Ok then, your method looks correct. Should work. The slave address you use looks also correct. So, I don't know. 3 things to check: 1/ to make sure the direction of the SDA line is effectively set to input on the 6522 when reading the ACK bits and for the read bytes, 2/ to indeed look at SDA with a scope, to see if the signal drops when it should be read as 0, but not enough to pass the low threshold. That may be the case here. Finally, 3/ make sure the DS3231 you use is actually working - do you have any MCU dev board or a I2C-USB bridge you could test it with?