I am using a 2,42-OLED-Display 128X64 SPI/IIC-Schnittstelle SSD1309 (display at Amazon) with an Wemos D1 and have to modify some resitors and wiring to switch to the I2C interface.
I found a video (https://www.youtube.com/watch?v=PcoylI7CkAY) with the following information how to modify the display supporting I2C:
move R4 to R3 (4.7kOhm)
bridge R5 (0Ohm)
and connect the Display Reset pin with the Wemos Reset pin and pullup this pin to VCC(4.7kOhm)
Without the connection of the Reset Pin it works, but sometimes the display gets dark and shows nothing.
With the Reset pins connected (externally pulled up to VCC) it works without a problem.
I want to understand, what could be the reason for this reset pin connection.
Regards Rainer
Reset pin has no weak pullup resistor and it is floating?
voltage of the not connected Wemos RST pin is about 3V.
But now I tested to connect the Reset pin of the display with the 3.3V and the 5V VCC voltage of the Wemos. Both seems to work well.
So connecting to the Wemos RST pin is not the point. The point seems to be keep the Reset pin of the display at high voltage, which keeps it running.
Correct. When reset is pulled low it halts (if kept low) or resets (if sent a low pulse) the processor. For the processor to run, it needs to be pulled-up high. The ESP8266EX is a 3.3V device, so the pull-up resistor should connect to the 3.3V rail. It would then be held in a known state, i.e. high. Without being connected to anything, the pin (or any wire connected to it) might pick up stray voltage from the environment or it might not and may therefore fluctuate between states, i.e. it would be "floating". The result is unpredicatble behaviour.