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Figure 5. A glitch with a less magnitude but occurs in a longer duration will trigger a reset signal as opposed to a short-duration glitch with greater magnitude.
Figure 6. Asserting of the reset signal will depend on the magnitude of the overdrive and its duration.
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behaviour shown in the blue-shaded area in Figure 4. This helps the supervisor in protecting the system from false resets.
Reset threshold overdrive vs. duration
Voltage glitches from external factors can occur in any system for either short or long periods. They can also have different magnitudes of voltage dip. Reset threshold overdrive vs. transient duration has something to do with the magnitude and duration of the voltage glitch or overdrive. A short-duration glitch with a greater magnitude will not trigger a reset signal to assert, while a less-magnitude overdrive with a longer duration will trigger a reset as shown in Figure 5.
Voltage transients in the monitored supply are ignored depending on their
duration. Disregarding these transients will protect your system from nuisance resets such as those caused by short-duration glitches. These glitches can falsely trigger system resets, to undesirable behaviour of the system. In the product data sheet, the reset threshold overdrive vs. duration is often illustrated in one of the typical performance characteristics plots such as in Figure 6. Any values above the curve will trigger a reset output while values within the curve will be ignored to prevent the system from false resets.
Manual reset setup period (tMR and debounce time (tDB
)
The reset timeout period, threshold overdrive vs. duration, and the threshold hysteresis address voltage glitches and transients associated with the monitored voltage, which is usually the power supply
)
of the system microcontroller. For the glitches brought by the mechanical contacts such as switches, the manual reset setup period and the debounce time alleviate the possible effects of the voltage transients and glitches.
The manual reset setup period (tMR ) is
the time required for the manual reset to hold and complete before it triggers a reset output. Some supervisors are made to have a long manual reset setup period to add protection to the system. These are common on consumer products that you need to hold on to the button for several seconds to reset your system. This method avoids accidental and unintended reset,
Figure 7. The manual reset setup period and debounce time diagram of a supervisor with a long manual reset setup period (MAX6444): (a) the manual reset setup period (tMR) needs to be completed first before a reset signal asserts; (b) to be considered as a valid input signal, debounce time (tDB) is required to be completed.
thus adding protection and reliability. With the manual reset setup period, all the short-duration transients and glitches when pushing on the switch are ignored, as shown in Figure 7a, thus helping the system to be glitch immune. The same concept applies to the debounce time. Like the setup period, debounce time (tDB
) ignores the high
frequency periodic voltage transients when pushing on or off a switch. These high frequency transients are considered invalid and do not trigger a reset as shown in Figure 7b. When the signal exceeds the debounce time, that is the time it will be considered a valid input signal from a switch or a push button.
Conclusion
Pinkesh Sachdev. “Adding Hysteresis for Smooth Undervoltage and Overvoltage Lockout.” Analog Dialogue, Vol. 55, No. 1, March 2021. References
1 2 3
“The Why, What, How, and When of Using Microprocessor Supervisors.” Maxim Integrated, April 2018. Greg Sutterlin. “Supervisors in Multivoltage Systems.” Analog Devices, Inc., November 2003.
32 December/January 2025 Components in Electronics
Without voltage supervisors, systems are at risk of brownout conditions and malfunction during voltage transients and glitches. Voltage supervisors solve this by putting processors into reset mode during such scenarios. All the parameters discussed above including reset timeout period, threshold hysteresis, threshold overdrive, manual reset setup period, and debounce time improve the reliability of voltage supervisors in monitoring power supply voltages by making them immune to glitches and transients. This gives stability and reliability to overall system performance.
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