测频2

fpga通过pll倍频产生200M时钟，来测量高频率信号源，不知道为啥加到500M就有问题了，测量的fx比实际值要小，正常的话每一兆有3hz的误差，40M的话大约有120Hz的误差，满足2015年产生10-4的要求，贴一下代码。

module    pinlv(
        //system    interface
        input                    clk,
        input                    rst_n,
        //sig        interface
        input                    sig_in,
        //user        interface
        output    reg    [31:0]        pinlv

);

parameter    F_1S = 199_999_999; //200M
//fgate
reg        fgate;
reg        [31:0]        fgate_cnt;
always @(posedge clk or negedge rst_n)
begin
    if(!rst_n)
        fgate_cnt <= 0;
    else if(fgate_cnt >= F_1S)
        fgate_cnt <= 0;
    else
        fgate_cnt <= fgate_cnt + 1;
end

always @(posedge clk or negedge rst_n)
begin
    if(!rst_n)
        fgate <= 0;
    else if(fgate_cnt >= F_1S)
        fgate <= ~fgate;
    else
        fgate <= fgate;
end

//fact    gate
reg        start_cnt;
always @(posedge sig_in)
begin
    if(fgate)
        start_cnt <= 1;
    else
        start_cnt <= 0;
end
//fx++
reg        [31:0]        fx_cnt_temp;
always @(posedge sig_in or negedge rst_n)
begin
    if(!rst_n)
        fx_cnt_temp <= 0;
    else if(start_cnt)
        fx_cnt_temp <= fx_cnt_temp + 1;
    else
        fx_cnt_temp <= 0;
end
//锁存
always @(negedge start_cnt)
begin
    pinlv <= fx_cnt_temp;
end



endmodule