Mahedee Hasan

Mahedee Hasan

I am a Microsoft MVP (11th times), programmer, software developer, software architect, consultant, author, researcher, technical blogger, technical speaker, trainer and learner. I have more than 17 years experience in the field of software design and development.

Getting Started with Embedded Systems Programming: Assembly Language Fundamentals

4 minute read

Introduction to Embedded Systems Programming

Embedded systems are everywhere around us - from the microwave in your kitchen to the sophisticated control systems in modern automobiles. At the heart of these systems lies the microcontroller, a small computer designed to perform specific tasks efficiently.

In this comprehensive tutorial, we’ll dive into the world of embedded systems programming using assembly language. While higher-level languages like C are commonly used today, understanding assembly language is crucial for:

  • Performance optimization - Direct hardware control for time-critical applications
  • Memory efficiency - Minimal resource usage in constrained environments
  • Deep understanding - Better comprehension of how microcontrollers actually work
  • Debugging skills - Ability to troubleshoot at the lowest level

What You’ll Learn

This tutorial series will cover:

  • Setting up the development environment (MPLAB IDE)
  • Understanding PIC16F84A microcontroller architecture
  • Writing your first assembly language program
  • Simulating circuits with Proteus
  • Controlling LEDs and creating light patterns
  • Register manipulation and I/O operations

Hardware and Software Requirements

Hardware:

  • Microcontroller: PIC16F84A (8-bit microcontroller)
  • LEDs: For visual output indication
  • Resistors: Current limiting resistors for LEDs
  • Breadboard: For circuit prototyping
  • PIC Programmer: For uploading code to the microcontroller

Software:

  • MPLAB IDE: Microchip’s integrated development environment
  • Proteus: Circuit simulation software
  • PIC Assembler: For converting assembly code to machine code

Understanding PIC16F84A Microcontroller

The PIC16F84A is an excellent choice for beginners due to its:

  • Simple architecture with 35 instructions
  • 1K words of program memory
  • 68 bytes of RAM
  • 64 bytes of EEPROM
  • Two I/O ports (PORTA and PORTB)
  • Built-in oscillator support

Pin Configuration:

  • PORTA: 5-bit I/O port (RA0-RA4)
  • PORTB: 8-bit I/O port (RB0-RB7)
  • VDD/VSS: Power supply pins
  • OSC1/OSC2: Crystal oscillator pins
  • MCLR: Master Clear (Reset) pin

Setting Up Your Development Environment

Step 1: Install MPLAB IDE

  1. Download MPLAB IDE from Microchip’s official website
  2. Install with default settings
  3. Configure for PIC16F84A target device

Step 2: Install Proteus

  1. Download Proteus simulation software
  2. Install and configure PIC library
  3. Set up simulation environment

Step 3: Create Your First Project

  1. Open MPLAB IDE
  2. Create new project: Project → Project Wizard
  3. Select device: PIC16F84A
  4. Choose assembler: MPASM
  5. Name your project

Your First Assembly Program: LED Blinker

Let’s start with a simple LED blinking program to understand the basics:

; LED Blinker Program for PIC16F84A
; This program blinks an LED connected to RB0

    LIST P=16F84A          ; Specify processor
    #include <P16F84A.INC> ; Include processor definitions
    
    __CONFIG _CP_OFF & _WDT_OFF & _PWRTE_ON & _RC_OSC

; Variable declarations
    CBLOCK 0x0C
        COUNT1
        COUNT2
    ENDC

    ORG 0x00           ; Reset vector
    GOTO MAIN          ; Go to main program

    ORG 0x04           ; Interrupt vector
    RETFIE            ; Return from interrupt

MAIN:
    ; Initialize ports
    BSF STATUS, RP0    ; Select Bank 1
    MOVLW 0x00         ; Set PORTB as output
    MOVWF TRISB
    BCF STATUS, RP0    ; Select Bank 0

LOOP:
    ; Turn LED on
    BSF PORTB, 0       ; Set RB0 high
    CALL DELAY         ; Call delay routine
    
    ; Turn LED off  
    BCF PORTB, 0       ; Set RB0 low
    CALL DELAY         ; Call delay routine
    
    GOTO LOOP          ; Repeat forever

DELAY:
    ; Delay routine (approximately 0.5 seconds)
    MOVLW 0xFF
    MOVWF COUNT1
DELAY_LOOP1:
    MOVLW 0xFF
    MOVWF COUNT2
DELAY_LOOP2:
    DECFSZ COUNT2, F
    GOTO DELAY_LOOP2
    DECFSZ COUNT1, F
    GOTO DELAY_LOOP1
    RETURN

    END               ; End of program

Understanding the Code

Configuration Bits:

  • _CP_OFF: Code protection disabled
  • _WDT_OFF: Watchdog timer disabled
  • _PWRTE_ON: Power-up timer enabled
  • _RC_OSC: RC oscillator selected

Key Instructions:

  • BSF/BCF: Bit Set/Clear File register
  • MOVLW/MOVWF: Move literal to working register/Move working register to file
  • DECFSZ: Decrement file, skip if zero
  • GOTO/CALL: Program flow control

Memory Banks:

  • Bank 0: Contains PORTB (data)
  • Bank 1: Contains TRISB (direction control)

Circuit Simulation with Proteus

  1. Create New Design: Open Proteus and create a new schematic
  2. Add Components:
    • PIC16F84A microcontroller
    • LED connected to RB0
    • Current limiting resistor (330Ω)
    • Power supply connections
  3. Load Hex File: Import your compiled .hex file
  4. Run Simulation: Start simulation to see LED blinking

Video Tutorial

Watch the complete tutorial to see the development process in action:

Advanced Example: Light Dance Pattern

For a more complex example, here’s a light dance pattern that creates an attractive LED sequence:

; Light Dance Pattern
; Creates a running light effect on PORTB

LIGHT_DANCE:
    MOVLW 0x01         ; Start with first LED
    MOVWF PORTB
    CALL DELAY
    
    MOVLW 0x02         ; Second LED
    MOVWF PORTB
    CALL DELAY
    
    MOVLW 0x04         ; Third LED
    MOVWF PORTB
    CALL DELAY
    
    ; Continue pattern...
    RETURN

Common Challenges and Solutions

Challenge 1: Timing Issues

Problem: LED blinks too fast or too slow Solution: Adjust delay routine loop counters

Challenge 2: Circuit Not Working

Problem: LED doesn’t light up Solutions:

  • Check power supply connections
  • Verify current limiting resistor
  • Ensure correct pin assignments

Challenge 3: Compilation Errors

Problem: Assembly errors during build Solutions:

  • Check syntax carefully
  • Verify include files
  • Ensure proper labels and directives

Best Practices for Assembly Programming

  1. Comment Your Code: Assembly can be cryptic - use plenty of comments
  2. Use Meaningful Labels: Make your code readable
  3. Modular Design: Break complex tasks into subroutines
  4. Test Incrementally: Start simple and add complexity gradually
  5. Understand the Hardware: Read the datasheet thoroughly

Next Steps

In the upcoming tutorials, we’ll explore:

  • Interrupt handling and real-time programming
  • Analog-to-digital conversion
  • Serial communication protocols
  • Advanced I/O techniques
  • Interfacing with sensors and displays

Source Code and Resources

Complete source code for this tutorial is available on GitHub:

📁 Source Code Repository

Additional Resources:

Conclusion

Congratulations! You’ve taken your first steps into embedded systems programming with assembly language. While the learning curve may seem steep initially, the deep understanding you’ll gain of microcontroller operation will prove invaluable throughout your embedded systems journey.

Remember that assembly language programming requires patience and practice. Start with simple projects like this LED blinker, and gradually work your way up to more complex applications.

In our next tutorial, we’ll dive deeper into PIC architecture and explore more advanced programming techniques. Stay tuned!

Have questions about this tutorial? Found any issues with the code? Feel free to leave a comment below or reach out through the contact page. Happy coding!

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