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// PentEvo project (c) NedoPC 2008-2009
//
// Z80 clocking module, also contains some wait-stating when 14MHz
//
// 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)
// FIXME FIXME FIXME FIXME FIXME FIXME FIXME FIXME FIXME FIXME FIXME FIXME FIXME FIXME FIXME
// CURRENTLY ONLY 3.5 and 7 MHz!!!! FIXME FIXME FIXME FIXME FIXME FIXME FIXME FIXME FIXME FIXME
// FIXME FIXME FIXME FIXME FIXME FIXME FIXME FIXME FIXME FIXME FIXME FIXME FIXME FIXME FIXME
`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 [1:0] turbo, // 2'b00 - 3.5 MHz
// 2'b01 - 7.0 MHz
// 2'b1x - 14.0 MHz
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 [1:0] int_turbo; // internal turbo, controlling muxes
reg old_rfsh_n;
`ifdef SIMULATE
initial // simulation...
begin
precend_cnt = 1'b0;
int_turbo = 2'b00;
old_rfsh_n = 1'b1;
end
`endif
// 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;
/* // phase zcount to take from it proper 3.5 or 7 MHz clock
always @(posedge fclk)
begin
if( half_precend )
zcount <= 3'd7;
else
zcount <= zcount - 3'd1;
end
*/
// switch between 3.5 and 7 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
/* always @(posedge fclk) if( h_precend_1 )
int_turbo <= turbo;
*/
always @(posedge fclk)
begin
if( (pre_cend && int_turbo[0]) || (h_precend_2 && !int_turbo[0]) )
zpos <= 1'b1;
else
zpos <= 1'b0;
end
always @(posedge fclk)
begin
if( (cbeg && int_turbo[0]) || (h_precend_1 && !int_turbo[0]) )
zneg <= 1'b1;
else
zneg <= 1'b0;
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)
if( int_turbo[0] ) // 7 MHz
zclk_out <= ~zcount[1];
else // 3.5 MHz
zclk_out <= ~zcount[2];
*/
always @(negedge fclk)
begin
if( zpos )
zclk_out <= 1'b0;
if( zneg )
zclk_out <= 1'b1;
end
endmodule