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why there is two pins for Vcc and two pins for ground

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sumendran
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2010/01/16 00:01:54 (permalink)
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why there is two pins for Vcc and two pins for ground

can anyone tell me y there is a need for two separate pins for both Vcc and gnd in PIC 40 pin controllers like 16F877, 18f4550 etc.....does it have any relation with the current drawn by the PIC...
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    MBedder
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    RE: why there is two pins for Vcc and two pins for ground 2010/01/16 02:50:59 (permalink)
    +2 (1)
    This is done to improve the dynamic current distribution inside the chip. You simply MUST connect all VDD and VSS pairs to your power supply and decouple each pair with 0.1 uF ceramic capacitor placing it as close as possible to the VDD/VSS pin pair.
    #2
    stanis
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    RE: why there is two pins for Vcc and two pins for ground 2010/01/16 11:42:01 (permalink)
    -2 (1)
    A common practice.
    Sounds like you're more software, than hardware engineer. I am a hardware EE, listen:

    it's very common and even advisable to have multiple power pins as they handle large currents and their poor connection would kill EVERYTHING.
    You don't have to solder more than one Vss + one Vdd, but it's advised to solder as many as chip makers guves you.

    ANOTHER REASON is for higher frequencies, more Vss (grounds) is better, why - too long to explxain.
    You don't have to come from RF/Microwaves fiedl within EE, and I can't write a 2-page article here to explain why, just accept it as a fact:

    More Ground pin connections i sbetter for higher clock frequencies.
    Don't worry about it, if you can't solder more than one pin per Vdd or Vss, though.
    #3
    stanis
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    RE: why there is two pins for Vcc and two pins for ground 2010/01/16 11:45:29 (permalink)
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    Should mention also, some chip packages offer you ENTIRE BOTTOM SURFACE (metallized) as Vss (ground) connection - which is equivalent to 20+ Vss pins!! And you worry about 2?

    This is better for high clock frequencies.

    P.S. Metal pad also serves another fucntion - better heat dissipation, if your PCB offers a matching metal pad where chip makes contact - connect it to Ground.

    And don't foget 0.1uF decoupling capacitors between Vdd & Vss pins on PCB baord AS CLOSE TO CHIP AS POSSIBLE.
    if you only write software, you don't have to worry - your hardware engineer would know why/what about Vdd & Vss pins.
    #4
    MBedder
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    RE: why there is two pins for Vcc and two pins for ground 2010/01/16 11:58:51 (permalink)
    +2 (1)
    ORIGINAL: stanis

    I am a hardware EE, listen:

    ...You don't have to solder more than one Vss + one Vdd...

    ...Don't worry about it, if you can't solder more than one pin per Vdd or Vss, though.

    I wonder where you have purchased your EE diploma then - must be in USSR, right?grin


    ...you don't have to worry - your hardware engineer would know why/what about Vdd & Vss pins.

    As perfectly shown above, some of such "engineers" wouldn'tgringringrin
    #5
    stanis
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    RE: why there is two pins for Vcc and two pins for ground 2010/01/16 13:00:33 (permalink)
    -2 (1)
    Diploma was purchased from Grove School of Engineering @CCNY - same school which gave us Andrew Grove (orignally Andras Graaf) CEO, Chairman, chief engineer & basically FOUNDER of INTEL Corp, Colin Powel (U.S. State SecretarY), CEO of Keyspan, Mario Runco - Astronaut, and so on.
    We haveNobel Prize winner at Dept. of Theoretical Physics.
    I hope you can swallow this pill now?

    AS OF VSS & VDD PIN SOLDERING NOW LISTEN FROM A DEGREED MSEE ENGINEER, WHOS EDIPLOMA WAS 'PURCHASED'
    YOU DON'T HAVE TO SOLDER MULTIPLES for CLOCKS UNDER 5MHz and LOW POWER MODES, IF YOU CAN'T, ESPECIALLY FOR PROTOYPING!!

    YOU DO SOLDER THEM FOR HIGH FREQUENCIES, HIGHER POWER DISSIPATION AND SIMPLY FOR RELIABILITY ON A FINISHED BOARD.

    My chip runs at 40Mhz so of course PCb tracks are as short as possible, all Vdd+Vss are soldered, SMT - no socket, and ceramic decoupling capacitors all over the place. I am not sure if that was the same case for this guy, if I run at 100KHz and sleep much of the time, I don't need multiple Vss connections if I simply can't or too lazy to connec ton a prototype.
    #6
    MBedder
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    RE: why there is two pins for Vcc and two pins for ground 2010/01/16 13:42:04 (permalink)
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    Needless to add a single word. Pity the school has degraded so much since then though[:'(]
    #7
    leon_heller
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    RE: why there is two pins for Vcc and two pins for ground 2010/01/16 13:51:05 (permalink)
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    He obviously has never been taught that signal rise and fall times are independent of the clock speed.

    Leon

    Leon Heller
    G1HSM

    #8
    MBedder
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    RE: why there is two pins for Vcc and two pins for ground 2010/01/16 14:03:34 (permalink)
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    This is forgivable because he might have been too busy teaching electronics to astronauts and state secretariesgringringrin
    #9
    stanis
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    RE: why there is two pins for Vcc and two pins for ground 2010/01/16 14:05:05 (permalink)
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    Agreed, pity on me
    #10
    stanis
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    RE: why there is two pins for Vcc and two pins for ground 2010/01/16 14:06:53 (permalink)
    +2 (1)
    I strongly recommend you don't connect any Vdd pins.
    New MCU's come with coal chamber, where miniscule amounts of coal can power device in case of nuclear war, an dpower disruption.

    As of rise time, it's getting longer with age.... :(
    Fall time is OK
    #11
    stanis
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    RE: why there is two pins for Vcc and two pins for ground 2010/01/16 14:07:39 (permalink)
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    Stop drinking Vodka in the morning.
    Focus on night drinking
    #12
    MBedder
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    RE: why there is two pins for Vcc and two pins for ground 2010/01/16 14:10:33 (permalink)
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    As of rise time, it's getting longer with age.... :(
    Fall time is OK

    You made my day, bro!gringringringringringringringringringrin
    #13
    Alex B._
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    RE: why there is two pins for Vcc and two pins for ground 2010/01/16 15:09:31 (permalink)
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    YOU DON'T HAVE TO SOLDER MULTIPLES for CLOCKS UNDER 5MHz and LOW POWER MODES, IF YOU CAN'T, ESPECIALLY FOR PROTOYPING!!

    OMG!!! [:-]
    #14
    MBedder
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    RE: why there is two pins for Vcc and two pins for ground 2010/01/16 15:17:40 (permalink)
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    I'd even have said OMJFC!!!gringringrin
    #15
    isa.guru
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    RE: why there is two pins for Vcc and two pins for ground 2010/01/16 15:24:24 (permalink)
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    ORIGINAL: stanis

    Diploma was purchased from Grove School of Engineering @CCNY - same school which gave us Andrew Grove (orignally Andras Graaf) CEO, Chairman, chief engineer & basically FOUNDER of INTEL Corp, Colin Powel (U.S. State SecretarY), CEO of Keyspan, Mario Runco - Astronaut, and so on.
    We haveNobel Prize winner at Dept. of Theoretical Physics.
    I hope you can swallow this pill now?

    AS OF VSS & VDD PIN SOLDERING NOW LISTEN FROM A DEGREED MSEE ENGINEER, WHOS EDIPLOMA WAS 'PURCHASED'
    YOU DON'T HAVE TO SOLDER MULTIPLES for CLOCKS UNDER 5MHz and LOW POWER MODES, IF YOU CAN'T, ESPECIALLY FOR PROTOYPING!!

    YOU DO SOLDER THEM FOR HIGH FREQUENCIES, HIGHER POWER DISSIPATION AND SIMPLY FOR RELIABILITY ON A FINISHED BOARD.

    My chip runs at 40Mhz so of course PCb tracks are as short as possible, all Vdd+Vss are soldered, SMT - no socket, and ceramic decoupling capacitors all over the place. I am not sure if that was the same case for this guy, if I run at 100KHz and sleep much of the time, I don't need multiple Vss connections if I simply can't or too lazy to connec ton a prototype.



    Nobody cares about diplomas. I have my MSEE since 1981, but I was learning, designing and even selling electronics a lot earlier. I started learning when I was 12.
    When I came to US 10 years ago I was asked once bout what diploma I had. Just asked, nobody wanted to see it or a copy of it.
    Since then nobody even asked me if I have a diploma at all. So diploma does not matter much.

    From your posts I can only guess that if you have any diploma at all it must be from those great circus schools in the old SSSR. I greatly respect and like Yurij Nikulin, for an instance. But may that is because he never tried to post here as an EE grin

    7th dan instrumentation and controls
    #16
    violin
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    RE: why there is two pins for Vcc and two pins for ground 2010/01/16 16:24:05 (permalink)
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    it's very common and even advisable to have multiple power pins as they handle large currents and their poor connection would kill EVERYTHING.
    You don't have to solder more than one Vss + one Vdd, but it's advised to solder as many as chip makers guves you.

     
    Hmm. I consider this a really stupid practice. No mater of its circumstances. If the devices support multiple VDD and/or VSS pins then you never ever leave them unconnected even when you are simply prototyping. Regards

     
    #17
    stumichaels
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    RE: why there is two pins for Vcc and two pins for ground 2010/01/16 20:15:56 (permalink)
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    I went to CCNY many years ago.  At that time they were teaching Engineering.  If OP is any indication, they've stopped.

    Obviously, OP doesn't know anything about IC design or construction, otherwise he would know why all Vdd and Vss pads MUST be connected to avoid internal voltage drops which can forward bias junction isolated tubs.

    If you don't connect all Vdd and Vss pads, the device may work or it may not.  If it does work, it may not work over the temperature range seen by many applications.  The way it may stop working can vary from device to device, sometimes it will completely stop, sometimes some peripheral will act funny.

    Depending on the layout of your circuit board, you may get away without putting 0.1uF on each Vdd-Vss pair.  On the other hand, you may find hard-to-trace problems from leaving them out.

    To be safe, connect each Vdd and Vss pad and use a 0.1uF cap on each pair!  For the cost of a few pennies and some solder, you have one less thing to worry about.
    #18
    rkarnik
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    RE: why there is two pins for Vcc and two pins for ground 2010/01/17 12:52:42 (permalink)
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    ORIGINAL: stanis
    ...
    Diploma was purchased from Grove School of Engineering @CCNY - same school which gave us Andrew Grove (orignally Andras Graaf) CEO, Chairman, chief engineer & basically FOUNDER of INTEL Corp, Colin Powel (U.S. State SecretarY), CEO of Keyspan, Mario Runco - Astronaut, and so on.
    We haveNobel Prize winner at Dept. of Theoretical Physics.
    ...


    The same Grove School described here?

    Rahul

    Recursion
    See Recursion
    #19
    ErnieM
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    RE: why there is two pins for Vcc and two pins for ground 2010/01/17 15:31:37 (permalink)
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    Wow, all this emotion and NO ONE yet has mentioned a PIC may well have both ANALOG and DIGITAL power inputs?

    #20
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