Starting a new thread as per suggestion given for my post in --"Problems calling ISP/IAP in a Philips P89C51RD2"
http://www.8052.com/forum/read.phtml?id=135547

We are using P89V51RD2 for long and we are programming it using a Flash Magic tool. We also program the serial number so that the code is no more readable.

After a year or so we are facing a comparatively strange problem. Some cards which are returned from site do not have our program (confirmed this by cross checking its functionality) but has a similar program in it.

We need to avoid the erasing & reprogramming of the chip. We have already planned to use potting technique to protect the chips, but they are ISP enabled. Initially we thought that as conventional ISP this chip will also use PSEN and if we cut it off then no body will be able to erase the chip, but it is not so. The chip does not require PSEN. SO potting & cutting off the PSEN still keeps chip re-programmable.

After going through the Flash Magic manual we thought of other solution of changing the Boot address from FC00H to something else so that nobody (we knew that with this even we will not be able to reprogram them) will be able to erase it. But the option is disabled in Flash Magic.

We do not have the liberty of changing the PCB & need to have some solution where nobody will be able to reprogram these chips, in short we want to make them OTP.

We thought of using P87V but these chips are not available in market and are comparatively costly, so the attempt fails on both grounds time & money.

We thought of changing it to AT89C51RD2 but the production has already procured 20,000+ chips and no body will take it back (they verified it).

I hope the problem is made clear enough, as I really do not know what more to explain.

First time out of options with 8051.