Can ground be shared amongst 2 voltages 3.3v and 5v

So quick question, Can a microprocessor which specifically cannot tolerate voltages above 3.6v tolerate a shared ground between a 3.3v and 5v system. I imagine not but I've read online in a few places that maybe so but online is somewhat vague so I want to be double sure before I blow up a microprocessor trying it.

From what I can gather online, ground is different and ok to share as long as the voltages aren't in the extreme differences but I can't find a clear answer and it doesn't feel right to me for obvious reasons so I want to ask here and hopefully get a clear cut answer that which I can't find elsewhere online because destroying a PCB board, components, and microprocessors isn't on my agenda of things to do today unfortunately.

If you are saying you have a 3.3v Arduino the GND pin on the board can be connected to a 5v sower supply GND, the 5v line going to Vin of the Arduino to power it.

The same 5v line can the be used to drive a 5v relay which is driven by a 3.3v Arduino out pins via a transistor.

5v signals from one system can be sent thru level translators to a 3.3v system.

3.3v to the 5 volt system should work without translating.

Other than this, don’t understand what you are asking.

How many power supplies do you use?

Usually earth gives a common ground potential (reference voltage). But without a separate neutral wire in mains outlets, or with 2 pole connectors, a voltage can exist between floating grounds of multiple devices.

To be sure you can measure the voltage between two ground wires before connecting them. Or connect the grounds using a 100R resistor and measure the voltage on that resistor, giving the compensation current between both devices. Apart from high voltages or currents, which indicate wrong choice of a ground wire, low currents can result from ground loops. Ground loops are very nasty, because even if broken the voltage between device grounds can vary, often depending on some device load.

Wired communication between devices usually requires a common ground. Use the resistor trick before connecting grounds. Use common power supplies when possible.

it is common in DC circuits to tie all the grounds of the design together regardless of the voltages (within reason), unless the circuits need to be isolated. on a pcb design oftentimes there is an entire layer dedicated to grounds.

to sum it up, it is 100% correct to connect all the grounds together, as long as there is only one main electrical feed in.

i.e. 12vdc adapter => 5v regulator => device
=> 3.3v regulator => device

but generally dont tie all the grounds together when:

i.e. 12vdc adapter #1 => 5v regulator => device
12vdc adapter #2 => 3.3v regulator => device

hope this helps

I have a 3.3v microprocessor on the same board as a few other components all 3.3v and I'm wanting to connect the 3.3v systems GND to the GND of a 5v system with it's own 5v components.

@super7800

Thank you for your answer! My board is getting power from an external 5v system so I used the quick and cheap method of using a resistor pair to shift the level down from 5v to 3.3v. The external system providing the 5v power also provides a ground that I directly connect my 3.3v system GND to as-is (making path of least resistance, hence no level shifters or protection from a 5v system)

So there is only 1 positive point (level shifted) and one negative point (unprotected) in/out and I'm gathering from your post it's A-OK because despite the voltage differences between the two systems, connecting the two GND is fine as long as theirs not two different sources of power which, in my case, theirs not, only one and it's coming from the 5v system.

@DrDiettrich

just saw your post and missed it in my initial reply, sorry. I'll keep it in mind to try before assuming it's safe. Thank you.

Without a common GND, how will the 5 resp. 3.3 volt system have any idea about the voltage of the other? GNDs should be connected in 1 unic point to awoid noice problems. GND is, like already said, the reference.

junebug12851:
I have a 3.3v microprocessor on the same board as a few other components all 3.3v and I'm wanting to connect the 3.3v systems GND to the GND of a 5v system with it's own 5v components.

@super7800

Thank you for your answer! My board is getting power from an external 5v system so I used the quick and cheap method of using a resistor pair to shift the level down from 5v to 3.3v. The external system providing the 5v power also provides a ground that I directly connect my 3.3v system GND to as-is (making path of least resistance, hence no level shifters or protection from a 5v system)

So there is only 1 positive point (level shifted) and one negative point (unprotected) in/out and I'm gathering from your post it's A-OK because despite the voltage differences between the two systems, connecting the two GND is fine as long as theirs not two different sources of power which, in my case, theirs not, only one and it's coming from the 5v system.

@DrDiettrich

just saw your post and missed it in my initial reply, sorry. I'll keep it in mind to try before assuming it's safe. Thank you.

Using resistors to divide supply voltage down is not good.

How would you recommend I lower the voltage then, I'm fairly new to electronics.

The issue is I have 32 pins I'm interfacing with and all are to a 5v system. The supply and RST is 5v and the data lines in and out are all 5v based TTL. The solution I need to use is also on a tiny PCB board that's somewhat cramped.

If resistors is a bad way to do this then how do I do I convert ~22 pins (excluding ground and output-only lines) to 3.3v on a tiny cramped pcb and keep it low cost.

I've seen some other solutions but I'm not sure what else to use, if you have better feedback or suggestions I would greatly appreciate feedback on this as it's been a huge problem I'm trying to figure out.

FYI

https://www.sparkfun.com/categories/361

junebug12851:
how do I do I convert ~22 pins (excluding ground and output-only lines) to 3.3v on a tiny cramped pcb and keep it low cost.

22 converter lines or simply use a 5v MCU...

Using resistors to divide supply voltage down is not good.

I think she wants to reduce the signal from a 5V system to feed into a 3V3 system. For that using resistors is fine. However keep the resistor values low, that is between 500R and 2K.

Connecting the grounds together is not only possible it is compulsory, if you want things to work.
See this Power Supplies

junebug12851:
How would you recommend I lower the voltage then, I’m fairly new to electronics.

The issue is I have 32 pins I’m interfacing with and all are to a 5v system. The supply and RST is 5v and the data lines in and out are all 5v based TTL. The solution I need to use is also on a tiny PCB board that’s somewhat cramped.

If resistors is a bad way to do this then how do I do I convert ~22 pins (excluding ground and output-only lines) to 3.3v on a tiny cramped pcb and keep it low cost.

I’ve seen some other solutions but I’m not sure what else to use, if you have better feedback or suggestions I would greatly appreciate feedback on this as it’s been a huge problem I’m trying to figure out.

level shifter for one of your queries, how much space you have is beyond control other than from your design I would imagine.

@larryd

Thanks so much for the links, it's quite a lot of information to take in but I'm sifting through it. While I can see a number of solutions I'm still not understanding of why a resistor pair is bad. I'll definitely bookmark the links, thanks for them.

@Slumpert

That's what I've already laid out in the schematic and board design, 22 converter lines using resistor pairs. I drew that out that days or weeks before I posted this question on here.

@Grumpy_Mike

Currently my resistor pairs are 1K and 2K. Should I do something else? In simulations any resistor value I entered 1K/2K or 5K/10K or higher or lower as long as the 2nd number was double the first number it all produced a 3.3v signal from a 5v signal so I didn't think there was any difference what the resistor pair was as long as it produced 3.3v based on simulations.

@bluejets

I know of a device that can linear shift a 5v or higher signal to a 3.3v and that it's not the most efficient method but it's cheap and it works. I also know the arduino series uses it. Is that what your talking about? I understand linear voltage converters only work for supply voltage and not for data lines.


Thanks for all the helpful answers

Currently my resistor pairs are 1K and 2K. Should I do something else?

No that is fine.
Myself I tend to use 510R and 1K but what you use is fine. Just don’t use things like 1M and 2M. Or 10K and 20K as they produce more degradation of the rise and fall time as the signals due to stray capacitance.

junebug12851:
I drew that out that days or weeks before I posted this question.

So in “all” that time, did you actually research the TTL signal your dealing with?
TTL is almost never 5 volts and may infact only be 3.3 volt as its “HIGH” output.

If it’s as low and the minimum required (> 2.7v ), your voltage divider might drop it too far.

@Slumpert

Keep in mind as of mid-july this year I literally didn’t even know what a resistor was much less ever heard of the name Arduino or the words microprocessor or PCB and had practically no math background of any kind so to come this far barely 30 days later is pretty exciting stuff for me. I’m still pretty deep in learning things.

With that said, it was all of a whopping 4 days ago that I realized after drawing out a schematic and PCB for my very first project and circuit board that I realized what’s considered a low and high have different competing specs the most popular being CMOS which is flexible and TTL which is the more older retro version that only works with 5v systems. Just 4 days ago I realized this and the system I’m working with has no published specs or any datasheet because it’s a proprietary closed system so I’m assuming by your question there’s differences in design between manufacturers on a TTL signal LOW and HIGH. All I know is a comment I read online somewhere that the LOW and HIGH are compatible with Microprocessors that use CMOS so that’s all I have to go on.

I do have a couple of websites bookmarked on the TTL specs and studied it a few times but all I know is you can’t connect a 5v wire to a 3v microprocessor unless it’s designed to handle it which mine can’t on any of it’s pins. I understand the voltage isn’t always going to be 5v but it can be and I don’t want the microprocessor damaged.

If it’s as low and the minimum required (> 2.7v ), your voltage divider might drop it too far.

No I think you need to not only read the TTL specifications but to understand then as well.

That conclusion is just wrong.

Simple explanation.
There is no 5V or 3.3V if there is no 0V / ground reference.

Any voltage is that value because it is measured with respect to a reference - usually 0V, which makes tat voltage +x or +y... or -z as designed.

If you decide the ‘return/ground’ reference is 8V, that terminal that was at 5V wrt zero, is now -3V and so on.

Power supply and ground/references are a critical part of initial development.
The wrong choice will cause all kinds of problems.

Where did my volts go?

In 95% of hobby projects, the 0V return paths are common across all elements of the project unless you know what you’re doing and why.

Grumpy_Mike:
No I think you need to not only read the TTL specifications but to understand then as well.

That conclusion is just wrong.

Maybe I am wrong, here I was thinking if the OP circuit is not getting +2volts into the Arduino pins, it’s not going to work.

I see this as reducing the voltage from a 5V Arduino to feed into a 3V3 Arduino or Raspberry Pi, without damaging the latter.
The values in the data sheet for the Arduino regarding minimum high output voltage apply when drawing 20mA from the pin. With a 3K load you are only going to draw a current of 1.6 mA so those voltage specifications are too pessimistic when only one tenth of the current are being drawn. Using 5V as an output value to calculate from will be just fine.