The NE555 chip is used to design a pulse square wave signal generator with an adjustable duty cycle.

				
					#include<reg51.h>  
#include"lcd.h"  

#define uchar unsigned char   
#define uint  unsigned int  
#define ulong   unsigned long  
uchar code EN_CHAR1[16]={"   CYMOMETER    "};  
uchar code EN_CHAR2[16]={"FREQ:         HZ"};  
ulong Freq, TimeCount;             
void main()  
{  
    uchar i, freqValue[6];  
    LcdInit();  
    TMOD=0x51;  
    //Set timer for 50ms when using a 12MHz crystal oscillator  
    TH0=0x3C;  
    TL0=0xB0;  
    //Enable interrupts  
    ET0=1;ET1=1;EA=1;  
    //Start timers  
    TR0=1;TR1=1;  
    for(i=0;i<16;i++)//Display on first line  
    {  
        LcdWriteData(EN_CHAR1[i]);  
    }     
    LcdWriteCom(0xc0);//Switch to second line for display  
    for(i=0;i<16;i++)  
    {  
        LcdWriteData(EN_CHAR2[i]);  
    }  
    while(1)  
    {  
        if(TR0==0)         //When the counter stops, it indicates counting is complete  
        {  
            Freq = Freq + TL1;       //Read the value of TL  
            Freq = Freq + (TH1 * 256); //Read the value of TH  
            LcdWriteCom(0xc8);  
            //Calculate the hundred thousands, ten thousands, thousands, hundreds, tens and units digits of the frequency             
            freqValue[0]='0'+Freq%1000000/100000;
            freqValue[1]='0'+Freq%100000/10000;   
            freqValue[2]='0'+Freq%10000/1000;     
            freqValue[3]='0'+Freq%1000/100;   
            freqValue[4]='0'+Freq%100/10;     
            freqValue[5]='0'+Freq%10;     
            for(i=0;i<5;i++)//Start from the highest digit  
            {  
                if(freqValue[i]==0x30)  
                {  
                    freqValue[i]=0x20;  //If it's 0, assign a space character  
                }                 
                else  
                {  
                    break;  
                }     
            }  
            for(i=0;i<6;i++)  
            {  
                LcdWriteData(freqValue[i]);   
            }     
            Freq=0;TH1=0; TL1=0;TR0=1; TR1=1;   
        }  
    }  
}  

void Timer0()   interrupt 1  
{  
    TimeCount++;  
    if(TimeCount==20)//Timing reaches 1 second  
    {  
        TR0=0;  
        TR1=0;  
        TimeCount=0;          
    }  
    TH0=0x3C;  
    TL0=0xB0;     
}  
void Timer1()   interrupt 3  
{     
    Freq=Freq+65536;          
}