DESIGN SOLUTION 7550

Is Your Application Protected from Glitches?

By: Tom Bui

Abstract:

Medical, industrial, and consumer devices require reliable operation, free from startup glitches. With the glitch-free operation available in the MAX16162, Maxim Integrated’s nanoPower supervisor IC, designers now have the means to prevent system startup glitches


Introduction

Microcontroller supervisor integrated circuits (ICs) provide a means to maintain reliable system operation during power-up, power-down, and brownout conditions. These protection ICs do this by accurately monitoring the system power supply and asserting or de-asserting the microcontroller's reset input to ensure the voltage level is above the microcontroller's minimum operating voltage. They also provide an adequate amount of time for the power supply to settle.

Microcontroller System Diagram

Typical microcontroller system block diagramFigure 1. Typical microcontroller system block diagram.

Typical battery-powered applications utilize a DC-to-DC converter to generate the supply rail from either a lithium or alkaline battery. The supervisor IC can be added between the DC-to-DC converter and the microcontroller to monitor the supply voltage and enable or disable the microcontroller.

Enabling and disabling the microcontroller is accomplished through the supervisor's reset output pin. This pin is typically an open-drain pin that is connected to a 10kΩ pullup resistor. The supervisor 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, the reset output remains asserted for the reset timeout period and then de-asserts, allowing the target microcontroller to leave the reset state and begin operating. But what happens to the reset output before the supervisor turns on and pulls the reset output low?

The Glitch

To answer this question, let us look at a typical power-up sequence (Figure 2). As the supply rail, VCC, begins to power up, both the microcontroller and the supervisor are off. Because of this, the reset line is floating and the 10kΩ pullup resister causes the voltage to track VCC. The voltage rise can be anywhere between 0.5V to 0.9V, potentially causing system instability.

Power-up sequenceFigure 2. Power-up sequence.

Once the supervisor 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 supervisor ICs.

Low-Power Technology Challenges

With the current trends for low-power devices operating at lower voltages, this becomes a major concern. Figure 3 shows three logic levels of 3.3V, 2.5V and 1.8V. For the 3.3V systems, Vol and Vil are between 0.4V and 0.8V, ensuring that the logic level is low. If a glitch occurred at 0.9V, then this would potentially cause the processor to become unstable by continuously switching off and on.

3.3V to 1.8V logic levelsFigure 3. 3.3V to 1.8V logic levels.

Now let us consider 1.8V systems. Vol and Vil are much lower at 0.45V to 0.63V, respectively, and the 0.9V glitch in this system represents a larger margin of error, so this type of system has a higher potential for a glitch causing a system error.

Glitch Impact

Next, let us look at how a glitch can impact your system's operation (Figure 4). In this example, the power-supply voltage (VDD) ramps up slowly to 0.9V and lingers for a short period of time at 0.9V. The voltage is not enough to turn on the supervisor IC, but the microcontroller could be enabled and running in an unstable state. Because the 0.9V is in an indeterminant state, the glitch can be interpreted by the RESET input as either a logic 1 or 0, which would enable or disable the microcontroller erratically. This causes the microcontroller to execute partial instructions or incomplete writes to memory, potentially causing a catastrophic issue.

3.3V to 1.8V logic levelsFigure 4. Unstable reset input.

Eliminating Supervisor Reset Glitches

To solve this issue, it requires a new generation of supervisor ICs that can prevent glitches from forming regardless of the voltage level during power down or brown outs. This requires a proprietary circuit that has the reset output to be held low when VDD is in the indeterminant range. The MAX16162 does exactly this. Whenever VDD is lower than the threshold voltage, the reset output is held low, preventing a voltage glitch on the reset line. Once the voltage threshold is reached and the delay has completed, the reset output de-asserts, which enables the microcontroller (Figure 5).

Glitch-free systemFigure 5. Glitch-free system.

Conclusion

Glitches are present in every application and in the past have not posed a significant issue for higher voltage applications, but with new lower power systems, supply voltages are moving lower, making the system less reliable with 0.9V glitches. A new generation of supervisor ICs offer glitch-free operation to provide the highest degree of system protection for today's and future low-power applications.