In this article, we’ll explore how to design and build a programmable timer using the D8253AC-2, a Programmable Interval Timer (PIT) chip. This versatile component is widely used in microprocessor systems for timing and counting applications, and it serves as a great tool for learning about timing circuits, frequency generation, and microcontroller interfacing.

We will cover:

· An introduction to the D8253AC-2 and its features

· Understanding programmable interval timers and their applications

· Step-by-step guide to building a programmable timer circuit

· Programming the D8253AC-2 for various timing modes

· Testing, troubleshooting, and expanding the project for further applications

1. Introduction to the D8253AC-2

The D8253AC-2 is a Programmable Interval Timer (PIT) originally developed for microprocessor systems to generate timing pulses and control clock signals. It features three independent 16-bit counters, each of which can be programmed for different modes such as event counting, frequency division, or time delay generation.

Some key specifications of the D8253AC-2 include:

· 3 Independent Timers: Each timer can operate in six different modes, making it suitable for diverse applications.

· 16-bit Counter Width: Provides a broad range of timing durations and precision.

· Compatibility: Designed to interface easily with 8-bit microprocessors like the Intel 8085, making it a great component for retro computing enthusiasts.

2. Understanding Programmable Interval Timers

Programmable interval timers are essential in microprocessor-based systems for tasks like:

· Generating delays or time intervals for controlling external devices.

· Producing square wave or clock pulses for synchronization.

· Acting as a counter for events such as pulses from sensors.

The D8253AC-2 can be configured in various ways depending on the desired timing behavior. Its three counters can be used independently or together to achieve complex timing schemes.

3. Designing the Programmable Timer Circuit

Components Required

To build a basic programmable timer using the D8253AC-2, you’ll need:

· D8253AC-2 Programmable Interval Timer (PIT): 1 piece

· Microcontroller (e.g., 8051 or Arduino): To interface and control the PIT

· Oscillator (1 MHz or similar): For the clock input to the D8253AC-2

· 74HC244 Bus Buffer: To manage data lines and communication with the microcontroller

· Resistors: Various values (10kΩ, 1kΩ, etc.)

· Capacitors: Various values (10µF, 0.1µF, etc.)

· LEDs and Seven-Segment Display: For visual output

· Breadboard or PCB, Wires, and Soldering Tools

Circuit Diagram

Before assembling the components, review the basic connections required:

1. Microcontroller Interface: Connect the D8253AC-2’s data and control lines (e.g., RD, WR, CS) to the microcontroller’s I/O ports.

2. Clock Signal: The D8253AC-2 requires an external clock signal. Use a 1 MHz crystal oscillator connected to its CLK pin.

3. Output Connections: The outputs of the counters (OUT0, OUT1, OUT2) can be connected to LEDs or other devices to visualize the timer operations.

4. Bus Buffer (74HC244): This chip helps in managing the data lines, ensuring that signals from the microcontroller are correctly routed to the D8253AC-2.

A standard microcontroller-based setup typically involves writing code to configure and control the timers, allowing the user to adjust timing intervals using inputs like buttons or a keypad.

4. Building the Circuit Step by Step

1.Set Up the Microcontroller:

o Choose a microcontroller like an 8051 or an Arduino. Connect its digital I/O pins to the D8253AC-2’s control and data lines.

o Make sure to connect the power and ground pins of the D8253AC-2 to the respective lines from the microcontroller.

2.Connect the Oscillator:

o Place a 1 MHz oscillator (or a suitable frequency) between the CLK pin and ground. This provides the clock input for the counters.

3.Install the Bus Buffer:

o The 74HC244 buffer manages the connection between the D8253AC-2 and the microcontroller. This buffer is necessary when dealing with multiple control lines to ensure signal integrity.

4.Connect the Output Devices:

o Connect LEDs or a seven-segment display to the counter output pins (OUT0, OUT1, OUT2). This will visually indicate when a counter is active or when a timer event occurs.

5.Power Up and Test Basic Connections:

o Double-check all connections and power up the circuit. Ensure the microcontroller recognizes the D8253AC-2 and that the clock signal is stable.

5. Programming the D8253AC-2

The D8253AC-2 operates using various modes, allowing each of its three counters to function independently. The modes include:

· Mode 0 (Interrupt on Terminal Count): Generates an output pulse when the counter reaches zero.

· Mode 2 (Rate Generator): Produces a steady output waveform with a programmable period.

· Mode 3 (Square Wave Generator): Generates a symmetrical square wave useful for clock signals.

· Mode 4 (Software Triggered Strobe): Output goes high for one clock cycle after the counter reaches zero.

Let’s program the microcontroller (e.g., an Arduino) to set the D8253AC-2 in Mode 3, generating a square wave:

6. Testing and Troubleshooting

Testing Procedures

1. Check the Clock Signal: Ensure the oscillator is providing a stable clock input. You can use an oscilloscope to verify the frequency.

2. Monitor the Outputs: Observe the output pins (OUT0, OUT1, OUT2) using an LED or oscilloscope. Verify if the programmed mode (e.g., square wave generation) is functioning correctly.

3. Adjust and Test Timer Intervals: Modify the timer values in your microcontroller code and observe the changes in output behavior.

Common Issues and Solutions

· No Output Signal: Verify that the clock input is correctly connected and that the microcontroller is sending the right commands to the D8253AC-2.

· Incorrect Timing: Check the configuration code for errors in setting the control words. Ensure that the microcontroller is properly synchronized with the D8253AC-2’s operation.

7. Expanding the Project

With the basic programmable timer circuit operational, you can expand its functionality by:

1.Adding User Input Controls:

o Use buttons or a keypad to allow real-time control of timer settings.

o Connect a display to show the current timer value or mode configuration.

2.Integrating with Other Systems:

o Connect the timer to a motor driver circuit, allowing you to create a simple motor controller.

o Use the timer outputs to control relays for activating and deactivating devices like lights or alarms at programmed intervals.

3.Multi-Mode Operation:

o Configure the microcontroller to switch between different modes of operation, demonstrating the versatility of the D8253AC-2.

o Implement an automatic mode that cycles through different timing modes and outputs patterns.

8. Conclusion

Building a programmable timer using the D8253AC-2 provides a hands-on experience with timing and counting circuits, crucial in many microprocessor applications. By interfacing the PIT with a microcontroller, you can create a flexible timing system capable of precise and varied output behaviors. This project serves as a foundation for more complex applications, including digital clocks, frequency generators, and automated control systems.

Experimenting with the D8253AC-2 allows you to explore both the hardware and software aspects of electronics, opening up numerous possibilities for further development and innovation.