I am currently trying to write a small interrupt driven Master TWI library for the ATxmega16A4u. The current issue that I am facing is my ISR is not firing off. After I write the address byte the WIF and CLKHOLD are high while RXACK is low. The BUSSTATE is 0x02 which I believe means it is the owner of the bus. To my knowledge this state means that I have received an ACK from the slave device, but the ISR code does not get run. I have enabled global interrupts sei() as well. I can post code if that would help. I also tried the following library as well adding in an sei() in main with the same results. The board is a custom board made not by me, I can find out more about it if that may be the issue. I am hoping I am just forgetting something dumb.