The selection of the three discrete components shown in the figure is straightforward and cost-effective, but it comes with a notable drawback. When the battery voltage gradually decreases to a level near the VT detection threshold, even a small glitch or noise on the supply voltage can cause the RST output to toggle between high and low states repeatedly. This instability prevents the microcontroller from reliably staying in the reset state, which could lead to unpredictable behavior.
To address this issue, it's essential to introduce a hysteresis voltage, Vhys, to the undervoltage detection threshold VT. This concept is similar to the hysteresis found in Schmitt triggers. Typically, the hysteresis voltage is set to around 0.2V. By doing so, the detection threshold splits into two levels: the upper threshold VTMAX (VT + Vhys/2) and the lower threshold VTMIN (VT - Vhys/2). The RST signal will only go high when the power supply voltage drops below VTMIN and will return to low only after the voltage rises above VTMAX. This hysteresis helps prevent frequent and unwanted resets due to minor fluctuations in the power supply.
Determining the upper and lower thresholds, setting the hysteresis voltage, selecting the appropriate dedicated chip, and understanding how the hysteresis voltage relates to the threshold voltage, regulator supply voltage, and the MCU’s nominal operating range are all critical steps. You can refer to the "Planning Reference Ruler" illustrated in the figure for guidance.
As shown in the figure, from left to right, we see the microcontroller’s nominal operating voltage range, the regulated power supply’s voltage range, the planned upper and lower thresholds for the undervoltage detection circuit, and finally the specific upper and lower threshold voltages along with the hysteresis value of the under-voltage detector model. Observing and analyzing this diagram provides several insights:
1. Most microcontrollers have a relatively wide operational voltage range. For example, the Philips P87C552 operates between 2.7V and 5.5V, while the ATMEL T89C51RD2-M works between 3V and 5.5V.
2. A regulated +5V power supply may have a nominal value of 5V, but it allows for small fluctuations based on load, temperature, and input voltage variations.
3. The output voltage of a regulated power supply can vary slightly around its nominal value. For instance, a MC7805 three-terminal regulator has an output range of 4.75V to 5.25V (±5%).
4. The upper threshold (VTMAX) for the undervoltage detection circuit should be set below the minimum output voltage of the regulator to detect early signs of voltage drop.
5. The lower threshold (VTMIN) should be higher than the minimum operating voltage of the MCU to ensure stable operation and allow time for protection mechanisms before the system shuts down.
6. Once the initial thresholds are determined, choosing a specific voltage detector from the market becomes easier. These detectors usually offer precise threshold voltages and narrow hysteresis ranges, often around 0.2V to 0.3V. For example, the ADM810M has a threshold of 4.375V and a hysteresis of 0.25V.
A practical design example of an undervoltage detection threshold is presented in the following table. It includes a specific microcontroller, a three-terminal voltage regulator, and a voltage detector. This example aligns with the information provided in the accompanying figure.
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