Design Feature: September 1, 1995

Many embedded systems use µP supervisors with on-chip watchdog timers. Some supervisors, such as the MAX691, allow you to select different watchdog and reset time-out periods, depending on how you connect pin 7 (Osc In) and pin 8 (Osc Sel). Table 1 in the MAX691 data sheet indicates the available watchdog and reset time-out choices. However, when you implement an extra-long watchdog time-out period by connecting a capacitor from pin 7 to ground and grounding pin 8, the reset time-out increases proportionally. In some embedded-system designs, such as a PC-compatible system with an LCD panel and a BIOS chip that takes a few seconds to initialize, the extended reset time can be undesirable. The circuit in Fig 1 allows you to set the watchdog time-out period independ-ently of reset time-out.

To maintain a short (200-msec) reset time-out and a long (18-sec) watchdog time-out, connect a 330-pF capacitor from pin 7 to ground and a FET (Q₁) from pin 8 to ground. Initially, RST is low, Q₁ is off, and pin 8 is floating. Approximately 200 msec after 5V is present at pin 3, RST goes high, grounding pin 8. If a 0-to-5V or 5-to-0V transition does not occur within approximately 18 seconds after reset, the cycle repeats. The watchdog time-out period is different during normal operation and immediately after reset (see Table 1).