AUTOMOTIVE 87C51FAFBFCFC-20

EPROM Program Lock

The 87C51FAFBFC program lock system when

programmed protects the onboard program against

software piracy

The 87C51FAFBFC has a 3-level program lock

system and a 64-byte encryption array Since this is

an EPROM device all locations are user program-

mable See Table 4

Program Lock Bits

The 87C51FAFBFC has 3 programmable lock bits

that when programmed according to Table 4 will

provide different levels of protection for the on-chip

code and data

Erasing the EPROM also erases the encryption ar-

ray and the program lock bits returning the part to

full functionality

Encryption Array

Within the EPROM array are 64 bytes of Encryption

Array that are initially unprogrammed (all 1’s) Every

time that a byte is addressed during a verify 6 ad-

dress lines are used to select a byte of the Encryp-

tion Array

This byte is then exclusive-NOR’ed

(XNOR) with the code byte creating an Encryption

Verify byte The algorithm with the array in the un-

programmed state (all 1’s) will return the code in it’s

original unmodified form For programming the En-

cryption Array refer to Table 3 (EPROM Program-

ming Mode)

Reading the Signature Bytes

The 87C51FAFBFC has 3 signature bytes in loca-

tions 30H 31H and 60H To read these bytes follow

the procedure for EPROM verify but activate the

control lines provided in Table 3 for Read Signature

Byte

Location

30H e 89H

31H e 58H

60H e FBH (for an FB part)

60H e FCH (for an FC part)

Table 4 Program Lock Bits and the Features

Program Lock Bits

Protection Type

LB1

LB2

LB3

1

U

U

U

No Program Lock features enabled (Code verify will still be encrypted by the

Encryption Array if programmed)

2

P

U

U

MOVC instructions 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

3

P

P

U

Same as 2 also verify is disabled

4

P

P

P

Same as 3 also external execution is disabled

Any other combination of the lock bits is not defined

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