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Be careful with MCLR/RE3

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lbodnar
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2006/01/11 14:30:53 (permalink)
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Be careful with MCLR/RE3

Just to reflect on my experience - a gotcha that may catch you when you are using MCLR pin as an input RE3.

Even being an input, any voltage spike on that pin will take the chip into programming mode which would look from the outside as a random reset. If you are not planning to use in-circuit programming and you could potentially have noise (e.g. long wires or traces running towards it), put a diode from it to Vdd to drain the excessive voltage off.

Keep this in the back of your mind - turning MCLR off does not mean the devioce cannot be rest through that pin (by taking it into programming mode and back).

We are only talking reasonable noise spikes, of course, but even a short blip may cause headaches.

Cheers!
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    RandySmith
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    Re: Be careful with MCLR/RE3 2014/08/11 08:59:30 (permalink)
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    Thanks for that note about RE3 being only a troublesome input if no load resistor is attached.
     
    I thought it was an I/O pin and had it set to output low thinking it was holding the Vpp line low via internal output transistor when not in use for ICSP. So i have it running with a 3-4 inch trace to a programming connector and it was causing random resets, damage to my hardware and or damage to the internal program firmware requiring reprogramming.
     
    My main question to Micro chip is why the hell didn't they equip this pin with an internal 100k pull down resistor so that it's never left floating when set up as RE3 input. ???
     
    Would have saved me a lot of time and energy over the years trying to find very rare and random problems that at times have been catastrophic failure of my product.
     
    Yours truly,
    Annoyed.
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    drazen
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    Re: Be careful with MCLR/RE3 2014/08/11 09:53:21 (permalink)
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    I had a different problem and MC did not helpped a lot.
    My problem was with 24F K series when I programmed MCLR as input. When I protected PIC RealICE was not able to earase it any more. Fortunately PICKit3 was able. I tried with several chips and same result.
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