理论基础
超声波模块的工作原理：单片机供给超声波信号端Trig一个最少10us长的高电平触发信号，模块自动发射8个40khz的方波，同时自动检测到信号是否返回，一旦有信号返回，Echo端输出一个高电平，高电平持续的实践就是超声波从发射到返回的时间。 对应的测试距离计算方法 ：（高电平时间_声速（340m/s））/2
超声波发射电路图

_*红外线和超声波选择电路图__

P10口时发射，P11口接收

核心代码如下

#include "reg52.h"  //定义51单片机特殊功能寄存器
#include "intrins.h"
#include "absacc.h"

#define somenop {_nop_();_nop_();_nop_();_nop_();_nop_();\
                 _nop_();_nop_();_nop_();_nop_(); _nop_();}
sbit TX = P1^0;  //发射引脚
sbit RX = P1^1;  //接收引脚

code unsigned char tab[] = { 0xc0,0xf9,0xa4,0xb0,0x99,0x92,0x82,0xf8,0x80,0x90,\
                             0xff
                           };
unsigned char dspbuf[8] = {10,10,10,10,10,10,10,10};  //显示缓冲区
unsigned char dspcom = 0;

unsigned int intr = 0;
bit s_flag;
unsigned int t = 0;

void send_wave(void);
void display(void);

void main(void)
{
    unsigned int distance;

    TMOD |= 0x11;  //配置定时器工作模式
    TH0 = (65536-2000)/256;
    TL0 = (65536-2000)%256;
    TH1 = 0;
    TL1 = 0;

    EA = 1;
    ET0 = 1;  //打开定时器0中断
    TR0 = 1;  //启动定时器

    while(1)
    {
        /** 200毫秒更新一次数据 */
        if(s_flag)
        {
            s_flag = 0;
            /** 关闭定时器0中断:计算超声波发送到返回的时间 */
//             ET0 = 0;
            send_wave();  //发送方波信号
            TR1 = 1;  //启动计时
            while((RX == 1) && (TF1 == 0));  //等待收到脉冲
            TR1 = 0;  //关闭计时

            //发生溢出
            if(TF1 == 1)
            {
                TF1 = 0;
                distance = 9999;  //无返回
            }
            else
            {
                /**  计算时间  */
                t = TH1;
                t <<= 8;
                t |= TL1;
                distance = (unsigned int)(t*0.017);  //计算距离
            }
            TH1 = 0;
            TL1 = 0;
        }
        /** 数据处理                      */

        dspbuf[5] = distance/100;
        dspbuf[6] = distance%100/10;
        dspbuf[7] = distance%10;
    }
}

//定时器0中断服务函数
void isr_timer_0(void)  interrupt 1  //默认中断优先级 1
{
    TH0 = (65536-2000)/256;
    TL0 = (65536-2000)%256;  //定时器重载

    display();  //2ms执行一次

    if(++intr == 200)
    {
        s_flag = 1;
        intr = 0;
    }
}

//显示函数
void display(void)
{
    XBYTE[0xE000] = 0xff;  //去除鬼影

    XBYTE[0xC000] = (1<<dspcom);
    XBYTE[0xE000] = tab[dspbuf[dspcom]];

    if(++dspcom == 8) {
        dspcom = 0;
    }
}

//TX引脚发送40KHz方波信号驱动超声波发送探头
//使用软件延时注意RC振荡器频率
void send_wave(void)
{
    unsigned char i = 8;  //发送8个脉冲

    do
    {
        TX = 1;
        somenop;
        somenop;
        somenop;
        somenop;
        somenop;
        somenop;
        somenop;
        somenop;
        somenop;
        somenop;
        TX = 0;
        somenop;
        somenop;
        somenop;
        somenop;
        somenop;
        somenop;
        somenop;
        somenop;
        somenop;
        somenop;
    }
    while(i--);
}