A Maxim MAX6710 IC (http://pdfserv.maxim-ic.com/arpdf/ MAX6710.pdf) can be used to monitor four supply voltages. If any monitored voltage drops below the threshold voltage set by the manufacturer, the IC triggers a Reset signal. This signal remains active for 140 ms after the voltages on all four inputs rise above the threshold voltage, in order to reliably reset the connected system. The fourth input (IN4) can be freely programmed using an external voltage divider. Its threshold voltage is set to 0.62 V. To calculate the values of the voltage divider resistors R1 and R2, you can choose a value for R2 (100 kΩ, for instance) and then calculate the value of R1 using the formula
The IC takes its power from the voltage applied to IN2, with a current consumption of only 35 μA. A faultless reset signal can be generated with a voltage of only 1 V present at IN1 or IN2. The IC is available in a SOT23 SMD package. A selection of the available types is shown in the table. If it is necessary to be able to generate a reset signal using a manually operated pushbutton, a pushbutton switch can be simply connected in parallel with resistor R2 in order to connect input IN4 to the ground when the button is pressed. In this case, R1 is required for current limiting.