The repository of switch bouncing traces has sparked a lively discussion among engineers about the most effective approaches to debouncing mechanical switches in electronic circuits. While the original collection provides valuable reference data showing the chaotic behavior of various switches during state transitions, the community's focus has shifted to debating optimal debouncing strategies for real-world applications.
Hardware vs. Software Debouncing Approaches
Engineers in the discussion have highlighted two primary schools of thought for handling switch bounce: hardware-based solutions using passive components and software-based algorithms. Some practitioners advocate for simple hardware solutions, with one engineer describing their approach: For my hobby projects I hardware debounce with a single tiny capacitor. The MCU already has schmitt trigger inputs (handles hysteresis). And it also has a high value pull up resistor. The tiny capacitor from input to ground complete the low pass filter. This method leverages existing microcontroller features to create a clean transition without requiring additional code. However, others point out that pure hardware solutions can introduce unwanted latency in user interfaces.
Common Debouncing Approaches Mentioned
- Hardware Solutions:
- Capacitor + pull-up resistor + Schmitt trigger inputs
- Analog low-pass filtering with electrical hysteresis
- Software Solutions:
- Two time constants approach (sampling for state change + ignore period)
- State machine with auxiliary variables
- Jack Ganssle's debouncing algorithms (referenced by multiple commenters)
Key Considerations for Debouncing
- Response latency vs. reliability trade-offs
- EMI rejection capabilities
- Environmental factors beyond mechanical bouncing
- Input capacitance effects on signal behavior
Balancing Latency and Reliability
A significant theme emerging from the discussion is the trade-off between response time and reliability. Several engineers described sophisticated software approaches that maintain both low latency and high reliability. One particularly interesting method uses two time constants - one to determine how many consecutive samples are needed to change state, and another to ignore subsequent inputs temporarily after a state change. This approach allows the system to register button presses quickly while still filtering out bounces, creating a responsive yet stable interface.
I do something very similar, but use two time constants. One to determine how many consecutive samples are needed to change from the established state, and a second one, as suggested here, to ignore subsequent input for a bit after a registered state change to allow for bouncing before going back to the receptive state. With well-chosen constants, I've found similar behaviour to the above; low latency and high performance at once.
Environmental Considerations Beyond Mechanical Bouncing
While the repository focuses on mechanical bouncing patterns, engineers emphasized that real-world implementations must consider additional environmental factors. Several commenters noted that electromagnetic interference (EMI) presents challenges beyond mechanical bouncing, particularly in systems where switches connect to long wires that effectively act as antennas. This highlights the importance of comprehensive approaches that address both mechanical bouncing and electrical noise. One engineer shared an anecdote about a colleague who successfully built a system combining a multi-kilowatt electric arc lamp and a CCD array with sensitive electronics, demonstrating that even extreme EMI challenges can be overcome with proper techniques.
The Complexity of a Simple Problem
Despite some engineers describing debouncing as trivial, the discussion reveals it remains a common source of defects in embedded systems. As one commenter noted, incorrect debouncing is the most common (and usually exceedingly obvious) defect in embedded systems. The repository's detailed traces show why - the seemingly simple on/off behavior of switches actually involves complex, chaotic transitions that vary significantly between switch types and even between individual actuations of the same switch. This complexity is further increased by the observation that the measurement equipment itself (with its 50-60 pF input capacitance) likely smoothed some of the transitions, meaning real microcontroller inputs might see even more chaotic behavior.
The discussion around this switch bouncing repository demonstrates that even seemingly basic electronic components require thoughtful engineering approaches. Whether implementing hardware filters, software algorithms, or a combination of both, understanding the physical behavior of switches remains fundamental to creating reliable electronic interfaces.
Reference: Switch bouncing traces