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Have seen external watchdogs used fairly frequently on automotive and industrial circuits where you want to be sure that power devices are shut down if the MCU is out of control. The external watchdog is typically routed to two places-- the RESET# input of the MCU, to try to get it started again; and to the RESET# inputs (or equivalent) of peripheral devices that control outputs. If the MCU recovers, good. But if not, at least the motors are shut off, or the switching power supply is shut off, or whatever, so as to do the least amount of harm, absent intelligent control. Use of an external watchdog is a valid practice IMHO if you are dealing with a system where failure of the MCU clock could lead to a hazardous situation. Outright failures of the 68HC11 clock oscillator are not very common, but the high impedance oscillator is actually one of the most susceptible parts of a typical circuit. Quartz crystals can fracture if an ECU is shocked mechanically. Crud can build up on a board in the oscillator area and stop the clock. Electrical noise events can cause pseudo-resets that put the internal state machine into a lock-up condition. The need for an external watchdog is strong enough that they make voltage regulators with built-in watchdog timers, which are sold into the automotive market. Best regards, Kerry Berland Silicon Engines 2101 Oxford Road Des Plaines, IL 60018 USA 847-803-6860 Fax 847-803-6870 ----- Original Message ----- From: "Tony Papadimitriou" <> To: <> Sent: Wednesday, June 05, 2002 7:25 AM Subject: Re: [m68HC11] Thinking... > ----- Original Message ----- > From: Roberto G Berner <> > To: <> > Sent: Tuesday, June 04, 2002 10:44 PM > Subject: Re: [m68HC11] Thinking... > > I don't think that the internal watch-dog timer has > > any kind of functional bug, but if the final product spec is very > tight, I > > would never trust the internal watch-dog by itself, just because it > depends > > on the MPU oscillator clock. > > The way I understand the internal watchdog circuitry makes me think > that the only possibility for the internal watchdog to not work is > that the MCU stops 'clocking'. An external watchdog using a different > clock reference would timeout even then but since the MCU is not > executing any code, it would be practically useless! Where would one > route the timeout signal from the external watchdog? Wouldn't you > need a second MCU to run the code for the external watchdog's trigger. > And then, the same problem repeats... > > > Roberto Guillermo Berner > > To unsubscribe from this group, send an email to:

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