user80654: I have a Arduino Mega with LCD TFT 2.4"" with reading 5v signal on A8 pin (Arduino mega pin) from a raspberry pi (just to test). They are grounded together. When the Arduino is without the LCD, the analog reading works fine (4.8V,same from multimeter). But when I assembly the LCD, even if I just assembly VCC and ground, the analog read increasce around of 0.2V measure. I already tried to change the voltage input with a potentiometer and I still receiving a measure error of 0.2V.  External energy supply to the LCD makes the things works, but I wouldn't like to have a external supply just to the screen.
What I'm doing wrong or what I can do ?


user4143: MCUFriend went through a number of revisions of the same 2.4" TFT shield over time. I guess it depended on what TFT screens they could get hold of cheapest at any given moment. I have two versions here, neither of which are the same as yours. Both have their own voltage regulator onboard for the 3.3V power (which yours doesn't). Both of them also have resistors present on the cathode of the back light - which yours doesn't.
I would imagine for this revision they mistakenly though the TFT screen's back light didn't require any current limiting resistors and omitted them, with the end result that connecting the TFT screen to the Arduino drags down the 5V voltage to that of the combined forward voltage of the LEDs in the back light circuit.
In short: your TFT screen is designed badly. Fixing it would require modifying the shield on a trace-level basis. Not easy.

80654: <p>I have a Arduino Mega with LCD TFT 2.4&quot;&quot; with reading 5v signal on A8 pin (Arduino mega pin) from a raspberry pi (just to test). They are grounded together. When the Arduino is without the LCD, the analog reading works fine (4.8V,same from multimeter). But when I assembly the LCD, even if I just assembly VCC and ground, the analog read increasce around of 0.2V measure. I already tried to change the voltage input with a potentiometer and I still receiving a measure error of 0.2V.  External energy supply to the LCD makes the things works, but I wouldn't like to have a external supply just to the screen.</p>
<p>What I'm doing wrong or what I can do ?</p>
<p><a href="https://i.stack.imgur.com/ipCHm.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ipCHm.png" alt="enter image description here" /></a></p>
 4143: <p>MCUFriend went through a number of revisions of the same 2.4&quot; TFT shield over time. I guess it depended on what TFT screens they could get hold of cheapest at any given moment. I have two versions here, neither of which are the same as yours. Both have their own voltage regulator onboard for the 3.3V power (which yours doesn't). Both of them also have resistors present on the cathode of the back light - <em>which yours doesn't.</em></p>
<p>I would imagine for this revision they mistakenly though the TFT screen's back light didn't require any current limiting resistors and omitted them, with the end result that connecting the TFT screen to the Arduino drags down the 5V voltage to that of the combined forward voltage of the LEDs in the back light circuit.</p>
<p>In short: your TFT screen is designed badly. Fixing it would require modifying the shield on a trace-level basis. Not easy.</p>
