I have done a port bit bashing interface ifor I2C to drive a serial EEPROM. I found that the ACK puse does have a maximum time for which it is available. In my case regular interrupts from an analogue output and serial RX where causing the timings to be greatly execded so as I2C read was only used at start up and write only very occasionally I had to stop all interrupts when readin or writing I2C