Hi Manfred,

I hate to say that but you can’t write a byte in the flash rom without erase entire memory for the AT89S52 (with some particular exceptions)
As for the “retention time “, I would like to live as much as the flash rom reliability is.
The retention time is more then 20 years ( but you can expect the twice ).
Thank you for your mail.
You can send the chocolate at the mail address : ( elcris@k.ro) J
Don’t forget to share it with the forum moderator for a bigger success J
Of course, I will help you to develop your project as much as I can.
But I cannot make all your design; I can advice you and ever make some software routines but not the entire project (unless a business proposition).
Send me details about your design and some particular questions.
I shall try to help you, as my limited time will allow me
Best regards,

Cristian