PIC doesn't start witout scope

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  1. - 25th April 2010, 04:07 #1
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    Default PIC doesn't start witout scope

    16F876A, I have a 1500 uf cap in front of an AP1084 5v regulator. On the 5v side, I have a 470uf cap. There are 24 1/4hp motors on the 12v side. They create alot noise so thats why I run the large value caps. Here is the deal, on power-up, quite often a PIC doesn't start. I couldn't figure out what was going on. Then I thought I needed to reprogram PIC, as soon as I connected the programmer, the PIC started. The next time it happened, I thought I would trace things out with my scope. As soon as I touch the scope lead to the MCLR pin, the PIC started again. Once it starts, it will keep going. I would say 75% of the time, I need to jump start with my scope.

    Any ideas? What is happening to MCLR when I touch it with my scope? Maybe a cap on the MCLR line?
    Thanks
    Toby
  2. - 25th April 2010, 04:18 #2
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    Schematic?
  3. - 25th April 2010, 04:27 #3
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    Here you go
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  4. - 25th April 2010, 04:55 #4
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    What happens if you turn the MCLR diode around?
    Dave
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  5. - 25th April 2010, 20:07 #5
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    Quote Originally Posted by mackrackit View Post
    What happens if you turn the MCLR diode around?
    Good call Dave. Is MCLR even a needed function ? Perhaps turn it off in the configs. . . just thinking out loud, if you are controlling motors, maybe an E-STOP and full reset would be safer ?
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  6. - 25th April 2010, 20:16 #6
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    MCLR OFF is not an option for that part. Would be nice if it was...

    Agreed about the full reset. Could save some problems down the road.
    Dave
    Always wear safety glasses while programming.
  7. - 25th April 2010, 21:56 #7
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    Quote Originally Posted by Joe S. View Post
    Good call Dave. Is MCLR even a needed function ? Perhaps turn it off in the configs. . . just thinking out loud, if you are controlling motors, maybe an E-STOP and full reset would be safer ?

    Good catch. Yes, Tobias definetely needs to turn that diode around or it won't work. He can even damage other components in his circuit with the ~12V going to Vpp from the programmer.

    I have a design with a 16F727 where I use a diode and a 4.7k resistor connected to MCRL. Now, reading Joe_S post I just realized that I don't really need this two extra components in my design. In the 16F727 you can configure pin 1 as MCRL or as RE3.

    Robert
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