Introduction

Microcontrollers are at the heart of countless DIY electronics projects, from embedded systems to robotics, home automation, and sensor interfaces. If you're looking to develop your skills in microcontroller programming and hardware design, using a classic microcontroller like the MC68HC11A1FN can be a fantastic starting point. While modern microcontrollers such as those from the Arduino or Raspberry Pi families have become widely popular, there is something uniquely educational about working with older chips like the MC68HC11A1FN, which was once a flagship product from Motorola (now NXP).

In this article, we will guide you through a DIY electronics project where you’ll learn how to use the MC68HC11A1FN microcontroller to build a simple embedded system. The project will cover the essential steps of using this microcontroller to control basic output devices, interface with sensors, and program it to execute simple tasks. This project is designed for beginners who want to learn the basics of microcontroller programming, circuit design, and system integration using a classic MCU.

What is the MC68HC11A1FN?

The MC68HC11A1FN is an 8-bit microcontroller from Motorola’s HC11 family of microcontrollers. Released in the early 1990s, the HC11 series was designed for embedded systems requiring low power, reliable performance, and flexible I/O capabilities. Despite its age, the MC68HC11A1FN remains a solid choice for hobbyists and students interested in exploring the fundamentals of microcontroller-based design.

Here are some key features of the MC68HC11A1FN:

● 8-bit Microcontroller: It uses an 8-bit CPU architecture, which is ideal for simple tasks.

● 4 MHz Clock Speed: A moderate clock speed that still allows the microcontroller to perform a variety of tasks efficiently.

● 2 KB Flash ROM: For storing the program code.

● 128 Bytes of RAM: For temporary data storage and variable manipulation during program execution.

● I/O Pins: A total of 32 I/O pins, which can be configured as inputs or outputs for connecting external devices like LEDs, sensors, and switches.

● PWM and Timer Features: For generating pulse-width modulation (PWM) signals and precise timing applications.

● Serial Communication (SCI): Built-in UART for serial communication, useful for interfacing with other devices or for debugging via a PC.

While the MC68HC11A1FN is not as popular as newer microcontrollers like the ATmega328 (used in Arduino boards), it remains an excellent tool for learning and understanding how microcontrollers function at a low level.

Components Required for the Project

In this project, we will build a simple embedded system that utilizes the MC68HC11A1FN microcontroller to perform basic tasks like controlling LEDs, reading switches, and outputting data over serial communication. The following components are required for the project:

1. MC68HC11A1FN Microcontroller

● The core of the project, responsible for controlling the input and output functions.

2. LEDs

● Used as visual indicators for various outputs. LEDs are simple, effective, and easy to control through GPIO (General Purpose Input/Output) pins.

3. Resistors

● For limiting current to the LEDs and other components, such as switches or sensors.

4. Push Buttons

● Simple input devices that the microcontroller can read to trigger specific actions or events.

5. Breadboard

● For prototyping the circuit without the need for soldering.

6. Power Supply

● The MC68HC11A1FN operates at a supply voltage of 5V, so you will need a regulated 5V power source, such as a USB to 5V converter or a regulated power adapter.

7. Capacitors

● For noise filtering and stabilizing the power supply to the microcontroller.

8. Serial Interface (optional)

● For serial communication with a PC or external device. This could be a USB to RS232 adapter for debugging or sending data.

9. External Components

● Depending on the complexity of your project, you may choose to include sensors (e.g., temperature sensor, light sensor) or actuators (e.g., motors, relays).

10. Programming Interface

● To load the firmware onto the MC68HC11A1FN, you will need a compatible programmer. The P&E Micro USB Multilink or USB HC11 Programmer can be used to connect your computer to the microcontroller for programming.

Step 1: Setting Up the MC68HC11A1FN Microcontroller

Before we start building the circuit, we need to set up the MC68HC11A1FN microcontroller. The chip is available in a DIP-28 package, which is easy to work with on a breadboard.

Powering the Microcontroller

● Connect Pin 1 (VDD) to a 5V regulated power supply.

● Connect Pin 28 (VSS) to ground (GND).

● Add decoupling capacitors (e.g., 100nF ceramic capacitors) between VDD and VSS to filter out any noise from the power supply.

Clock Source

● The MC68HC11A1FN requires an external crystal oscillator or clock source to generate the clock pulses that synchronize its operations. Typically, a 4 MHz crystal is used with capacitors to stabilize the frequency. Connect the crystal oscillator to Pins 14 and 15 (XIN and XOUT).

Reset Circuit

● The MC68HC11A1FN requires a reset circuit to ensure it starts correctly after power-up. A simple reset circuit can be implemented using a capacitor (e.g., 10µF) and a pull-up resistor (e.g., 10kΩ) connected to the reset pin (Pin 12).

Step 2: Building the Circuit

Once the microcontroller is set up, we can start building the circuit. In this project, we will create a simple system where the microcontroller reads input from a push button and turns on an LED when the button is pressed. Additionally, the microcontroller will send data to the serial port to indicate the button’s status.

1. LED Control

● Connect an LED to one of the GPIO pins (e.g., Port B pins on the MC68HC11A1FN). Use a current-limiting resistor (typically 330Ω to 1kΩ) in series with the LED to prevent excessive current draw.

● In the program, set the appropriate pin as an output and control the LED based on the input from the push button.

2. Push Button Input

● Connect a push button between one of the GPIO pins (e.g., Port A pin) and ground. Use a pull-up resistor (e.g., 10kΩ) to ensure the pin reads as high when the button is not pressed.

● When the button is pressed, the pin will read low (0V), and the microcontroller can take action based on this input.

Step 3: Writing the Code for the MC68HC11A1FN

The MC68HC11A1FN is typically programmed in Assembly or C. Here, we’ll write a simple program in C using the CodeWarrior IDE or HI-TECH C compiler.

1. Initial Setup

The code begins by initializing the microcontroller's I/O ports and setting up the serial communication for debugging.

2. Upload and Test

Once the code is written, you can compile it and upload it to the MC68HC11A1FN using a suitable programmer (e.g., P&E USB Multilink). Once the program is running, you can test the system by pressing the push button. The LED should light up when the button is pressed, and the serial monitor should display "1" when the button is pressed and "0" when released.

Step 4: Expanding the Project

This simple system can be easily expanded to include more complex functionality:

● Adding more LEDs or sensors: Use additional pins to read multiple sensors or control more LEDs.

● PWM for motor control: Use the built-in PWM feature to control motors or dim LEDs.

● Communication: You can interface the microcontroller with LCD displays, external sensors, or even Wi-Fi modules for more advanced projects.

Conclusion

In this project, we've demonstrated how to use the MC68HC11A1FN microcontroller to create a simple embedded system that can read input from a push button, control an LED, and communicate via the serial port. By following these steps, you've learned the basics of circuit design, programming, and microcontroller interfacing.

While the MC68HC11A1FN might seem like an older microcontroller, its simplicity and reliability make it an excellent choice for beginners. With this foundation, you can explore more advanced projects involving sensors, actuators, and even communication with other devices. The skills you've gained here will serve as a stepping stone to mastering more modern microcontrollers and embedded systems.