Introduction

In the world of embedded systems, managing input and output (I/O) efficiently is crucial for building effective and interactive devices. Whether you're building a microcontroller-based system, a data logger, or even a home automation project, you'll often need to interface with external peripherals such as sensors, buttons, LEDs, motors, or displays.

The MC68B21P, a Peripheral Interface Adapter (PIA) from Motorola, is a versatile I/O chip that can help expand the I/O capabilities of microcontrollers or other embedded systems. This chip provides a flexible way to interface with external components and can be used in many applications, including controlling multiple devices, handling communication, or managing digital inputs and outputs.

In this DIY project, we will walk you through the process of building an I/O Expansion System using the MC68B21P. We’ll cover the chip’s features, how to wire it up to a microcontroller, and how to use it in your projects. By the end of this guide, you'll have a solid understanding of how to integrate this powerful chip into your own designs, significantly expanding your microcontroller's I/O capabilities.

Components Required

To complete this DIY I/O Expansion Project, you’ll need the following components:

1．MC68B21P Peripheral Interface Adapter (PIA)

2．Microcontroller (e.g., Arduino, Raspberry Pi, or another MCU)

3．Resistors (typically 10kΩ for pull-up/pull-down configurations)

4．Capacitors for decoupling (e.g., 0.1µF)

5．Breadboard and jumper wires

6．LEDs (to indicate the state of output pins)

7．Push buttons or switches (to test input functionality)

8．External devices such as motors or displays (optional, for practical demonstrations)

9．3.3V or 5V Power Supply (depending on the microcontroller and peripheral voltage requirements)

Understanding the MC68B21P

The MC68B21P is a versatile I/O chip designed by Motorola, and it is often used to extend the I/O capabilities of microcontrollers, especially when more I/O lines are needed than the microcontroller can offer. It provides a 16-bit I/O interface, meaning it can control or read 16 separate input or output lines, or in some configurations, 8 lines each for inputs and outputs.

Here are the main features of the MC68B21P that you’ll be working with in this project:

1．16 I/O Pins: The MC68B21P provides a total of 16 I/O pins that can be individually programmed as either input or output. This gives you significant flexibility in designing your system.

2．Three Control Registers: The chip uses control registers to configure the I/O pins, including data direction (input/output), and can also manage interrupts.

3．Addressable by Microcontroller: The MC68B21P can be connected to a microcontroller via a parallel bus, allowing it to be easily interfaced with systems that communicate using digital signals.

4．Interrupt Handling: The chip has built-in interrupt functionality, which allows you to trigger specific actions in your system when a certain condition occurs (e.g., a change in an input pin).

5．3.3V or 5V Operation: The MC68B21P can operate with either 3.3V or 5V logic, depending on the system requirements.

Pinout and Connections

Before we get into the specifics of using the chip, let’s first take a look at the MC68B21P pinout. The chip has a 40-pin Dual In-line Package (DIP) configuration with the following key connections:

1．Pins 1-8: Data Lines (D0-D7) for the lower 8 bits of I/O.

2．Pins 9-16: Data Lines (D8-D15) for the upper 8 bits of I/O.

3．Pin 17: CS (Chip Select), used to enable the device.

4．Pin 18: RD (Read), used to read data from the chip.

5．Pin 19: WR (Write), used to write data to the chip.

6．Pin 20: RESET (active low), used to reset the chip.

7．Pin 21: INT (Interrupt), an output pin used to indicate that an interrupt has been triggered.

8．Pin 22: A0 (Address Line), used for selecting the I/O port or register.

9．Pins 23-40: Ground (GND) and Power Supply (Vcc), as well as additional address lines and control pins.

Step 1: Setting Up the Circuit

Wiring the MC68B21P to the Microcontroller

The MC68B21P communicates with a microcontroller via a parallel data bus. Here's how you can wire the chip to an Arduino (or any other microcontroller):

Data Lines (D0-D15): Connect these pins to 16 GPIO pins on the Arduino (or the appropriate microcontroller). If your microcontroller doesn’t have 16 available I/O pins, you can use techniques like multiplexing or shift registers to manage the I/O expansion.

Control Pins (CS, RD, WR, RESET):

①　CS (Chip Select): Connect this pin to one of the digital pins of the Arduino. This pin is used to enable or disable the communication with the MC68B21P.

②　RD (Read) and WR (Write): Connect these pins to digital pins on the Arduino. These pins determine whether the microcontroller is reading data from or writing data to the chip.

③　RESET: This pin should be connected to a high state (via a pull-up resistor) to keep the chip in normal operation mode. To reset the chip, you can pull this pin low momentarily.

Power and Ground:

①　Vcc: Connect to a 5V or 3.3V power source (depending on your microcontroller and the voltage rating of the chip).

②　GND: Connect to the ground of your microcontroller and the power supply.

Interrupt Pin (INT): This pin is used to trigger interrupts when specific conditions are met. You can connect it to an interrupt-capable pin on the microcontroller if you plan to use interrupt handling in your system.

Step 2: Configuring the MC68B21P

After wiring the circuit, the next step is to configure the MC68B21P's control registers. The MC68B21P provides a set of control registers that allow you to configure each of its 16 I/O pins as either input or output. You can also set the interrupt trigger conditions and configure the data bus.

(1) Data Direction Register: This register is used to set each pin as an input or output. For example, if you want pins D0 to D7 as outputs and pins D8 to D15 as inputs, you would configure the Data Direction Register accordingly.

(2) Data Register: This register stores the actual data for each pin. If a pin is configured as an output, you can write data to this register to control the pin. Conversely, if the pin is an input, you can read the value from this register.

(3) Control Register: The Control Register is used to manage interrupts, reset the chip, and handle other control tasks such as enabling or disabling specific features.

Step 3: Creating the Software Logic

Once the hardware is set up, it’s time to design the software that controls the MC68B21P. In this step, you’ll configure the chip's control registers to manage the I/O lines.

Here are a few tasks you might implement in the software:

Setting up the I/O Pins:

(1) Configure the pins that will be used as inputs or outputs using the data direction register.

(2) Set the output pins to their desired values (e.g., turning on LEDs, driving motors).

(3) Read from input pins (e.g., reading button presses, monitoring sensors).

Managing Interrupts:

(1) Set up interrupts using the INT pin. For example, if you want to trigger an action when a button is pressed (e.g., a pin state change), you can configure an interrupt on that pin.

Testing I/O:

(1) Create a simple program to toggle an LED connected to an output pin and monitor a button press on an input pin. This will give you a basic understanding of how the MC68B21P works.

Step 4: Advanced Use Cases

With the MC68B21P configured, here are a few advanced use cases for this I/O expansion system:

1. LED Control: Use the MC68B21P to control a set of LEDs. You could create a pattern of blinking LEDs based on external input or user-defined logic.

2. Motor Control: Expand the number of available motor control pins for driving multiple motors or actuators in robotics or automation projects.

3. Sensor Monitoring: Interface with a large number of sensors. For example, you could connect multiple temperature sensors, pressure sensors, or environmental sensors and read their outputs.

4. Data Acquisition System: The MC68B21P is ideal for expanding a data acquisition system by providing more analog-to-digital (ADC) or digital-to-analog (DAC) channels.

Troubleshooting and Optimization

As with any DIY electronics project, troubleshooting and optimization are essential steps:

1. Check Connections: Ensure all connections, especially data and control lines, are correctly wired.

2. Verify Power Supply: Ensure the chip is receiving the correct voltage and that there are no power supply issues.

3. Interrupt Handling: When using interrupts, make sure the interrupt pins are correctly configured and that there are no conflicts with other hardware.

4. Timing Issues: Ensure the timing between the chip and the microcontroller is synchronized, particularly when using the RD and WR signals.

Conclusion

The MC68B21P is a powerful and flexible I/O expansion chip that can help you build more complex embedded systems. By using this chip in your projects, you can significantly increase the number of I/O pins available for controlling external devices such as sensors, motors, LEDs, and other peripherals.

In this project, you’ve learned how to interface the MC68B21P with a microcontroller, configure its control registers, and implement software logic to manage I/O operations. With its flexibility and ease of use, the MC68B21P is an ideal choice for anyone looking to build more complex, scalable embedded systems.