Watchdog timer assumes varied roles

Terry Millward, Maxim Integrated Products, Lambourn Hungerford, UK -- EDN, 2/1/2001

The MAX6369-74 series of pin-selectable watchdog timers are designed to supervise µP activity and indicate when a system is working improperly. During normal operation, a µP should repeatedly toggle the WDI (watchdog input) before the selected watchdog-time-out period elapses to indicate that the system is properly executing code. If it fails to do so, the supervisor IC asserts a watchdog output   to signal that a problem exists. The cited family of watchdog supervisors are available in SOT23-8 packages and have selectable watchdog-time-out periods and delays of 1.7 msec to 104 sec in seven steps. The ICs also have selectable output-pulse widths of 1.7 or 170 msec, depending on part selection and the state of the: Set 0, Set 1, and Set 2 pins. You can use these devices for general-purpose timing funcions, especially when low current consumption is important. The ICs consume only 8 mA typical and 20 mA maximum over temperatures from a 2.5 to 5.5V supply. With WDI connected to ground or V_CC, the internal timer cycles, pulsing  low upon time-out. In addition to the lower current (20 versus 120 mA), the watchdog-timer IC takes less board space and uses no timing resistors or capacitors. The following circuits represent a few examples.

The circuit in Figure 1 uses a MX6373 to pulse

 low for 170 msec every 5.2 sec. The load is a front-panel power-on LED with a 1-k? current-limiting resistor. By pulsing the LED rather than powering it continuously, the average current decreases by a factor of 30 (88 µA versus 2.4 mA). The LED thus indicates that the equipment is on while minimizing battery drain. By changing the Set pins to Set 0=0V, Set 1=Set 2=V_CC, you can extend the off time to 17 sec, thus reducing the average current to 32 µA. The circuit in Figure 2 is similar to the one in Figure 1 but uses a MAX6371 to turn on a load for 170 msec every 104 sec. The load can be a battery-powered monitoring circuit that remains idle, saving power and then wakes up to make a measurement. The circuit in Figure 3 uses a MAX6373 with its Set inputs configured for timer disabled. If you hold Set 1 low for longer than the watchdog period (5.2 sec), then pulses low. You can use this circuit in applications in which a reset button is on a front panel, for example. You must deliberately depress the button for at least 5.2 sec to trigger a reset. This feature can prevent an accidental reset when someone inadvertently presses the button.