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CAT825RSDI-GT3 Datasheet(PDF) 6 Page - ON Semiconductor |
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CAT825RSDI-GT3 Datasheet(HTML) 6 Page - ON Semiconductor |
6 / 12 page CAT823, CAT824, CAT825 http://onsemi.com 6 Watchdog Timer The CAT823 and CAT824 provide a Watchdog input (WDI). The watchdog timer function forces RESET (and RESET in the CAT824) signals active when the WDI input does not have a transition from low−to−high or high−to−low within 1.12 seconds. Timeout of the watchdog starts when RESET (RESET on the CAT824) becomes inactive. If a transition occurs on the WDI input pin prior to the watchdog time−out, the watchdog timer is reset and begins to time−out again. If the watchdog timer is allowed to time−out, then the reset output(s) will go active for tRP and once released will repeat the watchdog timeout process. Figure 6 below shows a typical implementation of a watchdog function. Any processor signal that repeats dependant on the normal operation of the processor or directed by the software operating on the processor can be used to strobe the watchdog input. The most reliable is a dedicated I/O output transitioned by a specific software instruction. The watchdog can be disabled by floating (or tri−stating) the WDI input (see Figure 7). If the watchdog is disabled the WDI pin will be pulled low for the first 7/8th’s of the watchdog period (tWD) and pulled high for the last 1/8th of the watchdog period. This pulling low of the WDI input and then high is used to detect an open or tri−state condition and will continue to repeat until the WDI input is driven high or low. For most efficient operation of devices with the watchdog function the WDI input should be held low the majority of the time and only strobed high as required to reset the watchdog timer. Figure 6. Watchdog Timer CAT823 PIC μC DECODER ADDRESS GND WDI Supply Voltage VCC RESET MR MCLR Figure 7. Watchdog Disable Circuit mC OUTPUT Tristate 150 kW CAT823 GND WDI MR RESET 110 kW VCC VCC Figure 8. Timing Diagram – Strobe Input WDI INVALID STROBE VALID STROBE MIN. MAX. INDETERMINATE STROBE RESET tWD |
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