1. malahat

    Thread Starter New Member

    May 16, 2009
    2
    0
    hi

    I used PIC18F8720 with 24Mhz crystal in my project.I have 2 type of PCB(same pcb but different suplier).In one of them,my micro work correctly,but in other it doesn't work.All voltages are correct but i have not crystal signal.and when i touch the board,suddenly it sart to work.
    Please help me,if you have idea about that.
     
  2. debjit625

    Well-Known Member

    Apr 17, 2010
    790
    186
    Post the pictures of both the PCBs,both the sides.24MHz is high, normally from 10MHz is considered as a higher frequency and needs a good PCB design.

    Another thing many times the reset circuit causes this kind of problem, as you said their is no crystal signal. When you first power your circuit your crystal will be unstable slowly(compared to mcu’s speed) it will try to stabilize itself ,but for that moment of time you need to hold your mcu at reset state otherwise it will not work.

    Also check that you are powering your mcu properly, many mcu have two pins for power Vdd and Vss,you have to power them both.

    Good Luck
     
  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,386
    1,605
    It needs your hand to complete the circuit, and all your hand adds is capacitance. I would suspect something around your crystal: missing or open cap, incorrect value cap, something like that.

    The oscillator is on the edge of oscillating and needs a little kick to start.
    I doubt it has anything to do with which PCB manufacturer it is, but can't be ruled out either.

    How many boards are we talking about? Just the first one, or a pile of 10,000?
     
  4. malahat

    Thread Starter New Member

    May 16, 2009
    2
    0
    Last year we produced 100 boards and we had not any problem.
    this year we ordered 100 PCB to a good manufacturer.I test 15 boards and i saw this problem.Then i ordered 10 board to miscellaneous manufacturer and all of them were ok.
    fieber & cu thickness are different.but pcb file is same.
    when a board work correct,it had no dependence to capacitor and ....
     
  5. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,386
    1,605
    Two vendors give good results, one does not. So the one vendor did something different, what that is I could only guess. Perhaps they left out a via drill or mis-registered one or more holes so things you want connected are not connected.

    24MHz on a PIC circuit isn't very high; most of that signal is confined internal to the PIC anyway.

    Seeing the artwork may give us a better "guess."

    Good luck!