Sorry for the previous empty msg!. I am now having a problem simulating talking to an I2C device PCF8575, my main program works corrrectly with hardware so i know its doing the right thing but if i debug in sim mode and watch writes to IICOUT (vtreg) i keep getting 0x7F01 which im not sure of. 0xFFF=STOP, 0x0100=START, 0xFF00=ACK, 0xFF01=NACK, but why do i see 0x7F01 appear, sometimes twice. Does or has anyone seen 0x7F01 when I2C simulating?