PWM，脉冲宽度调制。顾名思义，是通过调制脉冲的宽度，即占空比，来实现的。可是使占空比逐渐由最小增加到最大，也可以使占空比由最大减少到最小来实现不同的现象。若用LED灯来显示现象，则可以称作：LED呼吸灯；
简单的代码如下：
/********************************Copyright************************************** **----------------------------File information-------------------------- ** File name :led_pwm.v ** CreateDate :2015.03 ** Funtions :pwm的试验，用led灯来显示pwm的效果 ** Operate on :M5C06N3L114C7 ** Copyright :All rights reserved. ** Version :V1.0 **---------------------------Modify the file information---------------- ** Modified by : ** Modified data : ** Modify Content: *******************************************************************************/ 001 module led_pwm (002 clk,003 rst_n,004 005 pwm_out006 );007 input clk; /* 24Mhz */008 input rst_n;009 // wire rst_n;010 output pwm_out;011 012 // assign rst_n = 1;013 //-------------------------------------014 /* ius */015 localparam t_1us = 5'd23;016 // localparam t_1us = 5'd6; /* 用于测试 */017 reg [4:0] cnt1;018 always @(posedge clk or negedge rst_n)019 begin020 if(!rst_n)021 begin022 cnt1 <= 0 ;023 end024 else 025 begin026 if(cnt1 == t_1us)027 cnt1 <= 0;028 else 029 cnt1 <= cnt1 + 1;030 end031 end032 033 034 /* 1ms */035 localparam t_1ms = 10'd999;036 // localparam t_1ms = 10'd19; /* 用于测试 */037 reg [9:0] cnt2;038 always @(posedge clk or negedge rst_n)039 begin040 if(!rst_n)041 begin042 cnt2 <= 0 ;043 end044 else 045 begin046 if(cnt1 == t_1us)047 begin048 if(cnt2 == t_1ms)049 cnt2 <= 0;050 else 051 cnt2 <= cnt2 + 1;052 end053 else 054 cnt2 <= cnt2;055 end056 end057 058 /* 1s */ 059 localparam t_1s = 10'd999;060 // localparam t_1s = 10'd19; /* 用于测试 */061 reg [9:0] cnt3;062 always @(posedge clk or negedge rst_n)063 begin064 if(!rst_n)065 begin066 cnt3 <= 0; 067 end068 else 069 begin070 if((cnt1 == t_1us)&&(cnt2 == t_1ms))071 begin072 if(cnt3 == t_1s)073 cnt3 <= 0;074 else 075 cnt3 <= cnt3 + 1;076 end077 else 078 cnt3 <= cnt3;079 end080 end081 082 reg flag;083 always @(posedge clk or negedge rst_n)084 begin085 if(!rst_n)086 begin087 flag <= 0;088 end089 else if((cnt1 == t_1us)&&(cnt2 == t_1ms)&&(cnt3 == t_1s))090 begin091 flag <= ~flag; 092 end093 else 094 flag <= flag;095 end096 097 assign pwm_out = flag?((cnt2 < cnt3)?0:1):((cnt2 < cnt3)?1:0);098 099 endmodule100101
仿真验证代码：
/********************************Copyright************************************** **----------------------------File information-------------------------- ** File name :led_pwm_tb.v ** CreateDate :2015.03 ** Funtions : led_pwm 的测试文件 ** Operate on :M5C06N3L114C7 ** Copyright :All rights reserved. ** Version :V1.0 **---------------------------Modify the file information---------------- ** Modified by : ** Modified data : ** Modify Content: *******************************************************************************/ 01 module led_pwm_tb;02 reg clk;03 reg rst_n;04 05 wire pwm_out;06 07 led_pwm led_pwm_1(08 .clk,09 .rst_n,10 11 .pwm_out12 ); 13 14 localparam tck = 24;15 localparam t = 1000/tck;16 always 17 #(t/2) clk = ~clk;18 19 initial 20 begin21 clk = 0;22 rst_n = 0;23 24 #(50*t) rst_n = 1;25 26 end27 28 endmodule29 30
仿真结果：
