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why two ground pins?

Discussion in '8051/8951' started by Rohitchampion, Feb 26, 2014.

  1. Rohitchampion

    Rohitchampion New Member

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    If you see 8086 microprocessor pin diagram, there are two GROUND pins, PIN1 and PIN20.
    Can anybody explain why two ground pins are used on 8086?

    I searched on google but all are giving different answers so I am posting here.
     
  2. JimB

    JimB Super Moderator Most Helpful Member

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    If you look at the PIC 16F887, you will see that that has two ground and two supply pins.

    I can think of two possible reasons:

    Two connections makes for a lower impedance connection.

    One connection for each of two different parts of the chip, makes life easier for the guy designing the silicon.

    JimB
     
  3. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    I can think of better reasons for the PIC:
    Analog ground, digital ground
    The pin for the ratiometric reference ground.
     
  4. dave

    Dave New Member

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  5. NorthGuy

    NorthGuy Well-Known Member

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    And then datasheet suggests that all ground pins have to be connected, which, if followed, makes PCB design harder. How come the guy designing the silicon is more important than the PCB guy?
     
  6. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Because in silicon, its harder. No layers. Wire bonds to anywhere on the chip. No insulated wire though.

    I did some manual ultrasonic gold wire bonds before.
     
  7. JimB

    JimB Super Moderator Most Helpful Member

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    A reasonable supposition, however, the PIC datasheet makes no mention (as far as I can see) of separate analogue and digital grounds.

    Northguy said:
    I did not intend to imply that anyone was more important than anyone else.
    Maybe it is a good idea to provide supplies to both sides of the chip, rather than having the supplies for one side flow through the other side.

    I don't know, just supposition* on my part.

    JimB


    * Supposition = A guess that has been to university.
     
  8. tronitech

    tronitech Member

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    I think you're right JimB. It's probably down to distributing clean power "across" the chip. Look at something like the i7 900 series processors, there are in the region of 200 of each supply and ground pins.
     
  9. atferrari

    atferrari Well-Known Member

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    :stop: An educated guess in short?
     
  10. Rohitchampion

    Rohitchampion New Member

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    Thanks JimB
    And yeah there are no separate analog and digital grounds.
     
  11. atferrari

    atferrari Well-Known Member

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    I stopped counting at 220. WOW!
     
  12. Little Ghostman

    Little Ghostman Well-Known Member Most Helpful Member

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    some pics have analogue and digital supplies! I forget the naming convention but for example the ADC's on some pics have there own supply pin's, even if you use the internal ref etc you still have to power the ADC pins
     
  13. rc3po

    rc3po Banned

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    Thanks JimB. I'm new to uControllers and I just assumed that the multiple Power & Ground pins were to allow the Chips to carry more current. So I guess I was wrong.
     
  14. Little Ghostman

    Little Ghostman Well-Known Member Most Helpful Member

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    In some ways your right, if the pic had to internally connect from one side to the other, then surely the current available would drop? Not too sure now Ive said it! to me what goes on inside is the pics business unless it smokes! :D. The bit that annoys me though is they tend to cross the VDD and GND pins on each side! they never seem to be opposite.
     
  15. JimB

    JimB Super Moderator Most Helpful Member

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    Just to keep you on your toes!
    Prevents complacency.:D

    JimB
     
  16. rc3po

    rc3po Banned

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    I'm still reading books and learning how to use my new Arduino so I haven't looked into the Chips internal structure.
    Having more Pow/ Grd pins may also help keep the Chip cooler instead of trying to send all of the current through 2 pins. I usually get yelled at around here for using common sense. Perhaps I'm a glutton for punishment?:D
     

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