Hi everybody, thanks for reading my post.
On my proyect I am using a 1.8" TFT SPI ST7735 Display on a NANO processor and there is a pretty strange effect regarding the powersupply. I am connecting a 5V external powersupply to Vin and GND of the processor and use the 3.3V output to supply the TFT Display. The sketch works fine with all funktions which I am using for the display an anything else. The strange effect happens when I have the USB port of the NANO connected in order to download the code. Then the screen of the display is awfully blurred and diffused but when I use an external powersupply after download the code an disconnect the USB everything looks nice and sharp on the display. I measured the voltage of the powersupply in both cases and I have 5.1V on the input and 3.37V on the output of the processor. GND is used commonly on all devices. I am using a cellphone charger as powersupply and everything is OK as long as the USB is unplugged from the NANO. I tried to connect the same USB plug from my computer using it as external power on the processor and everything is OK as long as the NANO is unplugged from it´s USB plug. Even when I don´t plug in the external powersupply and use only the power from the USB plugged in the NANO the screen is blurred. I have replaced the NANO, the display and all other HW components on the board and on top the computer I am using but the effect does not disappear.
I´m not sure about this, why is it named "Vin" if it is not the VCC supply input ? The other V pin is an output to supply external devices used. In spite of this the external powersupply is not my problem, the strange effect appears by connecting the USB to my PC.
...ok, I did some further research and came to the following:
1.) connecting 1,8" TFT ST7785 to a NANO on a testboard. Adafruit_ST7735.h library. Example sketch "graphictest"
2.) connecting the same to an UNO, same library same sketch.
3.) connecting the same TFT to a ESP32 Dev Module
4.) on all three configurations connecting an external powersupply (Mobilephone charger) to
the 5V and GND pins of the processors
5.) connecting the TFT power pins to 3,3V pin and GND pin of the processors.
6.) for code uploading connected a USB cable from the PC to the USB-plug on the processor. On three different processors three different USB cable types (old big plug on UNO, microUSB on one NANO and C-type on another NANO and on ESP32
Triplechecked the connections from the processors to the TFT using all indicated pins for the SPI.
Results:
USB cable connected to the processor gives this image:
this happens on UNO and NANO using the Adafruit TFT library. On a ESP32 with Bodmers TFT_eSPI everything is good.
On top of this, after changing the powersupply on the NANO from Vin to +5V the display does not work properly anymore on neither USB power nor external power.
I hope you don't intentionally try to kill your display!
I usually tell posters to provide a link to the site they have bought their display.
Then responders can see if the seller provides correct information. Your display is for 3.3V, supply and data lines.
Your Arduino NANO has strong driving capability of its IO ports.
It will feed nearly 5V to the display controller, through the protection diodes of the controller's data pins. The controller of your display will effectively run on about 4.5V, which is above specs.
You can measure this on the VCC pins of the display.
If you feed through Vin of the NANO, the effective voltage will be lower, as the LDO of the processor is rather weak. Vin is only for feeding 7..9 V to the processor, without any external load.
excellent feedback Mr. ZinggIM. I actually feed the power input VCC of the Display with 3.3V from the NANO or UNO but do not put resistors on the Data lines. I will do this today and try again.
I am German but live in Chile so I have limited acces to electronic parts but I buy my electronics parts from Aliexpress and never had neither logistic nor technical problems. The 1.8" TFT have a ST7735S chip as specified on the publication of the
...my 2.4" ST7789 Display is working as well now. On this one the same effect was even worse.
Now does that mean that on ESP32 processors there is no problem on the Datalines with 3.3V devices ???
Yes, because the voltage on the data lines is never higher than VCC.
BTW: 1k series resistors for TFT displays is ok, as backlight takes more current.
With e-paper displays I had disturbance even with 4.7k series resistors with UNO.
The issue arises as soon as supply is through data lines only, zero current or even backward current on VCC.
Yes, its good luck they make 3.3V displays for 3.3V processors. Or you could say its by design.
It takes an extra effort to make display shields for (5V) AVR Arduinos, e.g. by using level shifters and strong enough LDOs (series regulators).
Please mark this topic as solved, and terminate it.
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