//============================================================================
// Implementation of the Sinclair ZX Spectrum ULA
//
// This module contains the clocks section.
//
// TODO: Video RAM contention would cause a clock gating which would be
// implemented in this module. RAM contention is not implemented since we are
// using FPGA RAM cells configured in dual-port mode.
//
//  Copyright (C) 2014-2016  Goran Devic
//
//  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 clocks
(
    input wire clk_ula,         // Input ULA clock of 14 MHz
    input wire turbo,           // Turbo speed (3.5 MHz x 2 = 7.0 MHz)
    output reg clk_cpu          // Output 3.5 MHz CPU clock
);

//~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
// Generate 3.5 MHz Z80 CPU clock by dividing input clock of 14 MHz by 4
//~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
reg [0:0] counter;

// Note: In order to get to 3.5 MHz, the PLL needs to be set to generate 14 MHz
// and then this divider-by-4 brings the effective clock down to 3.5 MHz
// 1. always block at positive edge of clk_ula divides by 2
// 2. counter flop further divides it by 2 unless the turbo mode is set
always @(posedge clk_ula)
begin
    if (counter=='0 | turbo)
        clk_cpu <= ~clk_cpu;
    counter <= counter - 1'b1;
end

endmodule