16x2 LCD Interface with 8051 (4-bit Mode, Port 2 Only)

This tutorial explains how to interface a 16x2 LCD with the 8051 microcontroller using 4-bit mode, with both data and control lines connected to Port 2.

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📌 Overview

In this setup:

• D4–D7 → P2.4–P2.7
• RS → P2.0
• RW → P2.1
• EN → P2.2

This allows the entire LCD to be controlled using a single port (Port 2).

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🔌 Pin Configuration

LCD Connections

LCD Pin	Function	8051 Pin
RS	Register Select	P2.0
RW	Read/Write	P2.1
EN	Enable	P2.2
D4	Data	P2.4
D5	Data	P2.5
D6	Data	P2.6
D7	Data	P2.7

D0–D3 are not used in 4-bit mode

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⚙️ Bit Mapping of Port 2

P2 Pin	Function
P2.0	RS
P2.1	RW
P2.2	EN
P2.3	Unused
P2.4–P2.7	Data (D4–D7)

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⚙️ How It Works

• Data is sent in two nibbles
• Upper nibble → P2.4–P2.7
• Lower nibble → P2.4–P2.7 (shifted)
• Control bits must remain unaffected during data transfer

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🧠 Important Concept

Since control and data share the same port, we must:

• Preserve lower bits (P2.0–P2.3)
• Modify only upper nibble when sending data

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💻 8051 C Program

#include <reg51.h>

#define LCD P2

sbit RS = P2^0;
sbit RW = P2^1;
sbit EN = P2^2;

// Delay
void delay(unsigned int time) {
    unsigned int i, j;
    for(i = 0; i < time; i++)
        for(j = 0; j < 1275; j++);
}

// Enable pulse
void lcd_enable() {
    EN = 1;
    delay(1);
    EN = 0;
}

// Send 4-bit data safely
void lcd_send_nibble(unsigned char nibble) {
    LCD &= 0x0F;              // Keep lower bits (control pins safe)
    LCD |= (nibble & 0xF0);   // Send upper nibble
    lcd_enable();
}

// Send command
void lcd_cmd(unsigned char cmd) {
    RS = 0;
    RW = 0;

    lcd_send_nibble(cmd);        // High nibble
    lcd_send_nibble(cmd << 4);   // Low nibble
    delay(2);
}

// Send data
void lcd_data(unsigned char dat) {
    RS = 1;
    RW = 0;

    lcd_send_nibble(dat);
    lcd_send_nibble(dat << 4);
    delay(2);
}

// LCD Init
void lcd_init() {
    delay(20);

    lcd_cmd(0x02); // 4-bit init
    lcd_cmd(0x28); // 2 line, 4-bit
    lcd_cmd(0x0C); // Display ON
    lcd_cmd(0x01); // Clear
    lcd_cmd(0x06); // Entry mode
}

// Print string
void lcd_string(char *str) {
    while(*str) {
        lcd_data(*str++);
    }
}

// Main
void main() {
    lcd_init();
    lcd_string("Port2 LCD");

    while(1);
}