I have an Arduino Pro Micro(blue version) and I'm having trouble using the I2C. To test it, I use the code below and the result is an infinite square wave on SCL and SDA lines and I only get the number 1 on the terminal output. I have pullup resistors on both wires, 2k each. Since the board uses an ATMEGA32U4, I tried the same code on a bare MCU and the code works as expected. The difference between the two is the bootloader type. The bare PCB is flashed over USB, using the factory bootloader(ATMEGA32U4-AU, QFP44). By debugging the code, I found out that endTransmission does not return. Does anyone have an idea, why the I2C is not working as expected on the Pro Micro module?


#include <Wire.h>

void setup() {
  Serial.begin (115200);
  // Leonardo: wait for serial port to connect
  while (!Serial)
  Serial.println ();
  Serial.println ("I2C scanner. Scanning ...");
  byte count = 0; 
}  // end of setup

void loop() {
  byte count = 0;
  for (byte i = 1; i < 120; i++)
    Wire.beginTransmission (i);
    Wire.endTransmission ();
  • What do you mean by "infinite square wave"? Does that mean, that both SDA and SCL get continuously pulsed forever, when Wire.endTransmission() blocks?
    – chrisl
    Nov 26, 2020 at 13:38
  • Yes, and it does not stop. The waveform looks good, signal level is VCC and GND for 1 and 0.
    – therman
    Nov 26, 2020 at 14:12
  • 1
    Mhh, strange. Does this also happen, when nothing is connected to the bus?
    – chrisl
    Nov 26, 2020 at 14:24
  • 1
    Yes, that's right.
    – therman
    Nov 26, 2020 at 15:06
  • 1
    And nothing is printing on the Serial Monitor? then I guess your microcontroller is broken. Your sketch seems legit to me. And the lines should never just pulse continuously. A common problem is the bus getting stuck, because one line gets stuck at low, but I haven't seen your problem ever. Can you tell us the frequency of the pulses? Just to check
    – chrisl
    Nov 26, 2020 at 15:38

1 Answer 1


Seems like the module has a defect, since the same code on another board produces the expected output on the I2C lines

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