I am using ATmega8 with 12MHz crystal, on my breadboard. I can use SDA and SCL pins with default arduino library wire.h, and it works. But I want another pins to use I2C protocol.

I tried this library, even on real SDA and SCL pins, but it doesn't work.

I have pull up resistors, I tried with 10k and 4.7k without any success. (But those resistors worked with hardware I2C | TWI)

What am I doing wrong? (All connections is fine, because hardware TWI works but software does not.) Thank you for your help.

(Fuses are ok, because the Atmega won't respond if I remove the crystal oscillator, the LED blinking duration is ok.)

What i want to run (that doesn't work):

#define SDA_PIN 4
#define SCL_PIN 5

//#define I2C_TIMEOUT 100

#include <SoftI2CMaster.h>

uint8_t ADDR  = 0x68;

void setup(void) {

  Serial.println("Initializing ...");

  if (!i2c_start(ADDR | I2C_WRITE)) Serial.println(F("Device does not respond"));
  if (!i2c_write(0x00)) Serial.println(F("Cannot address reg 0"));

void loop (void) {
  unsigned int low0, high0, low1, high1;
  unsigned int chan0, chan1;
  unsigned int lux;
  int state1;
  int state2;

  i2c_start(ADDR | I2C_WRITE);
  i2c_rep_start(ADDR | I2C_READ);
  low0 = i2c_read(false);
  high0 = i2c_read(false);
  low1 = i2c_read(false);
  high1 = i2c_read(true);

And the code with hardware TWI that works:

#include "Wire.h"
#define DS3231_I2C_ADDRESS 0x68
// Convert normal decimal numbers to binary coded decimal
byte decToBcd(byte val)
  return( (val/10*16) + (val%10) );
// Convert binary coded decimal to normal decimal numbers
byte bcdToDec(byte val)
  return( (val/16*10) + (val%16) );
void setup()
  pinMode(13, OUTPUT);
  pinMode(5, INPUT);
void readDS3231time(byte *second,
byte *minute,
byte *hour,
byte *dayOfWeek,
byte *dayOfMonth,
byte *month,
byte *year)
  Wire.write(0); // set DS3231 register pointer to 00h
  Wire.requestFrom(DS3231_I2C_ADDRESS, 7);
  // request seven bytes of data from DS3231 starting from register 00h
  *second = bcdToDec(Wire.read() & 0x7f);
  *minute = bcdToDec(Wire.read());
  *hour = bcdToDec(Wire.read() & 0x3f);
  *dayOfWeek = bcdToDec(Wire.read());
  *dayOfMonth = bcdToDec(Wire.read());
  *month = bcdToDec(Wire.read());
  *year = bcdToDec(Wire.read());

Schematic is the same as: https://www.arduino.cc/en/main/standalone.

Except instead of 16MHz, I use a 12MHz and ch210x serial.

Arduino's bootloader works well, I compiled it from source.

  • It would help if you posted code and schematics of both situations (hardware and software I2C attempts). Also, to debug I2C effectively, you'd need a logic analyzer or at very least an oscilloscope.
    – Ricardo
    Jul 20, 2015 at 16:50
  • What do you get on the serial output?
    – Gerben
    Jul 20, 2015 at 18:04
  • Initializing ... Device does not respond Cannot address reg 0 SDA - SCL 0 0 Raw values: chan0=65535 SDA - SCL 0 0
    – ONSC
    Jul 20, 2015 at 18:34
  • actually i should get the ohur:minute:second day/month/year that i can get in hardware twi,
    – ONSC
    Jul 20, 2015 at 18:35
  • i ve removed some of code, just before he stop function , i get the pinstate to whats happening, so that output is showed
    – ONSC
    Jul 20, 2015 at 19:18

1 Answer 1


OK let me cry till morning...

i was having this issue during day, but the solution was sooo easy

i need to pay attention more ...

i2c_start(addr | R/W-bit) Initiates a transfer to the slave device with the (8-bit) I2C address addr.

what i was trying is to push 7 bit address.

i hope this might help some others.

Thank you for help.

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