The trick with a watchdog is that it needs to be edge triggered so the watchdog doesn't stay in reset should the processor crash during a reset. In an old MC6802 product I made a watchdog using a hex schmitt trigger (74hc14 or similar). It worked something like this... Make a standard inverting schmitt RC oscillator (capacitor to ground on input, resistor from output to input). Change it a bit by putting the capacitor to Vcc instead of ground. This way you'll get a low pulse out of the thing on power up. Make the frequency low (maybe 0.5 Hz or so). Put a series diode and resistor across the first resistor. Now the output is asymmetrical. Orient the diode so you have a short low pulse separated by periods of high (assuming an active low reset pin). The output of this circuit pulses low when the "bottom" of the capacitor goes above the upper threshold voltage of the schmitt. You can prevent a timeout by holding this voltage down. Do this by putting a diode between the bottom of the capacitor and the output of another schmitt. Since these are inverting schmitts, pulsing the input to this second schmitt high will reset the watchdog. Do not drive this second schmitt directly or you'll have the "crash during reset" problem. Instead, put a pulldown resistor on the input, then use a capacitor to connect the input to an output port. Pulsing that port bit high resets the watchdog. In my software on this circuit, I actually toggled the output bit each time around the loop. That way I didn't have to sit in a loop waiting for the bottom of the capacitor to be pulled down. A few hundred systems with this circuit have been working for almost 20 years. On the early ones I did not AC couple the watchdog reset. I once had to drive a hundred miles in Kansas to hit a reset button because of that... Harold On Tue, 27 Nov 2001 10:42:22 +0000 D Lloyd writes: > Hi, > > For a system with a micro with no on-board dog and where an external > wdog > ic "could" be eliminated, has anyone implemented anything cheaply > and > effectively to perform this function? I imagine a simple solution > would be > to have a CR arrangement where the C charges VCC and activates the > reset > line (for active high resets) if an output pin does not maintain a > specific > duty cycle. > > Any advances on that nastiness? > > Regards, > Dan > > -- > http://www.piclist.com hint: To leave the PICList > mailto:piclist-unsubscribe-request@mitvma.mit.edu > > FCC Rules Online at http://hallikainen.com/FccRules Lighting control for theatre and television at http://www.dovesystems.com ________________________________________________________________ GET INTERNET ACCESS FROM JUNO! Juno offers FREE or PREMIUM Internet access for less! Join Juno today! For your FREE software, visit: http://dl.www.juno.com/get/web/. -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu