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Problem with resetting CPU via external circuit

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GrandAlf

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Resetting CPU

I have several designs of mainly 8052 derivative CPUs. Usually I incorporate an external watchdog circuit, this is normally fed software generated pulses from a spare port pin. This usually works just fine, but on rare occasions the CPU simply does not respond to an sort of reset pulse. Only by a power recycle can I get it to start up again.

I have noticed this also on other types of processor, it is difficult to find out what is happening, as it is a fairly rare event. I cannot claim that this is a serious problem, but it would be interesting to hear comments about what is happening.

Anyone able to thow any light on this?.
 

stefk

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Hi,
I have had similar problems using external watchdog circuit. The main of them is that not always internal timers starts correctly?! I have made a simple test cycle in the begining of my programme , where I tested if the internal timers running and if not I stopped refreshing watchdog , to reset processor again. So after several resets the CPU started to work normaly but I still can't realize where the problem is. It will be helpful if someone tell me if 8051 clonings with internal watchdog have same strange behaviour.
Thanks
 

GrandAlf

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I suppose that could be it in some instances. I wish I could be there with a scope when it happens. I am not sure, but it seems like the CPU is just ignoring repeated reset pulses, until power down/up. I suppose it could be power related, or even the osc stopping. Beats me?
 

stefk

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In your case , when CPU does not starts at all hmmm .. I have just one crazy thought - use some small Microchip MCU like PIC 12C508 , just to controll the reset and power supply of your main processor :)))
 

stefk

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In yiur case , when CPU does not starts at all hmmm .. I have just one crazy thought - use some small Microchip MCU like PIC 12C508 , just to controll the reset and power supply of your main processor :)))
 

C-Man

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I know similiar problems under EMV conditions (for example when switching a motor or a stron relais) ...

All I can say that Microchip Pic parts work much more reliable under this conditions.

regards
 

GrandAlf

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I tend to use proprietry watchdog ICs that cover for brownout etc. I had thought as suggested, using another small micro to reset power. On reflection I considered that having 2 CPUs may well compound the problem. I do use relays on the same board as the processor, may be a layout thing. If I find the solution I will post it. Can't think that it is just me with this problem.
 

Bus Master

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About the relays problem, be sure to use a snubber circuit in parallel with the relay's coil. If you are using a DC relay, it's enough to put a reverse biased fast diode (like 1N4148 or BAT84) in parallel with the relay's coil. When driving an AC relay via an SCR or Triac, use a 2uF capacitor in series with 100 Ohms resistor and connect this snubber circuit in parallel with the relay's coil.

N.B., This Snubber circuit technique can be used with light current motors.

Hope this helps.
 

GrandAlf

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Thanks for that. All the usual protections have been complied with. This is not just one cct, but somthing that has been observed over many years of playing with various MCUs.
 

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