Fault Diagnosis and Redundant Technique for 24 Hours Clock Design Using VHDL

Abstract

In digital systems, precise timekeeping and fault tolerance are essential, especially in mission-critical applications. This project presents the design and implementation of a 24-hour digital clock using VHDL (VHSIC Hardware Description Language) with integrated fault diagnosis and redundancy techniques. The primary objective is to ensure the accurate display of time and the continuous operation of the clock even in the presence of faults. A redundant architecture is employed to provide backup operations in case of component failure, and diagnostic logic is incorporated to detect and isolate faults in real-time. Keywords: Test Vector Generator, Carry-out, Work mode, Repair mode, Test mode.

Introduction

This project presents a VHDL-based design of a fault-tolerant 24-hour digital clock suitable for critical applications such as transportation, automation, and industrial control. It addresses the issue of hardware failures and signal disturbances that affect system reliability by integrating fault diagnosis and repair mechanisms into the clock’s architecture.

Key Objectives

• Design a 24-hour digital clock using VHDL with built-in fault detection and correction.

• Ensure continuous and accurate timekeeping even in the presence of hardware faults.

• Use redundant architecture and a test mode to detect and isolate errors.

Core Features of the Proposed System

1. 24-Hour Clock Design:

   • Based on cascaded counters (MOD-10, MOD-6, MOD-3) for accurate tracking of minutes and hours.

   • Displays time in 24-hour format using BCD to seven-segment display conversion.

2. Fault Diagnosis Mechanism:

   • Incorporates a test vector generator and comparator to identify discrepancies in output.

   • Errors are flagged by comparing output with expected test patterns.

3. Redundancy and Self-Repair:

   • Employs modular redundancy to switch faulty modules with backup units.

   • Ensures real-time clock values remain accurate during repair.

4. Switching Logic:

   • Shifts counters from ripple mode to synchronous mode during fault repair.

   • Maintains system functionality without user interruption.

System Architecture Overview

• Working Mode: Normal operation of cascaded counters for timekeeping.

• Test Mode: Runs diagnostics on each module in parallel, detecting and isolating faults.

• Control Unit: Governs switching between modes and activates repair logic when needed.

• Display Unit: Uses seven-segment displays to show the real-time clock in 24-hour format.

Modules and Simulation Results

1. 60-Minute Counter:

   • Constructed using MOD-10 (units) and MOD-6 (tens) counters.

   • Accurate counting from 00 to 59, with waveform validation confirming proper operation.

2. 24-Hour Counter:

   • Uses MOD-10 (hours) and MOD-3 (tens of hours) counters.

   • Repairable version includes logic to detect and correct faults, maintaining a valid 0–23 count.

3. Faulty Module Detection & Repair:

   • Test mode signal initiates fault testing across all modules.

   • Faults trigger a repair signal, activating a redundant unit to replace the faulty one.

4. Fault Testing and Recovery:

   • Faults (e.g., in the “ten minutes” module) are detected through selector addressing.

   • Modular redundancy enables seamless module replacement and restoration of correct operation.

Conclusion

In this project, a fault-tolerant digital clock was successfully designed and implemented using VHDL, integrating key features such as testability and Two Modular Redundancy (TMR). The system was developed to maintain accurate timekeeping functionality while ensuring high reliability and fault resilience in digital hardware environments. The incorporation of TMR allowed the system to continue operating correctly even in the presence of single-module failures, thereby enhancing its robustness. The use of majority voting logic ensured that any faulty output from one of the redundant modules did not affect the final result, demonstrating the effectiveness of redundancy in fault masking. Furthermore, testability features such as self-checking mechanisms and internal signal monitoring were embedded into the design, enabling the detection and isolation of faults during both simulation and synthesis stages. These features significantly improve the maintainability and diagnostics of the clock system. Overall, the project showcases how digital systems can be made reliable and maintainable through thoughtful architectural design using VHDL. This design approach can be extended to other mission-critical digital systems where fault tolerance and testability are paramount.

References

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Copyright

Copyright © 2025 Ms. K. Pavani, Mr. Kurada V B C Sekara Rao, Dr. B. S. Rao. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.