DS87C520_1 Datasheet, PDF (24/42 Pages) Dallas Semiconductor – EPROM/ROM High-Speed Micro

DS87C520/DS83C520
SECURITY OPTIONS
The DS87C520 employs a standard three- level lock that restricts viewing of the EPROM contents. A 64-
byte Encryption Array allows the authorized user to verify memory by presenting the data in encrypted
form.
Lock Bits
The security lock consists of three lock bits. These bits select a total of four levels of security. Higher
levels provide increasing security but also limit application flexibility. Table 11 shows the security
settings. Note that the programmer cannot directly read the state of the security lock. User software has
access to this information as described in the Memory section.
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 Encryptio n Array can be discovered.
The Encryption Array is programmed as shown in Table 10. 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 10. The Option Register sets or reads these
selections. The bits in the Option Control Register have the following function:
Bit 7 -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.
Bit 2-0
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
30h
31h
60h
Value
DAh
20h
01h
Meaning
Manufacturer
Model
Extension
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