用C语言写51程序设计通过串口输入密码显示在LCD1602上，当密码正确时，显示屏显示Welcome, LED灯全亮。当密码错误时串0显示Plesase Try Again,延时3秒后，LCD1 602显示输入密码界面。当输入3次错误密码时，蜂鸣器发出声响。

以下是用C语言写51程序设计通过串口输入密码显示在LCD1602上的代码：

#include <reg51.h>
#include <stdio.h>

#define LCD1602_DB P0
sbit LCD1602_RS = P2^6;
sbit LCD1602_RW = P2^5;
sbit LCD1602_E = P2^7;

#define LED P1
#define BUZZER P3^7

void delay(unsigned int t)
{
    unsigned int i, j;
    for(i = 0; i < t; i++)
        for(j = 0; j < 125; j++);
}

void LCD1602_WriteCmd(unsigned char cmd)
{
    LCD1602_RS = 0;
    LCD1602_RW = 0;
    LCD1602_DB = cmd;
    LCD1602_E = 1;
    delay(1);
    LCD1602_E = 0;
    delay(1);
}

void LCD1602_WriteData(unsigned char dat)
{
    LCD1602_RS = 1;
    LCD1602_RW = 0;
    LCD1602_DB = dat;
    LCD1602_E = 1;
    delay(1);
    LCD1602_E = 0;
    delay(1);
}

void LCD1602_Init()
{
    LCD1602_WriteCmd(0x38);
    LCD1602_WriteCmd(0x0c);
    LCD1602_WriteCmd(0x06);
    LCD1602_WriteCmd(0x01);
}

void LCD1602_Clear()
{
    LCD1602_WriteCmd(0x01);
}

void LCD1602_SetCursor(unsigned char x, unsigned char y)
{
    unsigned char addr;
    if(y == 0)
        addr = 0x80 + x;
    else
        addr = 0xc0 + x;
    LCD1602_WriteCmd(addr);
}

void UART_Init()
{
    TMOD = 0x20;
    TH1 = 0xfd;
    TL1 = 0xfd;
    TR1 = 1;
    SM0 = 0;
    SM1 = 1;
    REN = 1;
}

unsigned char UART_Receive()
{
    while(!RI);
    RI = 0;
    return SBUF;
}

void UART_Send(unsigned char dat)
{
    SBUF = dat;
    while(!TI);
    TI = 0;
}

void main()
{
    unsigned char password[4] = {'1', '2', '3', '4'};
    unsigned char input[4];
    unsigned char i, j;
    unsigned char correct = 0;
    unsigned char tries = 0;
    
    LCD1602_Init();
    UART_Init();
    
    while(1)
    {
        LCD1602_Clear();
        LCD1602_SetCursor(0, 0);
        LCD1602_WriteData('P');
        LCD1602_WriteData('l');
        LCD1602_WriteData('e');
        LCD1602_WriteData('a');
        LCD1602_WriteData('s');
        LCD1602_WriteData('e');
        LCD1602_WriteData(' ');
        LCD1602_WriteData('E');
        LCD1602_WriteData('n');
        LCD1602_WriteData('t');
        LCD1602_WriteData('e');
        LCD1602_WriteData('r');
        LCD1602_WriteData(' ');
        LCD1602_WriteData('P');
        LCD1602_WriteData('a');
        LCD1602_WriteData('s');
        LCD1602_WriteData('s');
        LCD1602_WriteData('w');
        LCD1602_WriteData('o');
        LCD1602_WriteData('r');
        LCD1602_SetCursor(0, 1);
        for(i = 0; i < 4; i++)
        {
            input[i] = UART_Receive();
            LCD1602_WriteData('*');
        }
        for(i = 0; i < 4; i++)
        {
            if(input[i] != password[i])
            {
                correct = 0;
                break;
            }
            else
            {
                correct = 1;
            }
        }
        if(correct)
        {
            LCD1602_Clear();
            LCD1602_SetCursor(0, 0);
            LCD1602_WriteData('W');
            LCD1602_WriteData('e');
            LCD1602_WriteData('l');
            LCD1602_WriteData('c');
            LCD1602_WriteData('o');
            LCD1602_WriteData('m');
            LCD1602_WriteData('e');
            LED = 0xff;
            BUZZER = 0;
            delay(1000);
        }
        else
        {
            tries++;
            if(tries >= 3)
            {
                BUZZER = 1;
                delay(1000);
                BUZZER = 0;
                tries = 0;
            }
            else
            {
                LCD1602_Clear();
                LCD1602_SetCursor(0, 0);
                LCD1602_WriteData('P');
                LCD1602_WriteData('l');
                LCD1602_WriteData('e');
                LCD1602_WriteData('a');
                LCD1602_WriteData('s');
                LCD1602_WriteData('e');
                LCD1602_WriteData(' ');
                LCD1602_WriteData('T');
                LCD1602_WriteData('r');
                LCD1602_WriteData('y');
                LCD1602_WriteData(' ');
                LCD1602_WriteData('A');
                LCD1602_WriteData('g');
                LCD1602_WriteData('a');
                LCD1602_WriteData('i');
                LCD1602_WriteData('n');
                delay(3000);
            }
        }
    }
}

这个程序可以通过串口输入密码，然后在LCD1602上显示星号，如果密码正确，LED灯全亮，LCD1602上显示Welcome，如果密码错误，LCD1602上显示Please Try Again，延时3秒后再次显示输入密码界面，如果连续输入3次错误密码，蜂鸣器会发出声响。