Datasheet MAX6369, MAX6370, MAX6371, MAX6372, MAX6373, MAX6374 (Maxim) - 7

Pin-Selectable Watchdog Timers MAX6369–MAX6374
completion of the startup delay period, and no watch- delays from 3ms to 10s minimum. Two of the selectable dog output pulses will be asserted during the startup modes do not initiate the watchdog timer until the delay. When the startup delay expires, the watchdog device receives its first valid watchdog input transition begins counting its normal watchdog timeout period (there is no fixed period by which the first input must be and waiting for WDI transitions. The startup delay received). These two extended startup delay modes allows time for the µP system to power up and fully ini- are useful for applications requiring more than 60s for tialize before assuming responsibility for the normal system initialization. watchdog timer updates. Startup delay periods vary All the MAX6369–MAX6374 devices may be disabled between the different devices and may be altered by with the proper logic control pin setting (Table 1). the logic control set pins. To ensure that the system generates no undesired watchdog outputs, the routine
Applications Information
watchdog input transitions should begin before the selected minimum startup delay period has expired.
Input Signal Considerations
Watchdog timing is measured from the last WDI rising The normal watchdog timeout period countdown is initi- or falling edge associated with a pulse of at least 100ns ated when the startup delay is complete. If a valid logic in width. WDI transitions are ignored when WDO is transition is not recognized at WDI before the watchdog asserted, and during the startup delay period (Figure timeout period has expired, the supervisor asserts a 2). Watchdog input transitions are also ignored for a watchdog output. Watchdog timeout periods vary setup period, tSETUP, of up to 300µs after power-up or between the different devices and may be altered by a setting change (Figure 3). the logic control set pins. To ensure that the system generates no undesired watchdog outputs, the watch-
Selecting Device Timing
dog input transitions should occur before the selected SET2, SET1, and SET0 program the startup delay and minimum watchdog timeout period has expired. watchdog timeout periods (Table 1). Timeout settings The startup delay and the watchdog timeout period are can be hard wired, or they can be controlled with logic determined by the states of the SET0, SET1, and SET2 gates and modified during operation. To ensure smooth pins, and by the particular device within the family. For transitions, the system should strobe WDI immediately the MAX6369 and MAX6370, the startup delay is equal before the timing settings are changed. This minimizes to the watchdog timeout period. The startup and the risk of initializing a setting change too late in the watchdog timeout periods are pin selectable from 1ms timer countdown period and generating undesired to 60s (minimum). watchdog outputs. After changing the timing settings, two outcomes are possible based on WDO. If the For the MAX6371 and MAX6372, the startup delay is change is made while WDO is asserted, the previous fixed at 60s and the watchdog timeout period is pin setting is allowed to finish, the characteristics of the selectable from 1ms to 60s (minimum). new setting are assumed, and the new startup phase is The MAX6373/MAX6374 provide two timing variations entered after a 300µs setup time (tSETUP) elapses. If for the startup delay and normal watchdog timeout. the change is made while WDO is not asserted, the Five of the pin-selectable modes provide startup delays new setting is initiated immediately, and the new start- from 200µs to 60s minimum, and watchdog timeout up phase is entered after the 300µs setup time elapses. tWD WDI * * * * * tWD WDO tWD SET_ tSETUP tDELAY tWD tSETUP tDELAY tWD *IGNORED EDGE Figure 3. Setting Change Timing
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