Understand and apply supervisory ics to avoid low-voltage power-up glitch headaches
between 0.5 to 0.9 volts, potentially causing system instability. Once the supervisory IC turns on, the reset line is pulled down to prevent the microcontroller from inadvertently turning on. This glitch is common to all previous generations of supervisory ICs. Low-voltage systems magnify the problem This glitch scenario becomes a major concern with the trend toward low-power devices that are operating at ever-lower voltages. Consider systems with three logic levels of 3.3 volts, 2.5 volts, and 1.8 volts ( Figure 3 ). For the 3.3-volt system, the output low- voltage threshold (Vol) and the input low-voltage threshold (Vil) are between 0.4 volts and 0.8 volts. If a glitch occurs at 0.9 volts,
it would potentially cause the processor to become unstable by switching it off and on. The situation for a nominal 1.8-volt system is more sensitive. Now, Vol and Vil are much lower at 0.45 volts and 0.63 volts. A 0.9 volt glitch in this system represents a larger percentage, giving it a higher potential for error. How does this situation play out with the glitch impacting system operation? Consider a power supply voltage VDD which ramps up slowly to 0.9 volts and “lingers” there for a short period of time ( Figure 4 ). Although this voltage is not enough to turn on the supervisory IC, the microcontroller could still be enabled and running in an unstable state. Since the 0.9-volt value is in an indeterminant state, the glitch can be interpreted by the microcontroller RESET input as either a logic 1 or 0, which would erratically enable or disable it.
individual component variations in a batch of the same device, and other hard-to-determine factors. What is this glitch, and what is its source? Consider a system with a microcontroller and an associated supervisory/protection reset IC. The role of the latter IC is simple and focused: to maintain reliable system operation during power- up, power-down, and brownout conditions ( Figure 1 ). In a typical battery-powered application, the DC-DC converter generates the supply rail from a small, low-voltage battery. The supervisory IC is generally added between the DC-DC converter and the microcontroller to monitor the supply voltage and enable or disable the microcontroller. The supervisory IC ensures reliable operation by accurately monitoring the system power supply and then asserting or de-asserting the microcontroller's enable input. The enabling and disabling of the microcontroller is managed via the supervisory IC’s reset output pin. This pin is typically an open- drain that is connected to a 10 kilohm (kΩ) pull-up resistor. The supervisory IC monitors the power supply voltage and asserts a reset when the input voltage falls below the reset threshold. After the monitored voltage rises above the threshold voltage
This causes the microcontroller to execute partial instructions or incomplete writes to memory, as just two examples of what might happen, likely causing system malfunction and possible catastrophic system behavior. Solving the glitch problem Overcoming this problem does not require a return to higher voltage rails, or demand complicated system-level architectures to eliminate its occurrence or minimize its impact. Instead, it requires a new generation of supervisory ICs that recognize the unique aspects of the problem and prevent glitches from forming, regardless of the voltage level during power-up or brown-out conditions. Figure 4: As the power supply voltage VDD ramps up to 0.9 volts and lingers there, the microcontroller can be turned on and off erratically. (Image source: Analog Devices)
Figure 1: Understanding a glitch source begins with a look at a simple, typical arrangement of a microcontroller and its associated supervisory/protection reset IC, both powered by a battery and its regulator. (Image source: Analog Devices)
to its nominal value, the reset output remains asserted for a reset timeout period and then de-asserts. This allows the target microcontroller to leave the reset state and begin operating.
before the supervisory IC turns on and pulls it low? The answer is found by looking closely at a typical power-up sequence ( Figure 2 ). As supply rail VCC begins to power up, both the microcontroller and the supervisory IC are off. As a consequence, the reset line is floating and the 10 kΩ pullup resistor causes its voltage to track V CC . This voltage rise can be anywhere
But what happens to the reset line
Figure 2: In a typical power-up sequence, the reset line is floating, so its voltage tracks the rise in supply rail VCC. (Image source: Analog Devices)
Figure 3: Logic levels have shrunk from 3.3 volts down to 1.8 volts, and so have associated voltage thresholds. (Image source: Analog Devices)
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