// ZX-Evo Base Configuration (c) NedoPC 2008,2009,2010,2011,2012,2013,2014
//
// Z80 clocking module, also contains some wait-stating when 14MHz
/*
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/>.
*/
//
// IDEAL:
// fclk _/`\_/`\_/`\_/`\_/`\_/`\_/`\_/`\_/`\_/`\_/`\_/`\_/`\_/`\_/`\_/`\_/`\_/`\_/`\_/`\_/`\_/`\_/`\_/`\_/`\_/`\_/`\_/`\_/`\_/`\_/`\
// | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
// zclk /```\___/```\___/```\___/```````\_______/```````\_______/```````````````\_______________/```````````````\_______________/`
// | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
// zpos `\___/```\___/```\___/```\___________/```\___________/```\___________________________/```\___________________________/```\
// | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
// zneg _/```\___/```\___/```\_______/```\___________/```\___________________/```\___________________________/```\________________
// clock phasing:
// cend must be zpos for 7mhz, therefore post_cbeg - zneg
// for 3.5 mhz, cend is both zpos and zneg (alternating)
// 14MHz rulez:
// 1. do variable stalls for memory access.
// 2. do fallback on 7mhz for external IO accesses
// 3. clock switch 14-7-3.5 only at RFSH
`include "../include/tune.v"
module zclock(
input fclk,
input rst_n,
input zclk, // Z80 clock, buffered via act04 and returned back to the FPGA
input rfsh_n, // switch turbo modes in RFSH part of m1
output reg zclk_out, // generated Z80 clock - passed through inverter externally!
output reg zpos,
output reg zneg,
input wire zclk_stall,
input [1:0] turbo, // 2'b00 - 3.5 MHz
// 2'b01 - 7.0 MHz
// 2'b1x - 14.0 MHz
output reg [1:0] int_turbo, // internal turbo, switched on /RFSH
// input signals for 14MHz external IORQ waits
input wire external_port,
input wire iorq_n,
input wire m1_n,
input cbeg,
input pre_cend // syncing signals, taken from arbiter.v and dram.v
);
reg precend_cnt;
wire h_precend_1; // to take every other pulse of pre_cend
wire h_precend_2; // to take every other pulse of pre_cend
reg [2:0] zcount; // counter for generating 3.5 and 7 MHz z80 clocks
reg old_rfsh_n;
wire stall;
reg clk14_src; // source for 14MHz clock
wire pre_zpos_35,
pre_zneg_35;
wire pre_zpos_70,
pre_zneg_70;
wire pre_zpos_140,
pre_zneg_140;
`ifdef SIMULATE
initial // simulation...
begin
precend_cnt = 1'b0;
int_turbo = 2'b00;
old_rfsh_n = 1'b1;
clk14_src = 1'b0;
zclk_out = 1'b0;
end
`endif
// switch clock only at predefined time
always @(posedge fclk) if(zpos)
begin
old_rfsh_n <= rfsh_n;
if( old_rfsh_n && !rfsh_n )
int_turbo <= turbo;
end
// make 14MHz iorq wait
reg [3:0] io_wait_cnt;
reg io_wait;
wire io;
reg io_r;
assign io = (~iorq_n) & m1_n & external_port;
always @(posedge fclk)
if( zpos )
io_r <= io;
always @(posedge fclk, negedge rst_n)
if( ~rst_n )
io_wait_cnt <= 4'd0;
else if( io && (!io_r) && zpos && int_turbo[1] )
io_wait_cnt[3] <= 1'b1;
else if( io_wait_cnt[3] )
io_wait_cnt <= io_wait_cnt + 4'd1;
always @(posedge fclk)
case( io_wait_cnt )
4'b1000: io_wait <= 1'b1;
4'b1001: io_wait <= 1'b1;
4'b1010: io_wait <= 1'b1;
4'b1011: io_wait <= 1'b1;
4'b1100: io_wait <= 1'b1;
4'b1101: io_wait <= 1'b0;
4'b1110: io_wait <= 1'b1;
4'b1111: io_wait <= 1'b0;
default: io_wait <= 1'b0;
endcase
assign stall = zclk_stall | io_wait;
// 14MHz clocking
always @(posedge fclk)
if( !stall )
clk14_src <= ~clk14_src;
//
assign pre_zpos_140 = clk14_src ;
assign pre_zneg_140 = (~clk14_src);
// take every other pulse of pre_cend (make half pre_cend)
always @(posedge fclk) if( pre_cend )
precend_cnt <= ~precend_cnt;
assign h_precend_1 = precend_cnt && pre_cend;
assign h_precend_2 = !precend_cnt && pre_cend;
assign pre_zpos_35 = h_precend_2;
assign pre_zneg_35 = h_precend_1;
assign pre_zpos_70 = pre_cend;
assign pre_zneg_70 = cbeg;
assign pre_zpos = int_turbo[1] ? pre_zpos_140 : ( int_turbo[0] ? pre_zpos_70 : pre_zpos_35 );
assign pre_zneg = int_turbo[1] ? pre_zneg_140 : ( int_turbo[0] ? pre_zneg_70 : pre_zneg_35 );
always @(posedge fclk)
begin
zpos <= (~stall) & pre_zpos & zclk_out;
end
always @(posedge fclk)
begin
zneg <= (~stall) & pre_zneg & (~zclk_out);
end
// make Z80 clock: account for external inversion and make some leading of clock
// 9.5 ns propagation delay: from fclk posedge to zclk returned back any edge
// (1/28)/2=17.9ns half a clock lead
// 2.6ns lag because of non-output register emitting of zclk_out
// total: 5.8 ns lead of any edge of zclk relative to posedge of fclk => ACCOUNT FOR THIS WHEN DOING INTER-CLOCK DATA TRANSFERS
//
always @(negedge fclk)
begin
if( zpos )
zclk_out <= 1'b0;
if( zneg )
zclk_out <= 1'b1;
end
endmodule