校招Verilog——glitch free时钟切换电路

要求：

　　用 Verilog 实现 glitch free 时钟切换电路。输入 sel，clka，clkb，sel 为 1 输出 clka，sel 为 0 输出 clkb。

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

一、有毛刺写法

assign clk_o = sel ? clk_a : clk_b;

二、两个时钟是倍数关系

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、两个时钟是异步关系

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