2.5 主要代码片段及说明

首先是可调频率和占空比的PWM波，经过一阶低通滤波器后接入高速比较器负输入端。

always @(posedge sys_clk or negedge sys_rst_n) begin
    if(sys_rst_n == 1'b0)
        cnt <= 8'b0;
    else if(cnt == 8'b1111)
        cnt <= 8'b0;
    else
        cnt = cnt + 1'b1;
end

always @(posedge sys_clk or negedge sys_rst_n) begin
    if(sys_rst_n == 1'b0)
        pwm_out <= 1'b0;
    else if(cnt <= 8'd10)
        pwm_out <= 1'b1;
    else
        pwm_out <= 1'b0;
end

然后是频率测量模块。

always @(posedge clk_test or negedge sys_rst_n) begin
    if(sys_rst_n == 1'b0)
        freq_cnt <= 26'b0;
    else
        freq_cnt <= freq_cnt + 1'b1;
end

always @(posedge clk_1s or negedge clk_1s or negedge sys_rst_n) begin
    if(sys_rst_n == 1'b0)
        freq <= 26'b0;
    else begin
        if(freq_cnt >= freq_cnt_buff) freq = freq_cnt - freq_cnt_buff;
        freq_cnt_buff = freq_cnt;
    end
end

按键控制模式变换,同时被测信号频率大于1M时改显示单位，变为kHz。

//控制模式翻转
reg  h_or_l_r;
always@(posedge clk or negedge rst_n)
begin
	if(!rst_n)
		h_or_l_r <= 1'b0;
	else 
		begin
			if(key_out == 1'b1) h_or_l_r <= ~h_or_l_r;
		end 
end
assign h_or_l = h_or_l_r;

//检测到按键输入，切换模式
always @(posedge sys_clk or negedge sys_rst_n) begin
	if(sys_rst_n == 1'b0)
		date_buff <= 1'b0;
	else if(freq_or_count == 1'b0)
		date_buff <= count_buff - 1'b1;
	else if(freq_or_count == 1'b1 && freq_buff < 20'd1000000)
		date_buff <= freq_buff;
	else
		date_buff <= freq_buff_div;	
end
assign led_R = (freq_or_count == 1'b1) ? 8'b11110000 : 8'b00001111;

这里的计数器同样套用了按键消抖模块。

//计数加一
always@(posedge clk or negedge rst_n)
begin
	if(!rst_n)
       count <= 1'b0;
	else 
		if((key_out == 1'b1) && (freq_or_count == 1'b0)) count = count + 1'b1;
end

以上两个主要功能模块获取到频率和计数数据后，转换成BCD码的形式，方便OLED显示。

//当计数器等于20时，移位判断操作完成，对各个位数的BCD码进行赋值
always@(posedge sys_clk or negedge sys_rst_n)
    if(sys_rst_n == 1'b0)
        begin
            unit    <=  4'b0;
            ten     <=  4'b0;
            hun     <=  4'b0;
            tho     <=  4'b0;
            t_tho   <=  4'b0;
            h_hun   <=  4'b0;
        end
    else    if(cnt_shift == 5'd21)
        begin
            unit    <=  data_shift[23:20];
            ten     <=  data_shift[27:24];
            hun     <=  data_shift[31:28];
            tho     <=  data_shift[35:32];
            t_tho   <=  data_shift[39:36];
            h_hun   <=  data_shift[43:40];
        end

将OLED屏幕划分为4行，每行显示16个字符，在第一行显示模式（frequency/counter），第三行显示数值。第四行当单位为kHz时，显示余下的个十百位。

5'd0:	begin state <= INIT; end
5'd1:	begin if(freq_or_count == 1'b1) begin
                  y_p <= 8'hb0; x_ph <= 8'h10; x_pl <= 8'h00; num <= 5'd16; char <= "  frequency     ";state <= SCAN; end
              else begin
                  y_p <= 8'hb0; x_ph <= 8'h10; x_pl <= 8'h00; num <= 5'd16; char <= "  counter       ";state <= SCAN; end
        end
5'd2:	begin y_p <= 8'hb1; x_ph <= 8'h10; x_pl <= 8'h00; num <= 5'd16; char <= "                ";state <= SCAN; end
5'd3:	begin y_p <= 8'hb2; x_ph <= 8'h10; x_pl <= 8'h00; num <= 5'd16; char <= str_out           ;state <= SCAN; end
5'd4:	begin y_p <= 8'hb3; x_ph <= 8'h10; x_pl <= 8'h00; num <= 5'd16; char <= str_out_1         ;state <= SCAN; end