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watchdog with long delay

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wanchope

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picaxe

Hi,
I found one of my microcontroller systems hang from time to time. When it hangs, I need to remove the battery and wait for 20 seconds for the capacitor to drain. After that, I connect back the battery and the system will work again.
I would like to design a hardware watchdog with this 20 seconds delay. When the system is running, the mcu will give a heartbeat pulse every 1 second. If the pulse is not detected by this watchdog timer, it will pull high the gate of a Pmos and wait for 20 seconds before pulling down the gate pin.
Any suggestion to go about this?
 

hef4538 watchdog

wanchope said:
Hi,
I would like to design a hardware watchdog with this 20 seconds delay. When the system is running, the mcu will give a heartbeat pulse every 1 second. If the pulse is not detected by this watchdog timer, it will pull high the gate of a Pmos and wait for 20 seconds before pulling down the gate pin.
Any suggestion to go about this?
Click to expand...

My suggestion is...build what you have design. What is the problem? From my point of view, you are capable to do it...so, build it and submit the real problem.

Best Regard
Rikie
 

circuit of hef4538

Hi,
I did not quite understand why you need to discharge a cap for 20 secs. Is it not sufficient to reset the micro using a watch dog output?
Any how, if you need to do the way you wanted, use two monoshots. One of them has to be a retriggerable monoshot with a pulse time of above two sec., say, 2.2 sec. Trigger this retrig. mono using the 1 sec. system pulse. In the absence of the 1 sec. pulse, the mono will time out and use the falling edge of the pulse to trigger a second mono with a pulse period of 20 sec. Use this 20 sec high pulse to drive the PMOS.

Regards,
Laktronics
 

Thanks.
Attached a possible circuit.
But I foresee one problem.
If the system hangs at the very first time, ie. no heartbeat pulse at all, this circuit will not generate the reset pulse.
Any comments?
 

Hi,
For the problem raised by you , you can probably OR the Power On Reset pulse with your two sec clock and apply as trigger, assuming the clock will be stuck at Zero when fails.

Regards,
Laktronics
 

Thanks, Laktronics.
I built the circuit today, and found one mistake.
When the MCU hangs and the heartbeat pin stays at low, reset is not happening.
Any clue?
 

I think it’s a perfect job for something like the PICAXE-08 ..
Cutting story short, it’a 8-pin PIC based microcontroller with BASIC interpreter ..
Almost no external components are required, and if you stuff it up for the first time – just change the code ..

Look, mate, the 555 is still good but in applications that require a little bit of “intelligence” the “555” of the next millennium (PICAXE) seems to be much better option ..

Take a look at this:


Rgds,
IanP
 

Hi,
Please test the circuit applying a clock to its input and see if the reset is generated when the clock is stopped. By the way you were talking of generation of a 20 sec. pulse and not reset pulse in your requirements?.

Regards,
Laktronics
 

I used the function gen for the 0.5hz pulse. When i removed the clock input physically, the circuit works. But if i just switch off the function gen (leaving the clock pin intact), it does not.
I have another test again to confirm. At first I give a pulse at the clock input (pin 2), then ground the pin. The circuit does not work.
I think, the transitor always turns on, the cap always got discharged, and pin 6&7 is below the threshold value. That is why pin 3 of the first 555 stays high and does not generate the high-to-low trigger to the second 555. I guess a HEF4538 will solve the issue. But any better solution using the current circuit?
 

Hi,
You got the problem right. You may need to AC couple the trigger input and use a grounded emitter NPN transistor with its base driven directly by the pulse input through a base resistor.

Regards,
Laktronics
 

Hi,
I think this circuit can help you
Only re-check polarity of the signals
Enjoy
 

I am sorry. Some correction in attached circuits
 

wanchope,
The Maxim 6369 series of WD timers provides delays of up to 60 seconds.
**broken link removed**
Reagrds,
Kral
 

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