Using parallel port and I2C does not work (I think)

Hi there,

first I have to admit that I am a very new in terms of microcontrollers and Arduino and thus I hope my question is not too stupid and is placed at the correct place in this forum.

In my first steps I tried to realize a simple clock by getting time and temperature from a RTC (DS3231, connected via I2C) and display both on a TFT. It worked fine with my Nano and a 0.96 inch OLED-Display. Due to further development I wanted to use a larger display and bought an Arduino Uno and a 2.8 inch TouchScreen shield (Elegoo, ILI9341) which uses parallel communication. What I undestand is that parallel means I have to connect 8 pins for data transfer. But what are the LCD_CS, LCD_RS, LCD_WR and LCD_RD connections for? By googling this stuff I got the impression that this is a serial connerction (like CS, TXD, RXD) but it also have something to do with UART (which is a great mystery in my young ┬ÁC carrer). If I am right, why is this display using serial AND parallel communication?
There is also a SPI interface at the display for the SDcard reader but I do not need it so far.

The example sketches work fine but now to my essential question:

I would like to connect the shield for the first try by jumper cables (and it works fine). Later I want to use a serial/parallel interface and I think that should be possible.
BUT when I start the RTC in the sketch setup by "clock.begin();" the display does not do anything anymore. I do not understand whats going up there. The RTC is connected via I2C to the I2c pins which are not used by the display. What interaction (software/hardware) inhibits the use of both devices at the same time?

Hope you will not blow me apart! :confused:
Many thanks in advance!

Post a link to the actual display that you have.

Otherwise, we can only guess. Google finds this Elegoo 2.8 inch

I call this a "Blue 2.8 inch Mcufriend" style. Sometimes they have a White pcb.

This style of Shield uses the A4 pin for LCD_RST. (printed on the pcb)

Since the Uno shares A4 pin as Analog#4 and I2C SDA, you can't use any I2C devices on a Uno (or Nano)
It should be fine on a MEGA2560 because this uses different pins for I2C.


Well, I was not shure if it is allowed to post product links. :confused:
But you are right, this is exactly the shield I have.

I know that the Uno uses A4/A5 pins for I2C but it also has distinct SDA and SCL pins (I thought this should be the primary I2C connection due to that they are directly labled with SDA and SCL) which are not covered by the shield. And when I run the RTC on this pins I can see the values in my serial monitor. So the connection seems to be correct and working.
Yeah, so what if I connect the LCD_RST to the Arduino RST (in the sketch it is commented to be possible)? Could that be the solution?

It is ALWAYS wise to post full information. Then readers know which hardware, which software, ...

You have to do a hardware mod on the Shield. It has been described many times. Do it at your own risk.
Google "Mcufriend Uno I2C"


In the pinout diagram of my Uno I discovered that the A4/A5 pins seem to be connected in series. I think that is the core of my problem.
Thanks for the hint, David!