresetn == 1'b0) begin
		s_axis_ready <= 1'b1;
		s_axis_empty <= 1'b1;
	end else begin
		s_axis_ready <= (s_axis_raddr_gray[C_ADDRESS_WIDTH] == s_axis_waddr_gray_next[C_ADDRESS_WIDTH] ||
			s_axis_raddr_gray[C_ADDRESS_WIDTH-1] == s_axis_waddr_gray_next[C_ADDRESS_WIDTH-1] ||
			s_axis_raddr_gray[C_ADDRESS_WIDTH-2:0] != s_axis_waddr_gray_next[C_ADDRESS_WIDTH-2:0]);
		s_axis_empty <= s_axis_raddr_gray == s_axis_waddr_gray_next;
	end
end

always @(posedge m_axis_aclk)
begin
	if (s_axis_aresetn == 1'b0)
		m_axis_valid <= 1'b0;
	else begin
		m_axis_valid <= m_axis_waddr_gray != m_axis_raddr_gray_next;
	end
end

endmodule