Circuit provides brownout control of 80C31
By N Kannan, Mediatronix, Pappanamcode, India -- EDN, August 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 VCC. 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 VCC 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).
