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Help me design an external watchdog for my system

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Zedman

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I would like to make some external watchdog to my system.

I tought I'll use the 74HC123 monostable multivibrator as it's a discrete part and feed it from the PIC time to time with a pulse. And if the pulse doesn't come anymore the 74HC123 will generate a pulse and triggers the other multivibrator in the 74HC123 to generate a longer (5 sec) negative pulse to the SHUTDOWN pin of the voltage regulator to turn off the power.

Anyone made a circuit like this, or got a better idea doing external wd?

thanks,
Zedman
 

extern watchdog circuit

Hi,
Do not switch off power, just use it to discharge the reset cap.
Regards,
Laktronics
 

external watchdog circuit

I've read that PICs built in watchdog sometimes fails and the reset pin also.
So I decided to switch off the power for 5 sec.

This system will be installed hundreds of kilometers away so it should do all it can to keep itself alive.

Added after 11 minutes:

Look:

Added after 1 minutes:

the line from the second part's ~Q is going to the shutdown pin of the VReg.
 

external circuit of watchdog

Dear all

Can you confirm regarding giving external watchdog

What happen when uC get hang in any condition so watchdog refresh(pulse) pin may be high or low. Is in this condition watchdog give a pulse to reset uC
Can u explain in detail about giving ckt.
regards
RVG
 

external watchdog requirements

Hi,
There are two retriggerable monoshots connected in series. The first one is triggered by the micro and kept in triggered state with a high output by continously triggering at its input. When the micro fails to trigger, the output pulse of the first mono falls to zero and this falling edge is used to trigger the second mono. The output pulse of the second mono is used to reset the micro.

Regards,
Laktronics
 

74hc123 circuits

Hi Do it the easy way - use TPS3828 WD ic or you can use the 74HC4960 as a programmable WD


All the best

Bobi
 

ds1232 and pic

The internal watchdog should be very reliable, if you want you could use two PICs that share a heartbeat and if the primary fails the secondary PIC could take over till it's serviced.
 

watchdog with 74hc123

You can use this circuit, in this, RESET connect to RESET pin of microcontroller, Kreset connect to another pin to lock reset circuit. If microcontroller is halt, and the time is over, this circuit will attack on RESET pin to reset the system again.

Added after 1 minutes:

https://obrazki.elektroda.pl/74_1211787730.jpg
 

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