PIC/Resonator ground/Power question. Need help (Little Bit urgent)


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  1. #1
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    Nov 2009
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    Exclamation PIC/Resonator ground/Power question. Need help (Little Bit urgent)

    I am facing a very weird problem, I know it is related to hardware only but I wanted to get some ideas/corrections before I redesign my PCB.
    I have 4 x 16F676 as slaves and one 877A. My 877A supplies power to 4 x 676. I know maximum current from 877A should not exceed 125mA in total, and it doesn't, I have checked it.

    1) But is it OK to design a circuit in this way?

    Secondly, I have 8MHz 2 Pin resonators connected to every 676. I have provided ground to the 8 x 22pF capacitors which are connected to the resonators on the same track which provides common ground to all 4 x 676.

    2) Is it OK to provide the ground to the capacitors of the resonators from the same track which is providing the ground to the IC itself?

    Thirdly, I have my zero detection circuit which provides pulses to all 16f676. I have one common track from 0-detect circuit which is connected to one pin of each IC.

    3) Is it ok to do it this way or I should add some resistance between the signal track and the pic? I have one PIC814 opto for Zero-Detection. The arrangement is simple, Collector is connected to +5V via 4.7K. At 4.7k and collector junction, one track goes to PortA.2 of all 4 x 676. Emitter goes to ground.


    My circuit is behaving weird, I can't even explain the behavior because it changes as many times as I power the circuit. I don't know if it because of these factors or not. But I have try and redesign it but I want to know if what I am doing is technically ok or not.


    My circuit is the following:
    Name:  PCB.GIF
Views: 484
Size:  54.1 KB

    I have simulated the circuit and made it on breadboard and it all works like a charm. First attempt at PCB has failed so hopefully with some useful inputs second attempt should take me forward.

  2. #2
    Join Date
    Dec 2010
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    400

    Default Re: PIC/Resonator ground/Power question. Need help (Little Bit urgent)

    1) It may be possible to make this work, but it depends on your circuit. The current limitation is for a whole device, not a pin. Each pin has a much lower limit, and if your 676s are driving something else, that has to be added to the total pin current. It's a highly unusual configuration. If you need to turn the devices off (sleep won't work), use the control pin to switch a FET to power them, don't directly power them.

    2) It's not just O.K., it's pretty much mandatory. A good design will have a single ground. Keep tracks short and ground tracks fat.

    3) It's not clear what happens to this common track when the 676's are powering up - do the 676's make this pin an output briefly and mess up the signal? Resistors to give the connections a bit of elasticity are a good idea, but your powering is most likely the root cause of the problems. The whole area of initial states and power-up / reset causes designers to rip their hair out regularly.

    I'd guess a combination of 1 & 3 is the root cause of your problem.

  3. #3
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    Default Re: PIC/Resonator ground/Power question. Need help (Little Bit urgent)

    Hi Megahertz!
    I would agree with Charles and he is correct, power is typically the problem when things become "spritual" (technical term). The only thing I would question is the use of capacitors with the resonators. Are you certain the resonators require caps? Most resonators I have worked with do not require external capacitors. Second I am guessing that on the output of the voltage regulator you have a 1 uf cap as close as possible and then a larger value cap somewhere else? It is also useful to "sprinkle" .1 uf caps in various places on the power buss. Best, Ed

  4. #4
    Join Date
    Sep 2013
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    1

    Default Re: PIC/Resonator ground/Power question. Need help (Little Bit urgent)

    @Megahertz, not this design is technically not good enough. It might work under much specified conditions but it's just not the right way. You don't have to power up 4 slaves from one microcontroller. Instead, just power them up from the regular supply and give an option to the master controller to halt or reset them at any time. This not only gives you extra options for your design but also removes the vulnerability. Also, you can't provide one input to multiple controllers, it might lead to voltage dropping which will make the signal disappear, instead use simple buffers in between to ensure the signal reaches all the required pins. Hope that's helpful.

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