Pro micro Wire.endTransmission() does not return

Question

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?

Thanks!

#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; 
 Wire.begin();
} // end of setup
void loop() {
 byte count = 0;
 for (byte i = 1; i < 120; i++)
 {
 Serial.println(i);
 Wire.beginTransmission (i);
 Wire.endTransmission ();
 delay(10);
 }
}

• chrisl: What do you mean by "infinite square wave"? Does that mean, that both SDA and SCL get continuously pulsed forever, when Wire.endTransmission() blocks?
• therman: Yes, and it does not stop. The waveform looks good, signal level is VCC and GND for 1 and 0.
• chrisl: Mhh, strange. Does this also happen, when nothing is connected to the bus?
• therman: Yes, that's right.
• chrisl: 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

Answer 1

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