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released microcontroller's IO pins and noise

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MISU.RSG

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hi, dears.

I have a question about the pins that unused in microcontrollers.

I want to link all of them to each other directly and pull down them via a 100 ohms resistor. can direct linking of IOs make problems in my design?

notice you that all IOs are input by default and I let them still in this situation.

are there any better ways to protect my microcontroller and circuits from noise in industrial areas?

thank you
 

Re: released micomontroller`s IO pins and noise

I don't see an advantage of external pull-down. The only important point is to avoid floating digital inputs which cause extra current consumption. You can e.g. configure the pins as unconnected output, input with internal weak pull-up or even floating analog input.
 
Re: released micomontroller`s IO pins and noise

As FvM says, you can decare them as output
 

Re: released micomontroller`s IO pins and noise

which state is better?(Input,Output,Analogue)
So what about power consumption?
less noisy condition?
I`m with output but some companies suggest analogue input.
 

Re: released micomontroller`s IO pins and noise

It is not a case of which is best but which is worst.

Making the pins outputs but leaving them disconnected doesn't add any hardware and is completely safe.

Making them inputs and either linking to VSS or VDD is also OK but beware of what might happen at the moment of powering up before the software has configured the pins. For example, if the MCU powers up with the pins in a low state and you have them linked to supply, it might make a large current flow.

Making them inputs and linking to VSS or VDD through a resistor to prevent large current is also OK but need the extra components. A pull-up or pull-down resistor is needed and you have to make a compromise, too small in value could cause too much current, too high in value increases the noise pick-up risk. Somewhere around 1K - 10K is usually satisfactory.

Making the pins analog inputs is also OK as long as you disable the ADC. This is how most MCU start up automatically. It does leave the pins slightly more prone to static damage but the risk is fairly small.

I prefer to make the pins outputs and leave them disconnected. It allows the pin to be used later if needed and you can use the pins to give debugging or status information if needed.

Brian.
 
Re: released micomontroller`s IO pins and noise

Hi,

Making them inputs and linking to VSS or VDD through a resistor to prevent large current is also OK but need the extra components. A pull-up or pull-down resistor is needed and you have to make a compromise, too small in value could cause too much current, too high in value increases the noise pick-up risk. Somewhere around 1K - 10K is usually satisfactory.

I don't agree with the resistor value compromise.
"Making them as inputs" means they are high impedance. Thus any external (low value) resistor can't cause "too much current".
I agree with 1k ... 10k value. This is safe.

Making them as outputs ad leave them disconnected is OK.
I prefer "input with internal pullup, no external connection".

Klaus
 

Re: released micomontroller`s IO pins and noise

thanks all.
I'm not with internal pullup in industrial places as I used it before.
 

Hi,

Internal pullup for unconnected pins is for sure no problem.
I'm designing industrial equippment for decades now ... in really high power environment with high speed switching (500ns) up to 3500V and up to 8000A peak. The microcontroller is in close proximity to the power lines. Never encountered a problem.

Klaus
 

I don't agree with the resistor value compromise.
"Making them as inputs" means they are high impedance. Thus any external (low value) resistor can't cause "too much current".

I apologize for confusion. The OP has not given any information about the microcontroller type, there is a risk, albeit a small one, that the pin state defaults to an output state prior to initialization. It was to avoid a connection to the other logic state (VSS or VDD) and hence drawing excess current that I suggested not going below about 1K.

Brian.
 

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