Is it possible that all of reading and writing is OK but that the RTC seconds and sub seconds that you are comparing has changed due to the passage of time between the time you wrote and then read back?

It can be useful, when checking out code such as you are working with to use a fixed and known pattern instead of semi random data. Try using a 0x55/0xAA pattern and a counting pattern. Many times the pattern selected can help you to see what is wrong.

It may also be necessary for you to use a digital oscilloscope to evaluate the actual waveforms that your software is generating and make sure that they comply with the data sheet requirements of the memory part. It is very common that bit banged interfaces need to be tweaked to make the timing correct!!

Michael Karas