what to do with these pins on a PCB?

Community PCB Design
1

Hi there,

Where to connect the following pins of a microcontroller on a PCB?

unused ports ? leave them floating? ground them? what to do with them? I am not sure

AREF?

AVCC? VCC is to be connected to voltage source but what about AVCC?

NC?

I would appreciate your help. Thanks

Regards

2

Unused pins can be made outputs.

The data sheet should specify what is to be done with AREF. AVcc should be connected to Vcc and decoupled. It might need to be filtered if you are using the ADC.

NC means “no-connect”.

Leon

3

leon_heller:
NC means “no-connect”.

Leon

Hi Leon,

Does it mean that they have privided a physical pin that is not connected to internal circuitry? what to do with them on PCB? should we provide a pad for it?

Do you know any kind of tutorial about coupling and decoupling methods and techniques?

Thanks

4

Provide a pad for it, but leave it unconnected. It might be used during manufacture for testing, or it might simply be unconnected.

There might be tutorials about decoupling, but it’s mostly just commonsense.

Leon

5

I’ve seen datasheets for parts that specifically say to solder NC pins to pads to provide extra heat transfer. This was for switching regulator ICs like the [LM2574. It can’t hurt to solder NC pins to unconnected pads, and in some cases it’s apparently a good thing. So I’d just make a practice of always doing so unless the datasheet says not to.

As far as decoupling VCC or AVCC, a good general rule of thumb is that every IC’s VCC pin(s) should get a 0.1uF cap immediately adjacent to it with as short a trace as physically possible, with the other side connected to ground, preferably a via to a ground plane if you’re using one.

I’ve seen info on the web with more technical details that let you figure out the best capacitor values to use depending on frequencies and such. I found them some time back via google. I’d start with a search for ‘decoupling capacitor’. But unless you’re working with a lot of high frequency stuff, the 0.1 cap is probably good enough.

I’m too nice. :slight_smile: I just did the google search and here are a few useful looking links:

http://www.interfacebus.com/Design_Capacitors.html

http://www.seattlerobotics.org/encoder/ … asics.html

http://www.hottconsultants.com/techtips/decoupling.html

http://www94.web.cern.ch/HSI/s-link/dev … couple.pdf

Enjoy,

Keith](http://www.onsemi.com/pub/Collateral/LM2574-D.PDF)

6

khearn:
I’ve seen datasheets for parts that specifically say to solder NC pins to pads to provide extra heat transfer. This was for switching regulator ICs like the [LM2574. It can’t hurt to solder NC pins to unconnected pads, and in some cases it’s apparently a good thing. So I’d just make a practice of always doing so unless the datasheet says not to.

As far as decoupling VCC or AVCC, a good general rule of thumb is that every IC’s VCC pin(s) should get a 0.1uF cap immediately adjacent to it with as short a trace as physically possible, with the other side connected to ground, preferably a via to a ground plane if you’re using one.

I’ve seen info on the web with more technical details that let you figure out the best capacitor values to use depending on frequencies and such. I found them some time back via google. I’d start with a search for ‘decoupling capacitor’. But unless you’re working with a lot of high frequency stuff, the 0.1 cap is probably good enough.

I’m too nice. :slight_smile: I just did the google search and here are a few useful looking links:

http://www.interfacebus.com/Design_Capacitors.html

http://www.seattlerobotics.org/encoder/ … asics.html

http://www.hottconsultants.com/techtips/decoupling.html

http://www94.web.cern.ch/HSI/s-link/dev … couple.pdf

Enjoy,

Keith[/quote]

Keith,

Thanks a lot for the link. After I posted the question, I also did a quick search and came up with these links except one. Thanks a lot for your help.

I do not know why but I always seem to make things hard for myself. I always tend to go to the bottom of things. Although, I know what decoupling is but I wanted to start from scratch by reading a book or tutorials. Well, that is just the way I am. God help me. Thanks for your response.](http://www.onsemi.com/pub/Collateral/LM2574-D.PDF)

7

Soldering no-connect pins also improves the mechanical strength of the attachment of the IC to the board.

8

If you have the space, connect unused IO pins to either vias or to the pads of an unloaded 0805 part. That gives you a point to solder a wire for a change or debugging.

If the part in question is an AVR, I usually tie AVCC to VCC and connect a 100n cap from AREF to GND if I am not using the internal ADC (or am ok with the internal reference). This is covered in the data sheet for the processor.

9

In general, always start with the datasheet. They will usually tell you what’s important (like AVCC vs VCC, for example). It doesn’t hurt to look at app notes that use the chip in question to see what practices the manufacturer uses.

On unused port pins. This gets a lot of discussion but it’s general practice to leave them unconnected and set as output in the firmware.