The thing to avoid when sharing a ground pin, is using the same pin for a high current circuit (e.g. a power return) and a low level circuit, such as is often the case for a sensor.

@davee Thanks for this advice. 😀

I have decided to allocate 5 GND pins on the Pi to connect to the 5 GND pins on the display. 

The rest 3 GND pins on the Pi will remain free for using, while physical computing. (Although 1 GND pin is what I need) 

Okay then. I'm sticking to this plan and I'll surely inform about any developments. 😀 

  Yes, looking at the connector on your first photo, I assumed that only those pins shown would be connected ... the other pins were NC on the display board. But of course the R Pi pins covered by the display connector will be difficult to connect to for your sensors, etc.

Hence I suggested choices of soldering a connector on the back of the R Pi board (which understandably you declined) or using flexible wires, instead of pushing the R Pi and display boards back to back.

As for ground pins (and also power 5V or 3.3V), it is 'often permissable' to use 1 pin to go to two or more places, so you don't need to count them in the 'strict' sense required for I/O pins. The thing to avoid when sharing a ground pin, is using the same pin for a high current circuit (e.g. a power return) and a low level circuit, such as is often the case for a sensor.

P.S. What's the need of those pins labelled as NC in the datasheet?

@davee After a bit of research, I found that these NC pins(Not Connected) aren't actually connected to anything, although they are physically connected to GPIO pins. 

I was wondering, that in this way, 12 pins on the Pi, would be wasted for nothing.(When we connect Pi and display directly).

But by framing out our plan, we've saved 16 GPIO pins (2 5V, 2 3.3V, 12 NC pins) 😀 😀 

I completely forgot that the Raspberry Pi has 8 GND pins and not 5. 

I apologise for my carelessness. 

more than 1 ground return would be a better configuration.

@davee Okay then. I will surely follow your advice. 😀

But the problem is, I can allow only 4 GND connections for this purpose, as I'll be using this 5th one for physical computing. Will that be okay? 

  Without seeing your physical wiring layout it is difficult to answer your query, but I suspect more than 1 ground return would be a better configuration.

It is difficult to explain why, but I'll try to give you an insight.

Electrical current always flows in a complete loop circuit.

Think of a simple circuit like a torch (flashlight) using an old style filament bulb.

When the torch is lit, the current flows from one terminal of the battery, through the bulb, through the (closed) switch, and returns to the second terminal of the battery.

If the circuit is interrupted at any point, such as by opening the switch, then current flow will cease.

In the case of the torch, the single circuit loop is obvious.

In the case of the R Pi and the touch display there are many such loops, but they all obey the same principle. The ground wires form part of those circuit loops, roughly corresponding to the return connection to the second terminal of the battery for the torch.

For complex circuits with many loops, the return currents may have an opportunity to interact with each other, which in severe cases may actually prevent the performing its expected function. Hence, it is common in digital circuits for signal wires to have accompanying ground return wires, so that the signals do not become 'mixed up'.

In the case of the R Pi and the touch display, having separate ground returns for the 5V supply and for the data (signal) wires labelled TP-xx seems a very reasonable approach.

If a single wire ground wire were used for both tasks, it is possible that high current pulses associated with the 5V power demands could be 'mistaken' for data signals on the the TP-xx lines.

Sorry, this is a very vague, yet rather long explanation.

However, poorly designed 'ground' return strategies are one of the most common reasons new circuit designs do not behave as expected. With digital circuits, it is most commonly seen as circuit that works 'most of the time' but occasionally fails ... sometimes described as due to a 'glitch'.

And of course, you need the HDMI to HDMI link.

@davee Forgot to mention the HDMI connector. 

Posted by: @davee

You will also need to connect a ground to accompany the TP_XX wires...  preferably use the nearest ground pin to the TP_XX wires on the connector.

Only one ground connection will do, right? Because the display actually needs 5 GND connections. 

  You will also need to connect a ground to accompany the TP_XX wires...  preferably use the nearest ground pin to the TP_XX wires on the connector.

  And of course, you need the HDMI to HDMI link.

Otherwise, your plan concurs with my impression.

Good luck!

To "add" the "touch" functionality, you also need to connect some of the pins near the top of the connector in your board photo. These are the pins labelled TP_xx in your most recent post

@davee Then, I'll only need to connect 5 pins of the GPIO to the display.(The TP_xx ones)

5V, 3.3V and GND pins on the Pi shall remain free, as I'll power up the display using the USB cable. 

This is my plan. What's your opinion on this? 🙂

P.S. What's the need of those pins labelled as NC in the datasheet?

completely separate the cards, and reconnect using flexible wires

• This method obviates the need for a 'tricky' soldering job, and enables you to minimise the number of GPIO pins dedicated to the touchscreen, but leaves you with two 'dangling cards' that will need some mechanical support for continued usage.

I think this method shall work for me, as I have never soldered anything.

Posted by: @davee

+5V and ground (0V) ... for power. This can be either directly to the pins on the 28 pin connector, shown near the bottom of the connector in your photo, or by the microUSB socket. (You might be able to use 3.3V instead of 5V using different pins, but I doubt if that is helpful in your case .. I suspect that is for 3.3V processors, not the R Pi which is 5V.)

I want to use the micro USB socket for powering up the display, so that the 5v,3.3v and GND pins on the Pi are free for physical computing. 

Anyways, thank you very much for your solutions. 😀