T84C5112-RKSCV datasheet(73/97 Pages) ATMEL

73 / 97 page

Rev. B - November 10, 2000

Preliminary

T80C5112

16. ROM

16.1. ROM Structure

The T80C5112 ROM memory is divided in three different arrays:

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

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

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

16.2. ROM Lock System

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

16.2.1. Encryption Array

Within the ROM array are 64 bytes of encryption array. 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.

16.2.2. Program Lock Bits

The lock bits when programmed according to Table 41. will provide different level of protection for the on-chip

code and data.

U: unprogrammed

P: programmed

16.2.3. Signature bytes

The T80C5112 contains 4 factory programmed signatures bytes. To read these bytes, perform the process described

in Section ÒSignature bytesÓ, page 75.

16.2.4. Verify Algorithm

Refer to Section ÒVerify algorithmÓ, page 76.

Table 48. Program Lock bits

Program Lock Bits

Protection description

Securi-

ty level

LB1

LB2

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.

Same as 2, also verify is disabled

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

method.

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