87C5180C51BH80C31BH

Program Lock Bits

The 87C51 has 3 programmable lock bits that when

programmed according to Table 5 will provide differ-

ent 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

Reading the Signature Bytes

The 87C51 and 80C51BH have 3 signature bytes in

locations 30H 31H and 60H To read these bytes

follow the procedure for EPROM verify but activate

the control lines provided in Table 4 for Read Signa-

ture Byte

Location

Device

Contents

30H

All

89H

31H

All

58H

60H

87C51

51H

80C51BH

11H

Erasure Characteristics

(Windowed Devices Only)

Erasure of the EPROM begins to occur when the

chip is exposed to light with wavelengths shorter

than approximately 4000 Angstroms Since sunlight

and fluorescent lighting have wavelengths in this

range exposure to these light sources over an ex-

tended time (about 1 week in sunlight or 3 years in

room level fluorescent lighting) could cause inadver-

tent erasure If an application subjects the device to

this type of exposure it is suggested that an opaque

label be placed over the window

The recommended erasure procedure is exposure

to ultraviolet light (at 2537 Angstroms) to an integrat-

rating for 30 minutes at a distance of about 1 inch

should be sufficient

Erasure leaves the array in an all 1’s state

Table 5 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

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