symbiface_mouse.v

////////////////////////////////////////////////////////////////////////////////
//
//  SYMBiFACE II: PS2 Mouse
//  (C) 2018 Sorgelig
//
//  This program is free software; you can redistribute it and/or modify it
//  under the terms of the GNU General Public License as published by the Free
//  Software Foundation; either version 2 of the License, or (at your option)
//  any later version.
//
//  This program is distributed in the hope that it will be useful, but WITHOUT
//  ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
//  FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
//  more details.
//
//  You should have received a copy of the GNU General Public License along
//  with this program; if not, write to the Free Software Foundation, Inc.,
//  51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
//
////////////////////////////////////////////////////////////////////////////////

module symbiface_mouse
(
	input        clk_sys,
	input        reset,

	input [24:0] ps2_mouse,

	input        sel,
	output [7:0] dout
);

assign dout = data;
reg [7:0] data;

wire signed [8:0] dxp = {ps2_mouse[4],ps2_mouse[15:8]};
wire signed [5:0] dx  = (dxp > 31) ? 6'd31 : (dxp < -32) ? -6'd32 : dxp[5:0];

wire signed [8:0] dyp = {ps2_mouse[5],ps2_mouse[23:16]};
wire signed [5:0] dy  = (dyp > 31) ? 6'd31 : (dyp < -32) ? -6'd32 : dyp[5:0];

always @(posedge clk_sys) begin
	reg [2:0] avail;
	reg old_status, old_sel;

	old_status <= ps2_mouse[24];
	if(old_status != ps2_mouse[24]) avail <= {|dy, |dx, 1'b1};

	old_sel <= sel;
	if(~old_sel & sel) begin
		casex(avail)
			'b1xx: {avail[2],data} <= {3'b0_10, dy};
			'b01x: {avail[1],data} <= {3'b0_01, dx};
			'b001: {avail[0],data} <= {3'b0_11, 3'b000, ps2_mouse[2:0]};
			'b000: data <= 0;
		endcase
	end

	if(~sel)	data <= 8'hFF;
	if(reset) avail <= 0;
end

endmodule