Arduino Uno and OLED using i2c fails at first data byte

Projects Programming
1

I am trying to use the Arduino Uno (Atmega328p) to controll a small OLED display (SSD1306 driver) using the I2C protocoll. Sending the START signal and slave address both give ACK signal back, which is good. But when sending the first data byte, the TWINT flag in the TWCR register is not getting set (indicating transmission). The wiring is good, I've tried using the u8glib library, and it controls the display with success. I've also scanned for the display, and it shows up at the correct address. I've based my code on the C code given in the Atmega48a datasheet (which had multiple code bugs). Here is my code.

  // Send START signal to slave
  TWCR = (1<<TWINT) | (1<<TWSTA) | (1<<TWEN);

  // Block until transmitted
  while (!(TWCR & (1<<TWINT)));
  
  // Check if status is START (it is)
  if ((TWSR & 0xF8) != 0x08)
    i2c_err();

  TWDR = 0x79;
  TWCR = (1<<TWINT) | (1<<TWEN);
  
  // Block until ACK is received
  while (!(TWCR & (1<<TWINT)));
  
  // Check if address has been sent
  if ((TWSR & 0xF8) == 0x20)
    i2c_err();

  Send first data byte
  TWDR = B01000000;
  TWCR = (1<<TWINT)|(1<<TWEN);
  // This while loop never stops
  while (!(TWCR & (1<<TWINT)));

It freezes at the last line. Does anybody know why it doesn't transmit?

2

PelleS:
I am trying to use the Arduino Uno (Atmega48a)

That does not make sense... An Arduino Uno has an ATmega328p...

Why are you trying to mess with registers if you're using the Arduino IDE? Why not use Wire? Or even, why not stick with the excellent U8g2 library?

3

I've must have written wrong. I'm doing it to learn.

4

I still can't get it to work. It's kind of strange, as I followed the code example in the datasheet.