T84C5112-RKFEL datasheet(74/97 Pages) ATMEL

74 / 97 page

Rev. B - November 10, 2000

Preliminary

T80C5112

17. EPROM

17.1. EPROM Structure

The T80C5112 EPROM is divided into two different arrays:

· the code array: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Kbytes.

· the encryption array: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 bytes.

In addition a third non programmable array is implemented:

· the signature array: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 bytes.

17.2. EPROM Lock System

The program Lock system, when programmed, protects the on-chip program against software piracy.

17.2.1. Encryption Array

Within the EPROM array are 64 bytes of encryption array that are initially unprogrammed (all FFÕs). Every time

a byte is addressed during program verify, 6 address lines are used to select a byte of the encryption array. This

byte is then exclusive-NORÕed (XNOR) with the code byte, creating an encrypted verify byte. The algorithm, with

the encryption array in the unprogrammed state, will return the code in its original, unmodified form.

When using the encryption array, one important factor needs to be considered. If a byte has the value FFh, verifying

the byte will produce the encryption byte value. If a large block (>64 bytes) of code is left unprogrammed, a

verification routine will display the content of the encryption array. For this reason all the unused code bytes

should be programmed with random values.

17.2.2. Program Lock Bits

The three lock bits, when programmed according to Table 49., will provide different level of protection for the

on-chip code and data.

U: unprogrammed,

P: programmed

WARNING: Security level 2 and higher should only be programmed after EPROM verification.

Table 49. Program Lock bits

Program Lock Bits

Protection description

Security

level

LB1

LB2

LB3

No program lock features enabled. Code verify will still be encrypted by the encryption

array if programmed. MOVC instruction executed from external program memory

returns non encrypted data.

MOVC instruction executed from external program memory are disabled from fetching

code bytes from internal memory, EA is sampled and latched on reset , and further

programming of the EPROM is disabled..

Same as 2, also verify is disabled

This security level is available because ROM integrity will be veriÞed thanks to another

method..

Same as 3, also external execution is disabled.

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