DS87C520 datasheet(27/45 Pages) DALLAS

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DS87C520/DS83C520 EPROM/ROM High-Speed Microcontrollers

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Encryption Array

The Encryption Array allows an authorized user to verify EPROM without allowing the true memory to

be dumped. During a verify, each byte is Exclusive NORed (XNOR) with a byte in the Encryption Array.

This results in a true representation of the EPROM while the Encryption is unprogrammed (FFh). Once

the Encryption Array is programmed in a non-FFh state, the verify value will be encrypted.

For encryption to be effective, the Encryption Array must be unknown to the party that is trying to verify

memory. The entire EPROM also should be a non-FFh state or the Encryption Array can be discovered.

The Encryption Array is programmed as shown in Table 9. Note that the programmer cannot read the

array. Also note that the verify operation always uses the Encryption Array. The array has no impact

while FFh. Simply programming the array to a non-FFh state will cause the encryption to function.

OTHER EPROM OPTIONS

The DS87C520 has user selectable options that must be set before beginning software execution. These

options use EPROM bits rather than SFRs.

Program the EPROM selectable options as shown in Table 9. The Option Register sets or reads these

selections. The bits in the Option Control Register have the following function:

Bits 7 to 4

Reserved, program to a 1.

Bit 3

Watchdog POR default. Set = 1; watchdog reset function is disabled on power-up.

Set = 0; watchdog reset function is enabled automatically.

Bits 2 to 0

Reserved. Program to a 1.

SIGNATURE

The Signature bytes identify the product and programming revision to EPROM programmers. This

information is at programming addresses 30h, 31h, and 60h.

ADDRESS

VALUE

MEANING

30h

DAh

Manufacturer

31h

20h

Model

60h

01h

Extension

DS87C520 Datasheet(PDF) 27 Page - Dallas Semiconductor