Same ground

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Hi all,

 

I am looking to make a digital clock, using one of Microchip's RTCC (Real-Time Clock and Calendar) chip.  The chip can retain the time/data when the system is turned off via a 3.3V battery.  My question is: When the system power is supplied via the USB and the RTCC chip is also connected to the 3.3V battery.  What is the common ground here?  The ground of the USB power supply?

 

Please see the diagram.

 

Thank you!

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All of the "grounds" must be in common or there is no reference for the voltages present.

Ross McKenzie ValuSoft Melbourne Australia

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valusoft wrote:

All of the "grounds" must be in common or there is no reference for the voltages present.

 

Ok. So that would be common to the VCC, the USB +5V in this case then?

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unebonnevie wrote:

valusoft wrote:

All of the "grounds" must be in common or there is no reference for the voltages present.

 

Ok. So that would be common to the VCC, the USB +5V in this case then?

Yes.

Ross McKenzie ValuSoft Melbourne Australia

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unebonnevie wrote:

My question is: When the system power is supplied via the USB and the RTCC chip is also connected to the 3.3V battery.  What is the common ground here?  The ground of the USB power supply?

 

When it is connected, yes.  When it is disconnected, it will be 'floating'.  When reconnected it won't float anymore.  What the capacitors (Vbat, eg.) do is provide a voltage difference that allows the chip to keep running (until the capacitor/battery is empty).

 

Dunno if that helped.  S.

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 What is the common ground here?

What ground isn't common here?  You only show one ground type in your schematic.  Maybe you were questioning whether that was the right approach--yes, it is. 

Remember, voltage is ALWAYS measured as the difference between two points--that fact helps to answer a lot of questions. 

People tend to go off the rails, saying the voltage over here is 4.7V, the voltage over there is 9.53V...bad ways of saying it.

 

You can also see the common mistake they've made in their schematic (hint: maybe you can't spot it):

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Sat. May 30, 2020 - 08:25 AM