In the past I have seen two different schemes used to protect code in external memory. Although neither of them was used in an 8051/8052 design there is no reason that these approaches could have been used for an 8052 design today. But note, after I describe the schemes do not forget to read my final comment.

1) One clever scheme I saw was a specially designed socket into which a coin cell was afixed atop a spring contact. The socket was used to hold the external program memory which was a SRAM chip. The coin cell was used to backup power the memory chip content. If the The SRAM chip was outfitted with a label that made it look like a programmed EPROM. If a snooper tried to remove the PROGRAM memory chip it of course would lose power and the coin cell would shoot out of the bottom of the socket. (Note that although this scheme was not entirely fool proof from a hacking concept it did give the manufacturer a secondary revenue stream re-programming the units for nosey customers).

2) Another scheme I have seen was to connect the data bus and part of the address bus from the processor through an FPGA and then on to the EPROM/FLASH memory chip(s). The FPGA was programmed with an elaborate sequential decryption state machine that produced correct results only when the program ran through certain address ranges in the instruction fetch order (as opposed to linear address order). This scheme was effective for preventing a hacker from deducing the program by simply removing the chip and reading it in a device programmer. However it was possible to defeat the scheme by using a logic analyzer to probe and capture the actual address and data path values at the processor. A diligent effort would be required but none the less possible.

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My final comment.
Today if you want a code safety net then throw away your old board designs with external program memories. There are many cost effective processors with on-board protectable FLASH memory. Many many of these are our favorite 8051/8052 type processors. Whilest the processor of this type may be more expensive than the cheapest no ROM 80C31 chip with small external EPROM/FLASH it is for sure that the cost of additional procedures and circuits to try to code protect the external memory would more than offset the added cost of these more modern microcontrollers with on board FLASH. And size certainly should not be of an issue. There are 8051/8052 type clones with flash sizes from a few K bytes all the way up to 128K bytes. And I am sure even bigger parts are soon to be seen.

Michael Karas