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I am creating a circuit using an ATMEGA328P and i am unable to ground one of the 3 ground pins (other 2 are fine) due to limitations of my design. I have tested the IC with a multimeter and the pin does seem to be connected to the other ground pins which makes me think that it should be ok if it is left out. is there any problem with me doing this? thanks.

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Depends on what that pin is for. It may be required to link two grounds in the silicon, in which case it would cause an issue. You say it is linked, so it shouldn't be that bad. But ground pins are used for two other reasons.

Thermal: a ground pin gives a nice thermal link to a large copper plane which can be used to suck heat out of the IC and stop it over heating. Not connecting it up will cause there to be more heat in the IC, possibly spot heating, shortening the life of the IC as well as possibly causing damage straight away.

EMC: ground pins give you more routes for the current loops to follow. This reduces the chance of EMC problems and means that the IC should perform better. The ground link also reduces the inductive path for the current, allowing faster signalling and general improvement in performance.

As mentioned in the comments, and which I completely forgot about, the extra ground pins may also be carrying extra current. For every bit of current in, there must be a bit of current out. It can be surprising just how much current needs to flow in/out of these ICs.

So, while not connecting the pin may not cause immediate issues, it may cause a lot of problems down the line. If your design does not let you connect the pin, I would suggest your design needs to change so that it can. Connecting a ground pin is an easy task compared to working out why the whole product fails at some random point.

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  • \$\begingroup\$ Thanks very much for this. helps a lot. \$\endgroup\$ – user2105725 Jul 11 at 10:36
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    \$\begingroup\$ In addition, one ground pin may provide a high current ground for the outputs, etc., and omitting that may force the high currents out through the CPU ground or across a portion of silicon that isn't rated for that. Always ask yourself, "What extra information do I have that the manufacturer doesn't?" The answer is generally < 0. \$\endgroup\$ – Transistor Jul 11 at 11:08
  • \$\begingroup\$ Datasheet says to connect all of them. If you do not, you may see odd problems later on that can not be clearly attributed to anything. Same with all the VCC and AVCC pins - all must be connected to 5V (or whatever level you are powering it at, depending on the clock speed in use). \$\endgroup\$ – CrossRoads Jul 11 at 13:20
  • \$\begingroup\$ I would have sworn the AVR datasheets have said to connect all of them. But I could not find that part in the current datasheet any more. Can you post that here? \$\endgroup\$ – Justme Jul 11 at 15:12

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