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2 lots of Power input?

YT2095

Whilst working on another Pic project using a 16F877A, I noticed that it still worked with only the Gnd and +5 supply connected on the left hand side of the chip, so I reversed this and only connebted the right hand side and it still worked too.

so I`m wondering Why it has 2 lots of Gnd and +5v?

is it purely for board design convenience or something else?

 

Thanks image

  • 0

    It's not unusual for modern chips to have lots (>10) ground and power pins. This is because you can't get a low enough impedance connection between the power and ground and the chip using only one pin. The pins are usually connnected directly together on the chip.

    It may well be the case that you only get problems running the chip at high speed with a lot of IO switching so it is quite likely that it will work OK in many applications with some pins not connected.

     

    If you want designs that offer you the best chance of working to spec first time then connect all the pins and fit  a ceramic decoupling capacitor between power and ground close to each power pin. If you design with high end FPGAs and processors you will save yourself a lot of grief. My guess is that at least 90% of the decoupling capacitors I have designed in over the years were not necessary - the problem is the cost of finding out which ones were in the 10% which were needed !

     

    Michael Kellett

  • 0 in reply to michaelkellett

    Thanks!

    I`m going to test this out on an 18F4550 running at 48MHz, and using USB to see if it makes any difference.

    my Current build is using 3 of the 8 bit ports to drive LEDs (64 of them) at 20 MHz clock speed and also drive a speaker (it`s a toy for my Son).

    As for decoupling caps I get through 100`s here, and buy 1000 at a time, stuff can get really flakey without them, esp when (as I do often) use a Card and cardbus backplane design.

     

    I`ll let you know how I get on with the USB test, it would be neat to see how far these can be pushed image

  • 0 in reply to michaelkellett

    quick update, I tried this last night and it works at 48MHz and USB functions without any noticable problems.

    although, I tested the pins on the other side, and where 5V comes into the chip, only 4.97 exits?

    so On-Chip wiring seems to be the most likely cause for this resistance.

     

    it`s nice to know that at a real push, you don`t HAVE TO wire them up as long as you can at least de-couple image

  • 0 in reply to YT2095

    Hello,

     

    I always feel that since I have to pay for all the copper on the baord and then again to etch soem of it off I might as well connect as many pins as I can image

     

    Michael Kellett

  • 0 in reply to michaelkellett

    A good philosophy indeed! image

     

    I hardwire mine as all PCB attempts in the past have been Failures to say the least, I prefer hardwiring (wirewrap or point to point solder) anyway. although I expect this may be somewhat problematic at higher freqs in the 10`s of MHz, but non of my TTL stuff goes up that high anyway.