The design choice of making RESET/MCLR pins active low on most integrated circuits (ICs) is not arbitrary but stems from a combination of historical, practical, and logical reasons. This choice, while seemingly peculiar at first, has significant implications for the reliable and efficient operation of electronic circuits. Understanding the rationale behind this design decision provides valuable insights into the underlying principles of digital circuitry and how ICs interact with external components.
The History of Active Low Signals
The use of active low signals, where a low voltage level represents a logical "1" and a high voltage level represents a logical "0," predates the widespread adoption of CMOS technology. In the early days of digital electronics, transistors were primarily implemented using bipolar junction transistors (BJTs). These devices operated with active low logic, where a low voltage level at the base of the transistor enabled current flow and represented a logical "1." This approach naturally led to the adoption of active low signals in ICs, as it aligned with the inherent characteristics of the transistors used in their construction.
Practical Considerations and Advantages
Active low signals offer several practical advantages that have contributed to their prevalence in modern digital circuitry.
1. Robust Reset and Initialization
The active low nature of RESET/MCLR pins ensures that devices can be reliably reset even in the presence of noise or power-on transients. When power is first applied, the voltage levels on the pins may fluctuate before settling to their intended values. An active low reset signal ensures that the IC remains in a reset state until the voltage on the RESET/MCLR pin drops below the threshold for a logical "1." This robustness helps to prevent unwanted or unpredictable behavior during the power-up sequence.
2. Simplified Logic Design
Active low signals can simplify logic design, particularly in situations where multiple signals need to be combined to generate a reset condition. For instance, using a NOR gate with active low inputs makes it easy to generate a reset signal that is asserted only if any one of the inputs is low. This approach avoids the need for complex inverters or multiple logic gates, contributing to reduced circuit complexity and improved performance.
3. Compatibility with Traditional Logic Families
Active low signals have been a standard in traditional logic families like TTL (Transistor-Transistor Logic) and CMOS (Complementary Metal-Oxide Semiconductor). Many peripherals and supporting components have been designed to work with this convention. Switching to active high signals would have introduced compatibility issues and forced a complete redesign of existing systems.
Considerations for Active High RESET/MCLR
While active low RESET/MCLR pins are the dominant standard, there are situations where active high signals might be considered.
1. Simplifying Logic for Specific Applications
In some cases, active high RESET/MCLR signals might offer a more intuitive design approach, particularly when dealing with circuits where logic operations are primarily based on active high signals. However, this requires careful consideration to avoid potential compatibility issues with existing components.
2. Avoiding "Stuck-Low" Conditions
In circuits that rely on external pull-up resistors to maintain a high logic level, a "stuck-low" condition can occur if the resistor fails or is disconnected. With an active high RESET/MCLR, this situation would prevent the device from being reset, potentially leading to unexpected behavior. However, proper circuit design and component selection can minimize this risk.
Conclusion
The use of active low RESET/MCLR pins in ICs is a consequence of historical development, practical considerations, and design conventions. This approach offers advantages in terms of reset robustness, logic simplicity, and compatibility with established logic families. While active high signals can be advantageous in specific situations, active low RESET/MCLR remains the prevalent standard for its reliability and efficiency in a wide range of applications.