Author Topic: dealing with multiple grounds  (Read 1772 times)

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Offline stopgoTopic starter

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dealing with multiple grounds
« on: March 26, 2008, 08:12:53 PM »
I've recently come into a situation where I have two grounds.  My setup is as follows:  I have a Sparkfun USB-UART breakout board that I'm using to talk to my microcontroller, and another that I'm using to power it and various other peripherals (sonars, etc).  I think the different grounds are causing trouble because the breakout board doesn't understand the voltage pulses being sent to it 

Can I safely wire my two grounds together?   That seems like it could theoretically be dangerous - by wiring the two together I would be forcing quick shift in voltage levels in either my computer's USB port or the other power supply.

And one last question, I'm new to using my oscilloscope.  I have a 15V battery pack I'm using, and the scope doesn't give significantly different readings for the pack's positive and negative end (only using 1 probe, probing sequentially).  The scope is centered at 0V and when I first touch it to the 15V pack the signal shows a quick jump but then appears to continue resting at 0V.  Why would that be?

Update:  I just took a risk and wired the ground AND 3.3V connected to the USB port (that is the VCC on my UART board) and my alternate power source, and now it works.  That seems troublesome to me though - what would happen if I had wired a 5V source to a 3.3V source?
« Last Edit: March 27, 2008, 12:23:47 AM by stopgo »

Offline bulkhead

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Re: dealing with multiple grounds
« Reply #1 on: March 27, 2008, 01:31:56 AM »
I'm not sure if I understand the situation.  If you have a signal that you are sending you need a "common ground" between the send and receive end.  Are you dealing with two "grounds" that are at different voltages?  I know some serial ports use negative voltages and positive voltages, with respect to ground, but you can get level shifters to go to TTL.  Is your problem similar to this?

What do you mean when you say a "5V source"?  If you just mean a 5V signal, then you can safely go to 3.3V using a 5k ohm resistor or so (this works with the propeller, at least).  The resistor drops off some of the voltage to the point where it is safe, or at least safer.  There are I believe buffer IC's that can go from voltage X to voltage Y in a completely safe manner.

I could be understanding this completely wrong.  Perhaps it would help if you posted a schematic?

Offline paulstreats

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Re: dealing with multiple grounds
« Reply #2 on: March 27, 2008, 04:27:04 AM »
you are always supposed to put the ground's together if there is more than 1. This is to make 1 common ground

Offline TrickyNekro

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Re: dealing with multiple grounds
« Reply #3 on: March 27, 2008, 05:42:16 AM »
Look ground can get very complex... Since you have NO analog ground you can easily just connect the too grounds...
If you have an analog ground meaning that you play with high frequencies, acoustic frequencies and so on... you are in trouble with the analog circuitry, cause it's gonna get noisy from the digital circuit... But you are not worried about this at all...
Just simply wire grounds and only grounds...

Wiring a 3V3 to 5V!!! please avoid it at all costs... it will fry the 3V3 circuitry... why? Cause most of the drain comes from the higher voltage not the lower... for exp: If you have a 3V battery pack and a 1V5 battery pack and wire them in parallel you will get 3V but in the meanwhile the 1V5 battery pack drains power... in a circuit of such sensitivity it may have a catastrophic result...

Anyways... just wire the grounds, please...
For whom the interrupts toll...


P.S. I've been inactive for almost a year... Don't give promises but I'll try to complete my tutorials. I'll let you know when..

Cheers!

Offline stopgoTopic starter

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Re: dealing with multiple grounds
« Reply #4 on: March 27, 2008, 10:33:13 PM »
Thanks for the information guys!  So I'll just leave the VCC on my USB-UART unconnected, and wire that ground to the ground on my circuit.

Offline ALZ

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Re: dealing with multiple grounds
« Reply #5 on: March 30, 2008, 12:43:08 AM »
Hi:

 Sounds like you have your scope set on AC input and not DC input.



I've recently come into a situation where I have two grounds.  My setup is as follows:  I have a Sparkfun USB-UART breakout board that I'm using to talk to my microcontroller, and another that I'm using to power it and various other peripherals (sonars, etc).  I think the different grounds are causing trouble because the breakout board doesn't understand the voltage pulses being sent to it 

Can I safely wire my two grounds together?   That seems like it could theoretically be dangerous - by wiring the two together I would be forcing quick shift in voltage levels in either my computer's USB port or the other power supply.

And one last question, I'm new to using my oscilloscope.  I have a 15V battery pack I'm using, and the scope doesn't give significantly different readings for the pack's positive and negative end (only using 1 probe, probing sequentially).  The scope is centered at 0V and when I first touch it to the 15V pack the signal shows a quick jump but then appears to continue resting at 0V.  Why would that be?

Update:  I just took a risk and wired the ground AND 3.3V connected to the USB port (that is the VCC on my UART board) and my alternate power source, and now it works.  That seems troublesome to me though - what would happen if I had wired a 5V source to a 3.3V source?

 


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