要求：

　　用 Verilog 實現 glitch free 時鐘切換電路。輸入 sel，clka，clkb，sel 為 1 輸出 clka，sel 為 0 輸出 clkb。

module Change_Clk_Source
(
    input   rst_n,
    input   clk_a,
    input   clk_b,
    input   sel,
    output  clk_o
);

一、有毛刺寫法

assign clk_o = sel ? clk_a : clk_b;

二、兩個時鐘是倍數關係

reg clk_a_vld;
reg clk_b_vld;

 
always @(negedge clk_a or negedge rst_n)begin
    if(!rst_n)begin
        clk_a_vld <= 0;
    end
    else begin
        clk_a_vld <= sel & ~clk_b_vld;
    end
end

always @(negedge clk_b or negedge rst_n)begin
    if(!rst_n)begin
        clk_b_vld <= 0;
    end
    else begin
        clk_b_vld 
 
<= ~sel & ~clk_a_vld;
    end
end

assign clk_o = (clk_a_vld & clk_a) | (clk_b_vld & clk_b);

三、兩個時鐘是非同步關係

reg clk_a_vld;
reg clk_b_vld;
reg clk_a_vld_r;
reg clk_b_vld_r;

always @(posedge clk_a or negedge rst_n)begin
    if(!rst_n)begin
        clk_a_vld   <= 0;
        clk_a_vld_r 
 
<= 0;
    end
    else begin
        clk_a_vld   <= sel & ~clk_b_vld_r;
        clk_a_vld_r <= clk_a_vld;
    end
end

always @(posedge clk_b or negedge rst_n)begin
    if(rst_n == 1'b0)begin
        clk_b_vld   <= 0;
        clk_b_vld_r <= 0;
    end
    else begin
        clk_b_vld   <= ~sel & ~clk_a_vld_r;
        clk_b_vld_r <= clk_b_vld;
    end
end

assign clk_o = (clk_a_vld_r & clk_a) | (clk_b_vld_r & clk_b);

endmodule