AUTOMOTIVE 87C5487C54-20

EPROM Program Lock

The 87C54 program lock system

when pro-

grammed protect the onboard program against soft-

ware piracy

The 87C54 has a 3-level program lock system and a

64-byte encryption array Since this is an EPROM

device all locations are user programmable See

Table 4

Program Lock Bits

The 87C54 has 3 programmable lock bits that when

programmed according to Table 4 will provide differ-

ent levels of protection for the on-chip code and

data See Table 3

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 Modes)

Reading the Signature Bytes

The 87C54 has 3 signature bytes in locations 30H

31H and 60H To read these bytes follow the proce-

dure for EPROM verify but activate the control lines

provided in Table 3 for Read Signature Byte

Location

Contents

87C54

30H

89H

31H

58H

60H

54H

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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