Low-cost microprocessor supervisors reset controllers when power-supply voltages fall below given levels. As added protection, you can also reset the microcontroller when the power supply is too high by combining a low-cost shunt-voltage regulator with a supervisor that has a manual reset input. A simple overvoltage/undervoltage-protection circuit is easy to make (Figure 1). The circuit's output is active (low) when the monitored supply voltage, V_CC, is outside a predefined range. After the supply voltage returns to within the functioning limits, the reset output, remains active for a minimum of 140 msec. This interval gives the system time to stabilize before you remove the reset. The threshold voltage of the CAT811 microprocessor reset circuit, IC₂, sets the lower limit of the voltage range, V_LOW. This threshold can be 2.32 to 4.63V using standard products. Custom threshold devices with thresholds as low as 1.8V are also available. The 1.24V CAT431L shunt regulator, IC₁, and two resistors, R₁ and R₂, set the upper limit, V_HIGH: V_HIGH=V_REF(1+R₁/R₂), where V_REF is the internal reference voltage of IC₁ (V_REF=1.24V). The maximum V_HIGH that you can set is 5.5V, and the maximum supply voltage is 9V.

This design uses the CAT811 supervisor with a threshold voltage of 4.63V for tests. The upper limit of the voltage range is 5.5V using a 10-kΩ potentiometer with R₁=7.75 kΩ and R₂=2.25 kΩ. As the supply voltage rises into range, the reset pin becomes active (low) for a minimum of 140 msec (Figure 2). When the supply voltage falls into range, the reset pin also becomes active (low) for a minimum of 140 msec (Figure 3). A reset signal asserts when the supply voltage increases out of range (Figure 4). A reset signal also asserts when the supply voltage falls out of range (Figure 5).

Check out our Best of Design Ideas section!