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 示波器是非必须的，主要是为了观察由中断调制出来的时钟脉冲，因为ADC0808需要外部脉冲，这里采用的是软件模拟的方式，通过计时器中断，用P3.3口输出。

#include <reg51.h>
#include <intrins.h>
#include <stdio.h>
#include "LCM1602.h"

sbit OE=P3^0;
sbit EOC=P3^1;
sbit START=P3^2;
sbit CLK=P3^3;
sbit ALE=P1^0;
sbit ADDA=P1^1;
sbit ADDB=P1^2;
sbit ADDC=P1^3;

unsigned char buff[16];

void Init_ADC0808()
{
    ALE=0;
    ADDA=0;
    ADDB=0;
    ADDC=0;//select analog channel
    ALE=1;//address lock enable
    
    START=1;//clear registers
    _nop_();_nop_();_nop_();
    OE=0;//Do not allow output
    CLK=0;
    
 }

void main()
{
    float p;
    P2=0xff;
    TMOD=0x02;
    TH0=0xF6;
    TL0=0xF6;
    ET0=1;//Enable T0
    TR0=1;//T0 Run
    EA=1;
    Init_ADC0808();
    Init_LCM1602();
    //P2=0x00;
    WriteString_LCM1602(0x80,"DC VOLTMETER");
    while(1)
    {
        START=1;//clear registers
        _nop_();_nop_();_nop_();
        START=0;//start conversion
        _nop_();_nop_();_nop_();
        OE=0;
        while(EOC==0);//wait for signal of END OF CONVERSION
        OE=1;//output enable
        p=P2*5.0/256;
        sprintf(buff,"%1.2fV",p);
        WriteString_LCM1602(0xc0,buff);
        START=1;
    }
}

//the ADC0808 CLOCK is 10kHz~1280kHz, typical is 640kHz
//each 10us trig edge, so frequency is 1000*1000/10/2=50kHz
void INT_TIMER0() interrupt 1
{
    CLK=~CLK;
}