The following application note demonstrates alternative uses for the MAX6369-MAX6374 family of pin-selectable watchdog timers. Alternative uses include a power saving power-on LED indicator circuit, load connect/disconnect circuit and push-button reset circuit.
The MAX6369-MAX6374 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-timeout period elapses to indicate that the system is properly executing code. If it fails to do so, the supervisor IC asserts a active-low WDO (watchdog output) to signal that a problem exists. The cited family of watchdog supervisors is available in SOT23-8 packages and has selectable watchdog-timeout periods and delays of 1.7msec to 104sec in seven steps. The ICs also have selectable output-pulse widths of 1.7msec or 170msec, 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 functions, especially when low current consumption is important. The ICs consume only 8mA typical and 20mA maximum over temperatures from a 2.5V to 5.5V supply. With WDI connected to ground or VCC, the internal timer cycles, pulsing active-low WDO low upon timeout. In addition to the lower current (20mA versus 120mA), 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 MAX6373 to pulse active-low WDO low for 170msec every 5.2sec. The load is a front-panel power-on LED with a 1kΩ current-limiting resistor. By pulsing the LED rather than powering it continuously, the average current decreases by a factor of 30 (88µA versus 2.4mA). 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 = VCC, you can extend the off time to 17sec, thus reducing the average current to 32µA.
The circuit in Figure 2 is similar to the one in Figure 1 but uses a MAX6371 to turn on a load for 170msec every 104sec. 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 (5.2sec), then active-low WDO 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 5.2sec to trigger a reset. This feature can prevent an accidental reset when someone presses the button inadvertently.
A similar version of this article appeared in the February 1, 2001 issue of EDN magazine.