S03_CH04_AXI_DMA_OV5640摄像头采集系统信号延迟的问题

S03_CH04_AXI_DMA_OV5640摄像头采集系统的问题：


  我想问下  在count_reset模块中  parameter[19:0]num = 20'hffff0  我算了下这个参数 输入时钟25MHZ；  20'hffff 等于1048560 ; (1/25000000)*1048560 = 42ms;为什么延迟这么多呢？
   而 在cmos_decode模块中rst_n_i为什么反复寄存了四个时钟周期呢？

module count_reset#
(
    parameter[19:0]num = 20'hffff0
)(
    input clk_i,
    output rst_o
    );

reg[19:0] cnt = 20'd0;
reg rst_d0;

/*count for clock*/
always@(posedge clk_i)
begin
    cnt <= ( cnt <= num)?( cnt + 20'd1 ):num;
end

/*generate output signal*/
always@(posedge clk_i)
begin
    rst_d0 <= ( cnt >= num)?1'b1:1'b0;
end   

assign rst_o = rst_d0;

endmodule
我想问下    parameter[19:0]num = 20'hffff0  我算了下这个参数 输入时钟25MHZ；  20'hffff 等于1048560 ; (1/25000000)*1048560 = 42ms;为什么延迟这么多呢？
    在下面的模块中rst_n_i为什么反复寄存了四个时钟周期呢`timescale 1ns / 1ps
//////////////////////////////////////////////////////////////////////////////////
// Company: milinker corperation
// WEB:www.milinker.com
// BBS:www.osrc.cn
// Engineer:.
// Create Date:    07:28:50 09/04/2015  
// Design Name:  cmos_decode_v1
// Module Name:    cmos_decode_v1
// Project Name:  cmos_decode_v1
// Target Devices: XC6SLX25-FTG256 Mis603
// Tool versions:  ISE14.7
// Description:  cmos_decode_v1.
// Revision:  V1.0
// Additional Comments:  
//1) _i PIN input   
//2) _o PIN output
//3) _n PIN active low
//4) _dg debug signal  
//5) _r  reg delay
//6) _s state machine
//////////////////////////////////////////////////////////////////////////////
module cmos_decode(
//system signal.
input cmos_clk_i,//cmos senseor clock.
input rst_n_i,//system reset.active low.
//cmos sensor hardware interface.
input cmos_pclk_i,//input pixel clock.
input cmos_href_i,//input pixel hs signal.
input cmos_vsync_i,//input pixel vs signal.
input[7:0]cmos_data_i,//data.
output cmos_xclk_o,//output clock to cmos sensor.
//user interface.
output hs_o,//hs signal.
output vs_o,//vs signal.
output reg [15:0] rgb565_o,//data output
output vid_clk_ce
    );
parameter[5:0]CMOS_FRAME_WAITCNT = 4'd15;
reg[4:0] rst_n_reg = 5'd0;
//reset signal deal with.
always@(posedge cmos_clk_i)
begin
rst_n_reg <= {rst_n_reg[3:0],rst_n_i};
end
reg[1:0]vsync_d;
reg[1:0]href_d;
wire vsync_start;
wire vsync_end;
//vs signal deal with.
always@(posedge cmos_pclk_i)
begin
vsync_d <= {vsync_d[0],cmos_vsync_i};
href_d  <= {href_d[0],cmos_href_i};
end
assign vsync_start =  vsync_d[1]&(!vsync_d[0]);
assign vsync_end   = (!vsync_d[1])&vsync_d[0];
reg[6:0]cmos_fps;
//frame count.
always@(posedge cmos_pclk_i)
begin
if(!rst_n_reg[4])
begin
cmos_fps <= 7'd0;
end
else if(vsync_start)
begin
cmos_fps <= cmos_fps + 7'd1;
end
else if(cmos_fps >= CMOS_FRAME_WAITCNT)
begin
cmos_fps <= CMOS_FRAME_WAITCNT;
end
end
//wait frames and output enable.
reg out_en;
always@(posedge cmos_pclk_i)
begin
if(!rst_n_reg[4])
begin
out_en <= 1'b0;
end
else if(cmos_fps >= CMOS_FRAME_WAITCNT)
begin
out_en <= 1'b1;
end
else
begin
out_en <= out_en;
end
end
//output data 8bit changed into 16bit in rgb565.
reg [7:0] cmos_data_d0;
reg [15:0]cmos_rgb565_d0;
reg byte_flag;
always@(posedge cmos_pclk_i)
begin
if(!rst_n_reg[4])
byte_flag <= 0;
else if(cmos_href_i)
byte_flag <= ~byte_flag;
else
byte_flag <= 0;
end
reg byte_flag_r0;
always@(posedge cmos_pclk_i)
begin
if(!rst_n_reg[4])
byte_flag_r0 <= 0;
else  
byte_flag_r0 <= byte_flag;
end
always@(posedge cmos_pclk_i)
begin
if(!rst_n_reg[4])
cmos_data_d0 <= 8'd0;
else if(cmos_href_i)
cmos_data_d0 <= cmos_data_i; //MSB -> LSB
else if(~cmos_href_i)  
cmos_data_d0 <= 8'd0;
end
always@(posedge cmos_pclk_i)
begin
if(!rst_n_reg[4])
rgb565_o <= 16'd0;
else if(cmos_href_i&byte_flag)
rgb565_o <= {cmos_data_d0,cmos_data_i}; //MSB -> LSB
else if(~cmos_href_i)  
rgb565_o <= 8'd0;
end
assign vid_clk_ce = out_en ? (byte_flag_r0&hs_o)||(!hs_o) : 1'b0;
assign vs_o = out_en ? vsync_d[1] : 1'b0;
assign hs_o = out_en ? href_d[1] : 1'b0;
assign cmos_xclk_o = cmos_clk_i;
endmodule

不错，感谢