Introduction
A 16x2 monochrome LCD module is a widely used display in microcontroller projects. It can show two lines of text, each up to 16 characters long, making it ideal for simple user interfaces. These LCDs are based on the HD44780 controller (or compatible), which accepts commands and data via an 8-bit or 4-bit parallel interface.
The LCD works by receiving control signals (RS, R/W, E) and data signals (D0–D7 or D4–D7 for 4-bit mode). In this tutorial, we’ll use 8-bit mode for simplicity. The PIC16F84A will send initialization commands to the LCD, followed by ASCII characters to be displayed.
Understanding the control lines:
- RS (Register Select):
- 0 = Instruction/command (e.g., clear display).
- 1 = Data (character to display).
- R/W (Read/Write):
- 0 = Write to LCD.
- 1 = Read from LCD (rarely needed, we’ll keep it 0.
- E (Enable):
- A high-to-low pulse latches the data/command into the LCD.
Once initialized, the LCD can display text by sending ASCII codes as data.
Materials
- PIC16F84A microcontroller
- 16x2 HD44780-based monochrome LCD module
- 4 MHz crystal oscillator + 2 × 22pF capacitors
- 10kΩ potentiometer (for LCD contrast adjustment)
- Breadboard and jumper wires
- 5V regulated power supply
Code (Assembly for PIC16F84A)
We’ll walk through the full code, then explain each part.
Full Program
;=========================================================== ; Program: LCD 16x2 Display with PIC16F84A ;=========================================================== list p=16F84A include <p16f84a.inc> __CONFIG _FOSC_HS & _WDTE_OFF & _PWRTE_ON & _CP_OFF ;------------------------------- ; Pin connections ; LCD Data -> PORTB ; RS -> RA0 ; RW -> RA1 (tied low, write only) ; EN -> RA2 ;------------------------------- cblock 0x20 temp endc ;------------------------------- ; Reset vector ;------------------------------- org 0x00 goto main ;------------------------------- ; Subroutines ;------------------------------- delay_ms movlw d'250' movwf temp d1 nop nop decfsz temp, f goto d1 return lcd_pulse bsf PORTA,2 ; E=1 call delay_ms bcf PORTA,2 ; E=0 call delay_ms return lcd_cmd bcf PORTA,0 ; RS=0 movwf PORTB ; Send command call lcd_pulse return lcd_data bsf PORTA,0 ; RS=1 movwf PORTB ; Send data call lcd_pulse return lcd_init ; Function Set: 8-bit, 2 line, 5x8 font movlw 0x38 call lcd_cmd ; Display ON, Cursor OFF movlw 0x0C call lcd_cmd ; Clear display movlw 0x01 call lcd_cmd ; Entry mode: auto-increment movlw 0x06 call lcd_cmd return ;------------------------------- ; Main Program ;------------------------------- main bsf STATUS, RP0 movlw 0x00 movwf TRISB ; PORTB = output movlw 0xF8 movwf TRISA ; RA0-RA2 output, RA3-RA4 input bcf STATUS, RP0 call lcd_init ; Write "HELLO" movlw 'H' call lcd_data movlw 'E' call lcd_data movlw 'L' call lcd_data movlw 'L' call lcd_data movlw 'O' call lcd_data endless goto endless end
Simulate this code in PICSimulator
Code Explanation
1. Configuration and Setup
- Sets processor type and includes the register definitions.
- Configures the microcontroller for High-Speed crystal oscillator, watchdog timer off, power-up timer on, and code protection off.
2. LCD Pin Mapping
- PORTB is used for LCD data (D0–D7).
- RA0 = RS, RA1 = R/W (grounded), RA2 = E.
- Only writing to the LCD is needed, so R/W is tied to ground.
3. Delay Routine
A simple software delay loop to let the LCD process commands.
4. LCD Enable Pulse
The LCD latches commands/data on the falling edge of E.
5. Sending Commands and Data
These subroutines separate instructions (clear, cursor) from characters (ASCII).
6. LCD Initialization
These commands are required at startup to prepare the LCD.
7. Writing Text
Each ASCII character is written sequentially to the LCD.
Conclusion
In this tutorial, we connected a 16x2 monochrome LCD to a PIC16F84A microcontroller using an 8-bit parallel interface. We wrote initialization and display routines in assembly, giving full control of the LCD at a low level.
This project demonstrates how to:
- Initialize the LCD in 8-bit mode.
- Send commands and data separately.
- Display custom strings of text.
With this foundation, you can extend the program to display sensor data, menu options, or real-time feedback for embedded projects.
