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Topic: Is it necessary to connect all GND's and VCC's on a standalone board? (Read 2799 times) previous topic - next topic

bobthebanana

So I'm working on my first standalone board with an IC on it, in this case an atmega8 in a TQFP-32 package. I was wondering if it's absolutely necessary to connect every single VCC and GND pin on the IC, or if I can just hook up whichever power pins are most convenient. In my case it would be really nice if I only had to hook up one VCC and GND pin, as it would save the frustration of trying to hook all the power pins together.

winner10920

What kind of design is it that you can't manage to do that? I've never had that problem, even when I have boards literally packed with components on both sides,
you probably just need to rearrange things

Techone

Yes it is. All GND MUST to be connected together. The Vcc, well if they all the same voltage ( example +5 V ) , then connect them to 5 V line. Same goes with others Vcc., than connect to the same Vcc line.

bobthebanana

@winner10920 It's a hand drawn (with a sharpie as resist ink), home etched board, so using a TQFP is hard enough as it is and the less traces I have to run to the IC, the easier it will be to draw the board.

@Techone I decided to build up a breadboard arduino with a DIP atmega328, just to be completely sure. It seems, however, that I only have to hook up one GND and one VCC pin on the chip in order to get it to work, so I assume all the extra GND and VCC pins are internally connected together. Have you experienced any problem by not connecting all the power pins? Or is the atmega8 different in some way?


CrossRoads

Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

Techone

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I decided to build up a breadboard arduino with a DIP atmega328, just to be completely sure. It seems, however, that I only have to hook up one GND and one VCC pin on the chip in order to get it to work, so I assume all the extra GND and VCC pins are internally connected together. Have you experienced any problem by not connecting all the power pins? Or is the atmega8 different in some way?


Now that I know what you are trying to do.

1... You have to connect all VCC and all GDN of the ATmega328P to make it work. I did a lots of breadboard version, and they all work fine. Just read the datasheet of the chip.

2... I did a project that I use a DIY PCB board using trace & etch method. I did use a Sharpy to trace the PCB.

Here two pictures of the Arduino project I build.  My project is a Telephone Call Counter. The picture is the "motherboard with CPU"

As you can see, I connect teh Vcc and the gnd. And I did place a 0.1 uF cap between VCC and GND. And the Xtal with the two 22p cap.

Osgeld

Pin 8 and 22 are physically connected as ground, hooking both up is redundant (check it with a continuity meter)

pin 7 is VCC you need that to power the chip

pin 20 is for the ADC, if I dont use analog I dont hook it up and it works fine

so no you do not have to hook up ALL VCC's and GND's to get a basic functioning chip

but of course it depends on your needs, if you need the full capibilities of the chip then YES you do, if you do not care about analog then NO you do not.
http://arduino.cc/forum/index.php?action=unread;boards=2,3,4,5,67,6,7,8,9,10,11,66,12,13,15,14,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,86,87,89,1;ALL

Grumpy_Mike

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if you need the full capibilities of the chip then YES you do

And by full capabilities he means do you want it to work reliably, with the rated current output and not so susceptible to interference.

Osgeld

please explain

there is 1 VCC

there are 2 ground pins physically connected

there is one analog VCC, which if you dont need you dont use
http://arduino.cc/forum/index.php?action=unread;boards=2,3,4,5,67,6,7,8,9,10,11,66,12,13,15,14,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,86,87,89,1;ALL

Grumpy_Mike

We are talking about the DIL version of the 328
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there is 1 VCC

Only one marked Vcc but you need to connect both Vcc and Avcc together even if you are not using the analogue capabilities, it says so in the data sheet-
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AVCC is the supply voltage pin for the A/D Converter, PC3:0, and ADC7:6. It should be externally connected to VCC, even if the ADC is not used. If the ADC is used, it should be connected to VCC through a low-pass filter. Note that PC6..4 use digital supply voltage, VCC



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there are 2 ground pins physically connected

Ever thought why that is? Do you think the manufacturers do it for fun? It costs money to bond out a pin on a chip and they would save some if they did not do it. So where is the incentive?

It is because although the pins might look to be physically bonded there is:-
1) A current carrying capability involved with any bonding wire, by using one wire you are halving that.
2) There will be an impedance difference between the two bonds that could cause instability if only one is connected.

So yes it might function but it will not work like it is specified to.

Osgeld

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Only one marked Vcc but you need to connect both Vcc and Avcc together even if you are not using the analogue capabilities, it says so in the data sheet-
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AVCC is the supply voltage pin for the A/D Converter, PC3:0, and ADC7:6. It should be externally connected to VCC, even if the ADC is not used. If the ADC is used, it should be connected to VCC through a low-pass filter. Note that PC6..4 use digital supply voltage, VCC


any other reason WHY than the datahseet says so? I would like to see some test results on that one

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there are 2 ground pins physically connected

Ever thought why that is? Do you think the manufacturers do it for fun? It costs money to bond out a pin on a chip and they would save some if they did not do it. So where is the incentive?

It is because although the pins might look to be physically bonded there is:-
1) A current carrying capability involved with any bonding wire, by using one wire you are halving that.
2) There will be an impedance difference between the two bonds that could cause instability if only one is connected.

So yes it might function but it will not work like it is specified to.


Do I think its for fun? NO I think its cause they have to conform to a package specification and had X extra pins. and It apparently does not cost them that much to bond wires to a frame as the 328 in TQFP and MLF has 3 grounds, but yet the specs dont change.
http://arduino.cc/forum/index.php?action=unread;boards=2,3,4,5,67,6,7,8,9,10,11,66,12,13,15,14,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,86,87,89,1;ALL

Grumpy_Mike

#12
Feb 18, 2012, 10:40 am Last Edit: Feb 18, 2012, 11:51 am by Grumpy_Mike Reason: 1
You are just so wrong it is hardly worth arguing with you.
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any other reason WHY than the datahseet says so?

If you demand prof of the varsity of data sheets then there is no place for you in electronics. It shows a breath taking arrogance coupled with ignorance.
Lets look at common sense reason. With the analogue side unpowered you have powered circuits feeding into unpowered circuits, is that a good idea, I think not.

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I would like to see some test results on that one

And what sort of test results would convince you it is a bad idea? You would have to provide some results that says it is fine to do that, and I don't think you are equipped for that. It would require a complete simulation at chip level.

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NO I think its cause they have to conform to a package specification and had X extra pins.

Not true, if you look at the data sheet you will see the same chip is available in different packages. These have extra I/O not found on the 28pin DIL version so there is plenty of things to improve the chip to bond out if they wanted to.

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and It apparently does not cost them that much to bond wires to a frame

Wrong again, I have used ICs that have different part numbers and vastly different prices but the only real difference being that on the cheaper chip not all the pins were bonded out despite the packaging being the same.

So go ahead and make crappy circuits just don't whinge when they fail to work reliably.
But remember you are not in Kansas anymore.

winner10920

Doesn't the dip have alot more mass to it? Lol a smaller chip I think would need more connections to work the same

CrossRoads

Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

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