Circuit provides brownout control of 80C31

By N Kannan, Mediatronix, Pappanamcode, India -- EDN, 8/19/1999

In the reset and watchdog-timer circuit in Figure 1, IC₁ is a 74HC14 Schmitt-trigger inverter that, with R₁ and C₁, acts as an astable oscillator. The circuit provides an active-high reset for an 80C31 µC. The watchdog trigger (WDT) consists of watchdog-trigger pulses from a port line. At power-on, the voltage on C₁ is 0V, and reset=1. As C₁ charges, reset goes low, and the µC generates watchdog-trigger signals. These ac-coupled pulses periodically turn on Q₁ and charge C₁ to V_CC. This action prevents C₁ from discharging through R₁ when reset is low. If the watchdog-trigger pulses stop, Q₁ turns off and C₁ discharges through R₁. reset goes high, resetting the µC. Now, C₁ charges through R₁, and reset goes low after the reset period. D₁ prevents charge-pump action, and D₂ provides a fast discharge path for C₁ when the supply goes down. The Q₂-Q₃ combination acts as a low-voltage reset circuit. When V_CC decreases to less than approximately 4.5V, Q₂ turns off and Q₃ turns on, discharging C₁; reset then goes high. The circuit works with voltages as low as 1.5V. During power-up and -down, hysteresis of the inverter provides a clean reset signal. (DI #2400).