Subversion Repositories pentevo

// ZX-Evo Base Configuration (c) NedoPC 2008,2009,2010,2011,2012,2013,2014,2015,2016

//

// NMI generation

/*

    This file is part of ZX-Evo Base Configuration firmware.

    ZX-Evo Base Configuration firmware 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 3 of the License, or

    (at your option) any later version.

    ZX-Evo Base Configuration firmware 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 ZX-Evo Base Configuration firmware.

    If not, see <http://www.gnu.org/licenses/>.

*/

`include "../include/tune.v"

module znmi

(

        input  wire        rst_n,

        input  wire        fclk,

        input  wire        zpos,

        input  wire        zneg,

        input  wire        int_start, // when INT starts

        input  wire [ 1:0] set_nmi,   // NMI requests from slavespi and #BF port

        input  wire        imm_nmi,   // immediate NMI from breakpoint

        input  wire        clr_nmi, // clear nmi: from zports, pulsed at out to #xxBE

        input  wire        rfsh_n,

        input  wire        m1_n,

        input  wire        mreq_n,

        input  wire        csrom,

        input  wire [15:0] a,

        output wire drive_00, // drive nop to Z80 databus

        output reg         in_nmi,  // when 1, there must be last (#FF) ram page in 0000-3FFF

        output wire        gen_nmi, // NMI generator: when 1, NMI_N=0, otherwise NMI_N=Z

        output wire        nmi_buf_clr // clear ram read buffer in zmem during nmi entry

);

        reg  [1:0] set_nmi_r;

        wire       set_nmi_now;

        reg        imm_nmi_r;

        wire       imm_nmi_now;

        reg pending_nmi;

        reg in_nmi_2; // active (=1) when NMIed to ROM, after 0066 M1 becomes 0,

                      // but in_nmi becomes 1 -- ROM switches to #FF RAM

        reg [2:0] nmi_count;

        reg [1:0] clr_count;

        reg pending_clr;

        reg last_m1_rom;

        reg last_m1_0066;

        wire nmi_start;

        //remember whether last M1 opcode read was from ROM or RAM

        reg m1_n_reg, mreq_n_reg;

        reg [1:0] rfsh_n_reg;

        always @(posedge fclk) if( zpos )

                rfsh_n_reg[0] <= rfsh_n;

        always @(posedge fclk)

                rfsh_n_reg[1] <= rfsh_n_reg[0];

        always @(posedge fclk) if( zpos )

                m1_n_reg <= m1_n;

        always @(posedge fclk) if( zneg )

                mreq_n_reg <= mreq_n;

        wire was_m1 = ~(m1_n_reg | mreq_n_reg);

        reg was_m1_reg;

        always @(posedge fclk)

                was_m1_reg <= was_m1;

        always @(posedge fclk)

        if( was_m1 && (!was_m1_reg) )

                last_m1_rom <= csrom && (a[15:14]==2'b00);

        always @(posedge fclk)

        if( was_m1 && (!was_m1_reg) )

                last_m1_0066 <= ( a[15:0]==16'h0066 );

        always @(posedge fclk)

                set_nmi_r <= set_nmi;

        //

        assign set_nmi_now = | (set_nmi_r & (~set_nmi) );

        always @(posedge fclk)

                imm_nmi_r <= imm_nmi;

        //

        assign imm_nmi_now = | ( (~imm_nmi_r) & imm_nmi );

        always @(posedge fclk, negedge rst_n)

        if( !rst_n )

                pending_nmi <= 1'b0;

        else // posedge clk

        begin

                if( int_start )

                        pending_nmi <= 1'b0;

                else if( set_nmi_now )

                        pending_nmi <= 1'b1;

        end

        // actual nmi start

        assign nmi_start = (pending_nmi && int_start) || imm_nmi_now;

        always @(posedge fclk)

        if( clr_nmi )

                clr_count <= 2'd3;

        else if( rfsh_n_reg[1] && (!rfsh_n_reg[0]) && clr_count[1] )

                clr_count <= clr_count - 2'd1;

        always @(posedge fclk)

        if( clr_nmi )

                pending_clr <= 1'b1;

        else if( !clr_count[1] )

                pending_clr <= 1'b0;

        always @(posedge fclk, negedge rst_n)

        if( !rst_n )

                in_nmi_2 <= 1'b0;

        else // posedge fclk

        begin

                if( nmi_start && (!in_nmi) /*&& last_m1_rom*/ ) // fix for NMI page after execution of 0066 opcode everywhere

                        in_nmi_2 <= 1'b1;

                else if( rfsh_n_reg[1] && (!rfsh_n_reg[0]) && last_m1_0066 )

                        in_nmi_2 <= 1'b0;

        end

        assign drive_00 = in_nmi_2 && (!m1_n) && (!mreq_n) && (a[15:0]==16'h0066); // && last_m1_0066;

        assign nmi_buf_clr = last_m1_0066 && in_nmi_2;

        always @(posedge fclk, negedge rst_n)

        if( !rst_n )

                in_nmi <= 1'b0;

        else // posedge clk

        begin

                if( pending_clr && (!clr_count[1]) )

                        in_nmi <= 1'b0;

                else if( /*(nmi_start && (!in_nmi) && (!last_m1_rom))                      ||*/

                         (rfsh_n_reg[1] && (!rfsh_n_reg[0]) && last_m1_0066 && in_nmi_2) )

                        in_nmi <= 1'b1;

        end

        always @(posedge fclk, negedge rst_n)

        if( !rst_n )

                nmi_count <= 3'b000;

        else if( nmi_start && (!in_nmi) )

                nmi_count <= 3'b111;

        else if( nmi_count[2] && zpos )

                nmi_count <= nmi_count - 3'd1;

        assign gen_nmi = nmi_count[2];

endmodule